Special Flow Meters, Flow Gages, and Switches

CHAPTER X SPECIAL FLOW METERS, FLOW GAGES, AND SWITCHES

Chapter Outline 1.0.0 Introduction 2.0.0 Hall Effect Sensing and Flow Measurement 2.1.0 Theoretical Background of Hall Effect 2.2.0 Hall Effect Sensor Types 2.3.0 Magnetic System 2.4.0 Hall Sensor Features and Applications 2.5.0 Specification of Hall Sensors 3.0.0 Magnetic and Proximity Pickup and Flow Measurement 3.1.0 Magnetic Pickup and Flow Measurement 3.2.0 Proximity Pickup 3.3.0 Signal Conditioning Unit 4.0.0 Cryogenic Flow Measurement 4.1.0 Discussion on Cryogenic Flow Measurements 4.2.0 Differential Pressure Type Cryogenic Flow Measurement 4.3.0 Turbine Meter in Cryogenic 4.4.0 Vortex Meter in Cryogenic Applications 4.5.0 Coriolis Mass Flow Meter in Cryogenic Applications 4.6.0 Ultrasonic Flow Meter in Cryogenic Applications 4.7.0 Processing Electronics in Cryogenic Applications

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1.0.0 INTRODUCTION Starting from various principles of measurements, various kinds of flow meters have been discussed at length in previous chapters. The flow meters discussed so far are commonly found in various plants and industrial applications. Therefore, these can be considered as common flow meters. However, apart from various common flow meters there are a few flow meters which make use of some special physical phenomena for flow measurements. These are special flow meters. These are not very commonly used. However, under certain conditions and measurement constraints they are found to be extremely useful. Cryogenic condition is a special condition, and measurement of flow in that condition is not easy.

5.0.0 Flow Gages 5.1.0 Direct-Flow Gages 5.2.0 Sight Flow Indicator 5.3.0 Digital Local Flow Indicator 6.0.0 Mechanical Type Flow Meters 6.1.0 Mechanical Water Meters 6.2.0 Mechanical Oil and Other Flow Meter 7.0.0 Flow Switch 7.0.1 Definitions and Terminologies With Explanations 7.0.2 Flow Switch Types 7.1.0 General Requirements of Flow Switches With Explanations 7.2.0 Flow/No-Flow Switch: Paddle(/Vane) Type 7.3.0 In-Line and DP Type Flow Switches 7.4.0 Variable Orifice Type Flow Switches 7.5.0 Thermal Dispersion Type Flow Switch (Monitor) 7.6.0 Discussions on Miscellaneous Flow Switches 7.7.0 Discussions on Solid (Bulk) Flow Monitors List of Abbreviations References Further Reading

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Cryogenic flow meters are examples of some of the special flow meters. In some plants it is not always necessary to use flow meters for flow monitoring. Hall effect flow sensing is not a flow meter in the true sense. It represents a way to sense flow. Like magnetic pick, up this is another special way of flow sensing. Hall effect sensors are related to flow measurement in different ways. It can be used as a sensing element to compute flow in a turbine flow meter or paddle wheel flow (switch). If we look at solid flow measurement, a Hall effect sensor is utilized here also for sensing of conveyor speed measurement, which is directly related to solid flow measurement. Hall effect can be used to sense zero flow and this is also related to protection of solid flow

Plant Flow Measurement and Control Handbook. https://doi.org/10.1016/B978-0-12-812437-6.00010-X Copyright © 2019 Elsevier Inc. All rights reserved.

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measurement through a conveyor. Simple flow/ no-flow conditions would suffice, e.g., a big fan/ pump lubrication system, here there is a need to ensure that there is flow in lubrication line when the lube pump (in fact for that matter the main fan or pump) is started locally. Therefore, a simple flow/no-flow gage is sufficient. In this case, metering of flow by a flow meter would neither be cost-effective nor in many cases, on account of the short space installation of the flow meter, may it be feasible. When a big fan and/or pump is started through an auto-program or in running condition it is necessary to interlock the fan/pump with lube oil system for the safety of the fan/ pump. When the lube oil flow is less, first there may be a pretrip alarm followed by tripping of the concerned fan/pump. In all such cases switches (not the flow meter) are deployed. Some use a flow switch, some use a pressure switch. In many mixing and batch controls, the flow switch is necessary for the recipe to maintain quality of the output product. In all such cases flow switches are used. In this section these special flow instruments and flow gage flow switches have been covered to complete the discussions on flow monitoring. In many cases the operating principles of these flow gages and switches are similar to flow meters but the designs and/or installations are different, so these need separate treatment. It has been seen that Hall effect sensors are used for flow sensing in many instruments already discussed, such as turbine flow, to measure the speed of the rotating device. In the same way it can be used to detect the speed of a conveyor and/or zero speed switch for a conveyor which is used in solid flow measurement. Other than Hall effect sensors, often magnetic pickups and proximity pickups are also used. These are very useful in flow measurement. It is better to look into the details of Hall effect sensing along with other inductive pickups. The discussion starts with Hall effect sensing.

2.0.0 HALL EFFECT SENSING AND FLOW MEASUREMENT The Hall effect is a very effective sensing technology. It is named after its inventor, Edwin Herbert Hall, who invented it in 1879. When a

metal plate is connected across a power source then charges from the battery outlet go to the metal piece and pass through it in a straight path and return to the battery to complete the current path. When the same arrangement is placed in a magnetic field perpendicular to the direction of current flow it can be noticed that, on account of Lorenz force, charge careers are deflected and move towards the edges. This is the fundamental principle on which the Hall effect has been developed. The Hall element is a solid-state device developed from a thin sheet of semiconductor material. When it is supplied with a voltage source and is subjected to a magnetic field, it responds with an output voltage proportional to the magnetic field strength, with output connections perpendicular to both the direction of current flow and direction of magnetic field. The output voltage developed is very small (mV) and requires additional electronics to achieve useful voltage levels. Therefore, the Hall element combined with the associated electronics forms the popular Hall effect sensor. This magnetic field sensor has a very wide range of applications. It can be used as a sensor for speed, flow, current, temperature, pressure, position, etc. In this connection Section 9.0.0 of Chapter IV (where short discussions on this have been provided)may be referenced also. There are a few issues pertinent to the Hall effect sensor worth noting: 1. General sensor selection: For selection of the sensor there are certain fundamental principles to be followed. One important issue is to identify the input and output requirements, application requirements, and match these with the major sensing device components. Engineering judgment is the only tool, such engineering judgments come only after the strengths and weaknesses of each approach are weighed. The major issues are listed here: l Overall cost; l Device availability; l System and device complexity; l Tolerance of field conditions; l Compatibility with other system components [1]; l Reliability;

Special Flow Meters, Flow Gages, and Switches Chapter | X

System performance including repeatability; l Maintenance issues. 2. Hall effect as the preferred sensor: While selecting a sensing element, cost, performance availability, and mounting facilities are normally considered as primary issues. From all these considerations, Hall effect sensors are preferred mainly on account of the following reasons: l True solid state: Fast acting, low power; l Static, no moving parts; l Long life; l Good performance, including high repeatability; l Operates with stationary input (zero speed) [1]; l Available in analog and digital form; l Logic compatible input and output; l Wide temperature range for operation. 3. Design requirements of the Hall effect sensor with a system: As the Hall effect sensor is a magnetic sensing it requires a magnetic system capable of responding to the physical parameter to be sensed. Since silicon has a piezoresistive effect, the design should take care to minimize this effect. The physical parameter actually interfaces with the sensor. In most cases these are mechanical interfaces. The Hall effect sensor senses the changes in magnetic field to produce an electrical output. Here it is the responsibility of the output interface to match the output signal with the system requirements to fulfill the application objective. Therefore there are basically four blocks here, i.e., input interface (measuring parameter) with input interface, magnetic system, Hall element, and finally the output interface. This has been clearly depicted in Fig. X/ 2.0.0-1. Also, there should be a suitable interconnection amongst them according to the application requirements. It is worth noting that it is not mandatory that all four elements discussed above are necessary, e.g., for any measurement related to the magnetic field sensing system, does not require a magnetic system. l

MEASURING

929

QUANTITY

INPUT NPU NP UT INTERFACE HALL HA A ALL ELEMENT

SENSING G M MECHANISM ECHANISM HAN M OUTPUT OUTPU OU O UT UTP TPUT INTERFACE

ELECTRICAL OUTPUT

FIGURE X/2.0.0-1 General Hall effect device.

While defining the input to the sensor it is necessary to take care of the following: l Input parameter range values with possible rate of change (maximum and minimum); l Factors which can affect the measurement (temperature/EMI); l Safety factor to be chosen; l Probable sources of error; l Allowed tolerance limits; l Ambient conditions. Similarly, output characteristics are also guided by the following manner: l Electrical characteristics, i.e., output forms in current, voltage, pulse train, logic levels, etc.; l For digital output meaning and level of signals for 0 and 1; l Output at sensor OFF condition and interpretation; l Output load value and types (e.g., resistive); l Interconnection details with allowed cable length; l Output characteristics, i.e., sourcing/sinking; l Performance requirements; l Available space and weight. There is also requirement of system definition which includes but is not limited to the following: l Gap (minimum/maximum) between Hall element and magnet; l Maximum minimum allowed magnet travel;

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l l

l l

l

Plant Flow Measurement and Control Handbook

Mechanical linkage if required; Sensor output type (to match source/sinking type); Allowed operating and storage temperature; Selection of the magnetic mode, magnet, Hall effect sensor, and functional interface [1]; Matching of input/output requirements. We now examine the theory of operation for Hall effect sensing.

2.1.0 Theoretical Background of Hall Effect In this section the theory of operation of Hall effect sensing is discussed. The Hall element is a thin sheet of semiconducting material which can pass current through it. So when a voltage is applied to terminals 1 and 2 in Fig. X/2.1.0-1, the current will flow undisturbed from terminal 1 to 2 and if voltage is measured across terminals A and B there will be zero potential difference, when there is no magnetic field applied. When a magnet is brought near the Hall element, a Lorentz force is exerted on the current. This force disturbs the current distribution, based on the magnetic pole present near it, the charges are deflected from each other rather being shifted towards the sides of the Hall elements, as shown in Fig. X/ 2.1.0-1. As a result there will be positive and negative charges at the two ends. Therefore,

when voltage is measured across the Hall element there will be a potential difference between terminals A and B. This is Hall voltage, VH. It is worth noting that the voltage developed will be proportional to the vector cross-product of strength of current (in ampere) passing through the semiconductor and magnetic flux density (in Tesla) applied as shown in Eq. X/2.1.0-1. I VH ¼ RH $  B T

(X/2.1.0-1)

This also indicates that the output is perpendicular to both the direction of current and the magnetic field. RH is the Hall effect coefficient and T is the thickness in mm. The strength of voltage developed is in the order of a few microvolts, typically RH is 7 mV/Vs/Gauss. Naturally, in order to make the voltage workable for all practical applications it is necessary that there will be some signal conditioning units associated with the Hall element. Common mode voltage is an important issue here. If no magnetic field is applied, but there is some voltage at a terminal with respect to the ground, then it is common mode voltage and is the same at each output terminal. In order to get rid of this, like any other analog circuit, the first stage of the amplifier is a differential amplifier. A typical Hall element with signal conditioning unit(s) is discussed in the following section. 2.2.0 Hall Effect Sensor Types Hall Effect sensors are of two types: analog and digital. In this section both types are discussed, starting with the analog type.

N

2

S A

-

-

-

-

-

-

+

+

+

-

+

+

+

+

+

B

1

VH

FIGURE X/2.1.0-1 Hall effect sensing principles.

2.2.1 ANALOG TYPE SENSOR The analog sensor is depicted in the first part of Fig. X/2.2.0-1 i.e., up to the first output. As shown in the figure, a regulated voltage source drives the current through the Hall element, which is subject to a magnetic field. As the voltage is regulated it is constant, thus in analog sensors the output voltage is proportional to the strength of the magnetic field to which it is

Special Flow Meters, Flow Gages, and Switches Chapter | X

REGULATED

ANALOG TYPE

931

ONLY FOR DIGITAL TYPE

POWER UNIT

OUTPUT

HE HI GAIN

SCHMITT

AMPLIFIER

TRIGGER

HE: HALL ELEMENT

OUTPUT

ONLY FOR DIGITAL TYPE FOR DIGITAL TYPE ADDITIONAL

NULL POINT CROSSING

VS

ANALOG TYPE MAGNETIC FIELD

ANALOG TRANSFER FUNCTION

ANALOG OUTPUT

NULL POINT

ANALOG OUTPUT

SCHMITT TRIGGER STAGE IS NECESSARY

BIPOLAR DIGITAL TYPE

0 MAGNETIC FIELD

DIGITAL TRANSFER FUNCTION

FIGURE X/2.2.0-1 Hall effect sensing types.

exposed. Depending on magnetic polarity there will be different ways that the charges are migrated towards the periphery of the Hall element. As a result of this the output of the amplifier will be either positive or negative. This would then necessitate both plus and minus power supplies. In order to avoid this, a fixed bias is introduced into the differential amplifier. Therefore, when a positive magnetic field is sensed, the output increases above the null voltage and when a negative magnetic field is sensed, the output decreases below the null voltage, but remains positive. Therefore, output is always positive. Naturally, when there is no magnetic field applied, the bias value would appear on the output. This is referred to as null voltage, which is the crossing point of the voltage output curve with the magnetic field axis, i.e., where the positive side increasing output curve meets with the negative side increasing curve in Fig. X/2.2.0-1. As is seen in this figure, as the magnetic strength is increased

in either side the output voltage changes proportionately across the null point—but how long can it go? Theoretically it can go up to the supply voltage level, but before that it is saturated as shown. Transfer function represents a graph or equation of output in terms of input. An analog type sensor transfer function is characterized by sensitivity, null offset, and span as shown. Sensitivity is defined as the change in output resulting from a given change in input, i.e., the slope. Span and null points are also very well shown in Fig. X/ 2.2.0-1 and can be easily arrived at. 2.2.2 DIGITAL TYPE SENSOR As the name implies, digital type sensors give digital output. It is the same as an analog type with an additional stage as shown in Fig. X/2.2.0-1, i.e., the second output after the Schmitt trigger stage. A basic analog circuit has been modified with the use of a Schmitt trigger. A Schmitt trigger

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works basically as a comparator. It compares the output of the differential amplifier with a set point/ reference. As long as the output is greater than the set point the output is logical 1 and below that it is logical 0. Therefore, the sensor has an output that is just one of two states: ON or OFF. Hysteresis is included in the Schmitt trigger circuit for jitter-free switching [1]. Like any other hysteresis loop, here also there are two distinct reference values at times referred to as set and reset depending on whether the sensor is turned ON or OFF. The transfer function for a digital output Hall effect sensor with hysteresis is shown in Fig. X/2.2.0-1. As the magnetic field is increased, no change in the sensor output will occur until the reference/set point is attended. Once the operate point is reached, the sensor will change state, e.g., from the OFF state to the ON state and any further increases in magnetic input beyond the operate point do not have an effect. The point where the changes occur is referred to as the set point. When the magnetic field is decreased it will remain in the ON state until a point is reached when the output changes state from the ON to OFF state. Other than an ideal system this point will be separate from the set point and is called the reset point. The differential between the set and reset points is the hysteresis and serves the useful function of eliminating false triggering. The input characteristics of a digital sensor are defined in terms of set and reset points, and differential. Depending on the set and reset points the sensitivity and resolution are determined. Since these characteristics change with temperature and from sensor to sensor, they are specified in terms of maximum and minimum values. Maximum operate point refers to the level of magnetic field that will insure the digital output sensor turns ON under any rated condition. Minimum release point refers to the level of magnetic field that insures the sensor is turned OFF [1]. Digital output may be unipolar or bipolar (as shown in Fig. X/2.2.0-1). When unipolar both poles of magnet output are positive but values are different. On the other hand, when bipolar it is around the zero point as shown.

2.3.0 Magnetic System In Hall effect sensors, physical parameters such as position, speed, and flow are converted into electrical output in the presence of a magnetic field. Therefore, it is needless to say that magnetic field strength has a good influence on the operation of the sensor. This concept has been depicted in Fig. X/2.3.0-1A. Naturally the configuration and orientation of the magnetic field with respect to the Hall element are extremely important. There are two types of magnetic field: unipolar and bipolar. 2.3.1 UNIPOLAR MAGNETIC SYSTEM In a unipolar magnet only one pole is towards the sensor and the other is away, as shown in Fig. X/ 2.3.0-1B. Unipolar system can be two types viz. “head on” type and “slide by” type as shown in Fig. VII/2.3.0-1B. 1. Unipolar head-on mode: In case of a head on, the magnet’s direction of movement is directly toward and away from the sensor, with the magnetic lines of flux passing through the sensor’s reference point. In Fig. X/2.3.0-1B the south pole of the magnet will approach the sensing face of the Hall effect sensor. In the unipolar head-on mode, the relation between Gauss and distance is given by the inverse square law. Distance is measured from the face of the sensor to the pole of the magnet, along the direction of motion. Magnetic filed versus distance have been shown in Fig. X/2.3.0-1B. 2. Unipolar slide-by mode: In this mode, the sensor and magnet have a vertical gap and a magnet is moved in a horizontal plane sidewise. Distance in this mode is measured relative to the center of the magnet’s pole face and the sensor’s reference point in the horizontal plane of the magnet. The magnetic field versus distance relation in this mode is a bell-shaped curve, which is also shown in Fig. X/2.3.0-1B. 2.3.2 BIPOLAR MAGNETIC SYSTEM In a bipolar system, as the name suggests, there will be two poles of the magnet approaching and

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(A)

(B)

(C)

FIGURE X/2.3.0-1 Magnetic system of Hall sensor. (A) Effect of magnet on sensor. (B) Unipolar magnetic system. (C) Bipolar magnetic system. (C) Developed based on Hall Effect Sensing and Application, Honeywell (Technical internet document). https://sensing.honeywell.com/hallbook.pdf. Courtesy: Honeywell.

going away from the sensor. In a bipolar system a slide-by mode is possible. A bipolar system with bipolar slide-by mode has been depicted in Fig. X/2.3.0-1C. Here there are basically two sets of magnets moving in the same fashion as the unipolar slide-by mode. In this mode,

distance is measured relative to the center of the magnet pair and the sensor’s reference point. The Gauss versus distance relationship for this mode is an “S”-shaped curve as shown in Fig. X/ 2.3.0-1C [1]. There is another possibility of a magnet in ring form in this manner.

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For further reading and comparison the chart in Ref. [1] may be referenced. 2.4.0 Hall Sensor Features and Applications Hall effect sensors have a number of features and applications worth noting. However, our discussions are limited to flow applications only. As speed measurements and zero speed monitoring form a part of solid flow measurements these have been included here. 2.4.1 FEATURES OF HALL EFFECT SENSORS In flow sensing the following features of Hall effect sensors are important:

5. 6. 7. 8. 9. 10. 11.

Wide working voltage (3.8e30 VDC); Low power consumption; Maximum current drawn is <20 mADC; Performance of accuracy around 1% possible in some cases but normal accuracy is lower than this; Highly durable; Open collector output possible; Wide range of temperature 40 to 150 C; High humidity-withstand capability; All SS construction possible; Nonintrusive measurement; Suitable for hazardous applications with FM/ other certification from approving authorities. HALL SENSOR ASSEMBLY

MAGNET

1. 2. 3. 4.

2.4.2 APPLICATIONS IN FLOW MEASUREMENT It is worth noting that the Hall effect sensor is always used with any flow transducers as measurement of any desired parameter of the flow transducer, e.g., the impeller speed of a turbine flow meter is a measure of the flow rate. The Hall effect sensor can be used to measure the flow in an indirect way. Therefore, there are several different types of flow transducers that can be used in conjunction with the sensor to measure/ compute flow rate in the conduit. Fig. X/2.4.0-1 illustrates a typical flow measurement scheme. The fluid to be measured flows through the sensor and is directed past the paddle wheel. The paddle wheel rotates and one Hall effect sensor as shown is mounted at the sensor case near the vicinity of the paddle wheel. As each of the paddle wheel edges passes the sensor it interrupts the magnetic field created by the magnet with the sensor being interrupted, hence VH of the sensor results small AC pulses at the sensor output. This small AC output of the sensor through the signal conditioning unit, including the Schmitt trigger, produces a digital signal output proportional to the flow. As the flow rate through the meter increases, there will be more pulses per second. As there are multiple pulses per rotation and the sensor is linear with regard to the number of pulses per volume irrespective of flow rate it is easy to interface with the

HALL ELEMENT PADDLE WHEEL

NS

FIGURE X/2.4.0-1 Hall effect sensor in flow measurement.

Special Flow Meters, Flow Gages, and Switches Chapter | X

microcontroller, an essential part to run the embedded system. A embedded microcontroller regulates the entire operation of the Hall effect sensor. 2.4.3 APPLICATIONS IN SPEED SENSING AND MEASUREMENT The speed sensor is one of the most common applications for a Hall effect sensor. It is even used to measure the speed of large turbines. This is an example of magnetic measurement. The required magnetic flux to operate the sensor may be furnished by magnets mounted on the shaft or hub or by a ring magnet, typically as shown in Fig. X/2.4.0-2. In this figure it can be seen that in one case a magnet is mounted on the shaft, whereas in the other case a magnet is embedded on the hub. In both cases, as the shaft rotates there will be a change in the magnetic flux density so that the Hall effect sensor can measure the number of rotations per minute of the shaft. Alternatively, without mounting a magnet on the shaft/ hub, it is also possible to measure the speed in a similar fashion shown for flow in the paddle

wheel (or turbine meter). The main issue is to bring about the change in the magnetic field due to rotation of the shaft/flow so that it can be measured by the Hall effect sensor. Most of the RPM sensor functions of interest to us are listed here: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Speed control; Control of motor timing; Zero speed detection; Under or over speed detection; Disk speed detection; Shaft rotation counter; Bottle counting (in dispensing machine); Flow-rate meter; Tachometer pick-ups.

2.5.0 Specification of Hall Sensors A brief specification of a Hall effect sensor has been enumerated in Table X/2.5.0-1. Now we look for another kind of pickup used in flow meters and for speed measurement of conveyor related to solid flow measurement, for which magnetic pickup and a proximity sensor are commonly used.

S N S N S N S N

N

S

S N S

N

N

S

S

935

FIGURE X/2.4.0-2 Hall effect sensor for speed measurement.

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TABLE X/2.5.0-1 Specification of Hall Effect Sensor SL

Specifying Point

Standard/Available Data

1

Supply voltage

3.8e30 VDC

2

Current

Normally <20 mADC

3

Operating frequency

0e20 KHz (typical)

4

Rise time

To specify

5

Fall time

To specify

6

Types

Analog/digital

7

Output

Voltage/current (analog) or open collector source/sink (digital)

8

Output function

Described in main body

User Spec.

Remarks Sensor specific

Materials of Construction (MOC) for 9

Body

Stainless steel SS 316

Connection and Mounting Details 10

Process

Threaded

11

Electrical

½00 NPT/ET

Performance and Other Details 12

Accuracy

1 FSD

13

Certification

Necessary certification from appropriate authority for hazardous applications

14

Application of interest

Flow/speed

15

Operating Temperature

40 to 150 C

16

Special feature

To specify

3.0.0 MAGNETIC AND PROXIMITY PICKUP AND FLOW MEASUREMENT Like Hall effect sensors, magnetic pickups and proximity pickups are commonly used in flow meters as well as for measurement of speed for conveyors necessary for solid flow measurements. Both principles are quite close, and so are covered in this section. In this section brief discussions on each have been presented. In this connection Section 2.1.3 of Chapter V may also be referenced for short discussions on pick ups. 3.1.0 Magnetic Pickup and Flow Measurement When one recalls the discussions on turbine/PD flow meters, magnetic pickups are extensively

To specify

used. Also, for speed measurement of conveyor belts, magnetic pickups are used extensively. We now start the discussions on magnetic pickup working principles. 3.1.1 MAGNETIC PICKUP WORKING PRINCIPLES The magnetic pickup produces a voltage output when any magnetic material moves through the magnetic field at the end of the pickup. Any device which produces a dynamic discontinuity of magnetic material in the field of the pickup will produce an electrical voltage. A magnetic pickup consists of a coil wound around a permanently magnetized probe. When any ferromagnetic objects—such as gear teeth, turbine rotor blades, slotted discs, or shafts with keyways—cross the

Special Flow Meters, Flow Gages, and Switches Chapter | X

probe’s magnetic field, the flux density is modulated. On account of flux density modulation or cutting of the lines of force by the object, an AC voltage will be induced in the coil. One complete cycle of voltage/pulses will be generated for each object passed. If the objects are evenly spaced on a rotating shaft, the total number of cycles will be a measure of the total rotation, and the frequency of the AC voltage will be directly proportional to the rotational speed of the shaft. The magnetic speed pickup (MPU) is used to detect the speed of the prime mover. Therefore, in the area of flow measurement it can be used to measure the speed of the conveyor in solid flow measurement or it can be used with fluid flow meters, such as turbine and PD meters, to measure fluid flow in the conduit. Output waveform is a function of the following: 1. 2. 3. 4. 5.

Rotational speed of the moving object; Gear-tooth dimensions; Gear teeth spacing (gaps between gears); Dimension of the pole-piece (diameter); Air gap (between the pickup and the moving object).

Therefore, for optimum response the above parameters need to be adjusted, and the pole piece should be less than or equal to both the gear width and the dimension of the tooth’s top. The air gap should be as low as possible, normally around 0.25 to 1 mm depending on the cases. 3.1.2 INFLUENCING FACTOR FOR MAGNETIC PICKUP As discussed above, the performance of a magnetic pickup is influenced by a number of factors. The output voltage of a magnetic pickup is affected mainly by three factors: 1. Generated voltage is proportional to the surface speed of the monitored magnetic material.

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2. With a decrease in the air gap between the magnetic pickup and the surface of the rotating object, voltage increases. 3. Voltage waveform is determined by the size and shape of the gear tooth in relation to the size and shape of the pole piece. For any given speed and clearance conditions, there will be a maximum power output when the path is filled with a relatively infinite mass of ferromagnetic material. Therefore, it is filled with an infinite mass of ferromagnetic material at one instant and a complete absence of ferromagnetic material (i.e., air) the next. As a result of this there will be quick changes in the lines of force, which results in the generation of AC voltage. The following are reasonably good conditions: l

l

When the cross-section of the exciting masses is equal to or greater than that of the pole piece; When the gap is equal to or greater than three times the diameter of the pole piece.

3.1.3 INSTALLATION OF MAGNETIC PICKUP Magnetic pickup is mounted radially to the outside diameter of the desired gear made up of ferromagnetic either through the housing or on a rigid bracket. The mounting can be in the vertical or horizontal plane as shown in Fig. X/3.0.0-1. In order to get rid of EMI and other pickups, a shield of nonmagnetic material may be installed between the gear and the pickup if necessary for physical shielding. Additionally, there may be a threat that electromagnetic force may be generated by eddy currents in the shielding material. Therefore, suitable protection should be taken care of to stop this from happening. There is another kind of pickup called proximity pick up, which is mostly used for zero speed detection, as discussed in the next section.

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ELECTRICAL CONNECTION

MAGNETIC PICKUP

SPEED GEAR FOR MEASUREMENT

ELECTRICAL CONNECTION MAGNETIC PICKUP

POLE PIECE HORIZONTAL PLANE POLE PIECE VERTICAL PLANE

SPEED GEAR FOR MEASUREMENT

FIGURE X/3.0.0-1 Magnetic pickup.

3.2.0 Proximity Pickup Proximity sensors have versatile applications, and are used for vibration and speed measurement. In our case it is mainly used for zero speed sensing for large pieces of equipment because of its abilities to operate with a large air gap and at low surface speeds. Zero speed sensing by proximity switch is important for solid flow measurement by (say) Belt feeder or weigh feeder etc. The output of these pickups depends solely on the position of ferrous discontinuity such as gear teeth. Short details of the operating principle are given here. 3.2.1 OPERATING PRINCIPLE OF PROXIMITY PICKUP Proximity sensing is inductive sensing. It consists mainly of four blocks. These blocks are the oscillator, magnetic/inductive coil, detector, and

output stage, which could be a Schmitt trigger. With a DC power supply, the oscillator produces an oscillating AC signal to generate a fluctuating magnetic field around the coil at the sensing surface. This magnetic field is like a donut and leaves the face surface. When any metal object moves into the inductive proximity sensor’s field of detection, on account of the cutting of the magnetic lines of force by the metal object, an Eddy current/circuit is built on the metallic object and this will try to oppose the oscillator circuits’ oscillating magnetic field. The detector circuit continuously monitors the magnetic field strength to produce output. The Schmitt trigger is used for production of output or stops output, depending on the NPN/PNP transistor configuration of the output stage. Thus when the magnetic field strength goes below the set point, a switching action in the form of output as described above will take place.

Special Flow Meters, Flow Gages, and Switches Chapter | X

3.2.2 INSTALLATION PROXIMITY PICKUP The proximity pickup is mounted outside the diameter of the desired gear made up of ferromagnetic material either through the housing or on a rigid bracket. As shown in Fig. X/3.0.0-2, the proximity pickup can be mounted for radial as well as axial measurements.

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4e20 mADC in two wires to connect to an external device, e.g., a PLC. Since these are loop powered, these are connected to the external 24 VDC with a wire and through another wire they provide the 4e20 mA signal output. Generally they are made up of ABS/polycarbonate blend [2]. With this the discussions on various pickups come to an end and we now investigate another

RADIAL GAP

AXIAL SENSING

PROXIMITY SWITCH

PROXIMITY SWITCH

AXIAL SENSING

RADIAL SENSING

FIGURE X/3.0.0-2 Proximity pickup.

3.3.0 Signal Conditioning Unit Normally these pickups need a loop-powered flow monitor that converts a meter’s pick signal to a 4e20 mA current output. The linear output is proportional to the meter’s flow rate. These signal conditioning units should be small enough to fit inside any meter box. Also setting should be easy and they should be compatible meters. Often this is referred to as the preamplifier stage, when there is separate signal conditioning electronics. This connects the meter’s register with the pickup through a cable. Otherwise these can generate

special type of flow measurement—cryogenic flow measurement.

4.0.0 CRYOGENIC FLOW MEASUREMENT There have been drastic changes in cryogenics industry in the past few years. There has been greater demand for cryogenic and liquefied gases, which means that there has been an increased demand for effective measurement. At cryogenic conditions, liquids offer little lubrication for moving parts and the thermal shock of fluids at

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Cryogenic liquid and cryogenic treatment: Any liquid which boils under normal condion at a temperature below –90°C is referred to as cryogenic liquid. Most of the gases such N2, H2, O2 inert gases (e.g. Ar, He), liquefied natural gas (LNG) and ethane etc. have their boiling points below that. So, in liquid condion, these fall under this category. All these gases are transported and stored in liquid condions for facilitang their handling. From this point of view study of cryogenic liquid and their flow measurements are important. Cryogenic materials have special property and cryogenic treatment refers to processing cryogenic liquids.

FIGURE X/4.0.0-1 Cryogenic liquids and their treatment.

these low temperatures is very problematic for transportation. Keeping elements cold, below boiling points—and also ensuring that these materials are in their purest forms—is challenging [3]. On account of the temperature and two-phase nature, cryogenic fluids show different behavior and special treatment is called for. For idea on Cryogenic liquids and their treatments Fig. X/ 4.0.0-1 may be referenced. As such, two-phase flow behavior is not easily predictable, and in case of cryogenic liquid the operating temperature is too low. So handling of cryogenic liquid is more tough than normal two phase fluids. Cryogenic fluids find their applications in a wide variety of applications, including propellants for aerospace applications, storage of industrial and life support gases, and coolants for superconducting magnets. Natural gas (NG) is also important for the energy system and power generation. Natural gas is handled cost-effectively in liquefied form. Therefore, flow measurement of liquefied natural gas (LNG) is a common example for cryogenic flow measurement for storage and handling. In aeronautics and aerospace, cryogens are commonly seen as fuels for propulsions systems. In spaceshuttles liquid hydrogen (LH) and liquid oxygen (LO) are used, with liquid nitrogen being used (LN) in coal mines. The German V2 rocket developed in World War II was the first aerospace application developed on a large scale that used cryogenic fluids as propellants [4]. Since then, numerous propulsion systems using cryogens have been developed. As indicated above, because of the two-phase nature of cryogens and the presence of bubbles within the liquid interference, it is very difficult to have precise measurement. Orifice, Venturi in DP method, turbine flow meters, Coriolis flow meters, vortex flow meters, and US meters have been applied for cryogenic applications

but there are many limitations to their use. From the literature, deployment of microwave flow meters with suitable DAS has also been noted. 4.1.0 Discussion on Cryogenic Flow Measurements Various constraints associated with cryogenic instrument types and brief selection criteria are discussed in this section. 4.1.1 CONSTRAINTS OF VARIOUS CRYOGENIC FLOW METER TYPES As indicated above, several types of conventional instruments have been applied for cryogenic applications, but each has some limitations. A few of these limitations have been discussed in this section. These constrains include but are not limited to the following: 1. Orifice plate DP meter: Unnecessary resistance is offered to liquid flow and this creates additional permanent pressure drop than under normal situations. Extra permanent pressure drop limits its use in many cryogenic applications under saturated conditions. 2. Venturi DP meter: In the case of a Venturi, due to extra pressure drop there can be cavitations. 3. Turbine flow meter: During cooling there is the possibility of higher oscillation, which damages the meter. Also at cryogenic conditions, when the flow alternates between bubbles and liquid there can be serious damage to the bearing. 4. Vortex flow meter: Vortex flow meter performance is acceptable only at lower Reynolds number, otherwise the accuracy deteriorates. For that reason, a noninvasive ultrasonic flow meter is also a good choice. However, in the majority of cases, especially for thermal performance assessment and

Special Flow Meters, Flow Gages, and Switches Chapter | X

calculations, mass flow rate is required to be assessed and to be used for mass and heat balancing. Naturally, in all the above cases of metering, density measurement is required in conjunction with the flow measurement to get the mass flow rate. Therefore, to obtain better performance and accuracies expensive and complicated calibrations at working temperatures need to be performed [5]. Mass flow measurement in that case is a preferred one as in such cases density measurement is not required. 5. Coriolis flow meter: With a Coriolis meter, measurement is carried out with the help of vibration of the tube, whose modulus of elasticity changes with temperature. Liquid Nitrogen, Carbon dioxide, Argon, LNG and LPG are some of the examples where Coriolis mass flow meters are used extensively. Therefore suitable temperature compensation cannot be avoided. However, in most cases these can be a constant factor and so expensive calibration arrangements can be avoided but temperature measurement and compensation will be necessary. 6. Summary of major constraints: From the discussions above one can infer that there are major issues/criticality associated with cryogenics, which are summarized here: l Measurement difficulties and inaccuracies come from the two-phase nature of cryogenic liquid; l Selection of materials for operating temperature; l Proper and actual calibration at operating temperature; l Density correction for volumetric flow measurement and temperature correction factor modulus of elasticity in the case of a Coriolis mass flow meter. We now investigate various instruments normally used for cryogenic liquids, including LNG. 4.1.2 SELECTION OF FLOW METER IN CRYOGENIC APPLICATIONS Choice of a correct and accurate meter is extremely important for making an investment.

941

Also, taking the decision calls for a good knowledge of the industry. This is especially important when transporting cryogenic materials [3]. For meter selection, the following points need to be considered. 1. Quality: The designs and calibration of the meters must meet the highest standards of quality, with independent verification of the same. In the case of cryogenic applications, meters must be precision-crafted from the best available materials by skilled machinists so that they are able to withstand harsh conditions, including the capability to handle a wide range of temperature, [3] with a lower temperature limit <200 C. 2. Reliability: Material quality, workmanship, proper installation, and calibration of the meter in operating conditions are key issues for reliable operation of the meter. Any meter with no moving parts always has an edge over these with moving parts in cryogenic applications. Therefore, meters with moving parts should have quick fault detection and replacement of parts. 3. Precision: Turbine meters should be equipped with a specially designed turbine rotor that spins freely. As the rotor spins, it affects the magnetic field provided by the magnetic pick-up, which is interpreted by the flow monitor and expressed as a flow-rate readout. In addition to the need for a high-quality meter, it is also important to have a monitor that displays in real time. 4. Accuracy: Meter accuracy is also important for meter selection. Independent calibration provides a greater level of accuracy and reliable operation. In the following sections short discussions on various types of instruments used in cryogenic applications are given. It is worth noting that all these meters are conventional meters already discussed in previous chapters and so are not repeated again here. In the following sections, only special relevant parts required for cryogenic applications have been discussed. For details about any of these meters the relevant sections and chapters in this book may be referenced.

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Plant Flow Measurement and Control Handbook

4.2.0 Differential Pressure Type Cryogenic Flow Measurement

4.2.2 DP TYPE FLOW METERING AND CRYOGENIC APPLICATIONS

The DP method is one of the oldest methods applied for cryogenic application. We now discuss its working method with its pros and cons.

Cryogenic plants in earlier days used to have DP type meters installed at room temperature connected through long impulse lines. In such cases the accuracies suffer due to the following reasons:

4.2.1 DP TYPE FLOW MEASUREMENT METHODS When differential pressure/head type flow meters discussed in Chapter II are recalled it can be seen that the measuring principle of these types of flow meters uses Bernoulli’s equation to measure the flow of fluid in a conduit. On account of a restriction introduced in the conduit, as per Bernoulli’s equation, there will be a pressure drop across the restriction. From Bernoulli’s energy balance equation, this pressure drop across the restriction is proportional to the square of the flow rate. In the case of an orifice plate this differential pressure is measured between the upstream and downstream of the orifice plate. In the case of a Venturi, DP is measured between the upstream and throat section. The nonlinear relationship between flow and DP, as stated above, can have a detrimental effect on the accuracy and turndown of DP meters. The major advantage of DP type flow meters comes from low cost, with multiple versions possible for different fluids and measurement objectives and it being approved for custody transfer applications. Also, the measurement scheme is easily understood and well established. Added to the above, these can be easily coupled with temperature/pressure sensors to provide mass flow for steam. However, the measurement scheme is not direct, instead flow is inferred from DP. Also, rangeability and accuracies are not good because of the nonlinear relationship, and accuracy is dependent on both the flow element (mainly) and DP measuring instrument (highly accurate smart transmitters are available). These meter types can and have been applied for cryogenic applications also.

l

l

Lack of proper calibrating equipment operating at cryogenic temperature, the calibrations are done at room temperature and then calculated/estimated by extrapolating for cryogenic conditions from reading at room temperature. As stated during the initial discussions, there can be a two-phase issue, as a result liquid vapor interfaces inside the impulse lines and they may be changing states, giving rise to fluctuation in reading, especially because liquid vapor interfaces in two tubes will not be the same.

Modern transmitters are available to withstand cryogenic temperature and most of these are tested at NIST using liquid nitrogen (LN2) as a flowing media, with a boiling point of 77K (321 F/196 C). This is especially so when an integral orifice is used, e.g., 3051SFC Compact Conditioning Flow meter of Emerson (Rosemount). Large temperature gradients present concerns when O-rings, glands, welds, or dissimilar metals are present in the flow stream. Therefore, suitable care should be applied. At cryogenic conditions, liquids lose lubrication property for moving parts and this is another challenge for instruments with moving parts, such as turbine and positive displacement flow meters. It is important that the measurement does not have any moving parts, and hence it should be inherently reliable and often can be used in space applications (without manning). Venturi in liquefied helium (LHe) is common. However, on account of the sharp restriction, a Venturi is always preferred because of the larger pressure drop which can locally trigger flashing of liquid

Special Flow Meters, Flow Gages, and Switches Chapter | X

and/or give rise to cavitations to reduce accuracy and bring in instability into the measurement. As a corollary to this it could be argued that the inner surface of the flow element must be highly polished. Standard wrought austenitic stainless steels are used extensively for cryogenic applications, even for temperatures as low as the boiling point of liquid helium (269 C). Depending on the availability in the particular form or size required, the most widely used wrought stainless steels for cryogenic service are AISI types 304 and 304L, while types 316, 316L, 321, and 347 are also used. These materials are used for flow elements and impulse lines, as well as transmitter wetted parts. This is applicable for other instrument types also. When instruments are tested individually, an accuracy of 1.5% is achievable [6]. This will also ensure better repeatability. Testing of the meter and straight length in operating conditions is extremely important. 4.3.0 Turbine Meter in Cryogenic As already discussed in Section 2.4.0 of Chapter V, in turbine flow meters, on account of fluid flow, the rotor of the turbine rotates in a suitable bearing. The rotational speed of the turbine is measured by magnetic (Hall effect) pickups. The turbine meter has moving parts and, as stated earlier, liquids offer little lubrication for moving parts in cryogenic conditions. Therefore, it is vulnerable when subjected to cryogenic conditions unless highly precise design, sizing, workmanship for manufacturing with most suitable materials, proper installation, and calibration of the meter are undertaken. Also, as the turbine meter bearing and other parts may get damaged, the importance of quick fault prediction and detection by associated electronics and ways and means for quick replacement of parts should not be overestimated. There should be well-protected

943

insurance against low performance; suppliers who calibrate both in-house and independently offer a more conservative and reliable option [3]. Turbine meters with accuracy of 0.5% FSD or better and repeatability of 0.1% FSD are available, only it must to be ensured that it is of good quality and that the manufacturer has a track record of manufacturing turbine meters for cryogenic applications. Material selection for cryogens is also important. Normally, a stainless steel body with nickel, 17-4 PH rotor and ceramic ball bearing are common choices of materials, as indicated in Section 2.4.1 of Chapter V. Wide variations in operating temperature are available, e.g., () 448 to þ450 F (267 to 232 C) [7]. 4.4.0 Vortex Meter in Cryogenic Applications As discussed in Section 3.1.1 of Chapter V, when a shedder bar is placed in a flow, Karman vortices are generated on the downstream side of the bar. The Karman vortices are detected. The vortex frequency is proportional to the flow velocity. The vortex meter does not have any moving parts and so vulnerability due to low lubrication at cryogenic temperature is not applicable. No zero adjustments are necessary. In order to have lower pressure drop, a single shedder bar may be used. Vortex meters are susceptible to pipe vibration. In order to prevent measurements being influenced by noise due to strong piping, vibration may affect the accuracy of vortex frequency detection. In many instruments piezoelectric elements are installed to detect vibration to adjust the output, e.g., digital YEWFLO of YEL. Such signals are duly processed by digital signal processing (DSP). For cryogenic applications the operating temperature range could be from 200 to 40 C. Normally vortex meters are available with builtin temperature or RTD sensors (equivalent to Pt1000, Class A) for temperature monitoring function and a mass flow rate calculation

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Plant Flow Measurement and Control Handbook

function. For this reason this meter is often referred to as a multivariable flow meter. Normally these meters are equipped with DSP, so as to calculate and provide outputs such as mass flow rate, temperature, pressure, volumetric flow rate, and fluid density. Such processing functions facilitate highly accurate measurement of flow rate over a wide range, even under radically fluctuating temperatures. An accuracy of 1.0% AR with repeatability 0.2% AR is possible in available meters. 4.5.0 Coriolis Mass Flow Meter in Cryogenic Applications A Coriolis mass flow meter (refer to Section 2.1.0 of Chapter VI) consists of a manifold which splits the flow into two parallel tubes (commonly U shaped). The tubes are vibrated at a resonant frequency of the system. With passage of flow through the tubes, due to Coriolis force there will be a phase shift (Dt). The delta t is directly proportional to the mass flow rate. Coriolis meters are insensitive to fluid parameters (i.e., density, swirl, viscosity, etc.) and find their uses in a wide range of applications. A typical Coriolis meter makes a temperature measurement for compensation of the vibration characteristics of the sensing element. Proper account must be taken of the nonlinear temperature dependence of the Young’s modulus of the vibrating tube in the Coriolis meter. As discussed earlier, the change in modulus of elasticity is well characterized, hence the modulus of elasticity can be corrected at various operating temperatures. Here it is important to note that the slightest warming can cause these fluids to flash, generating bubbles and causing measurement error. To save on cost, extra cooling below boiling point is normally avoided in the main system design. Therefore, it is critical to size the meter in such a way that sizing is optimum, and there is no extra pressure drop to cause flashing.

As already discussed, as the Coriolis meter directly measures mass and temperature, the effect on the vibration tube can be easily compensated for, so Coriolis mass flow meters in many ways are well suited for cryogenic fluids. However, it is possible that these cryogens and subzero fluids freeze the internal measuring components to create a restriction of meter motion and deterioration of measurement. It is essential that suitable materials must be selected for the sensor, the driver, and coil components for cryogenic operating conditions, i.e., performance and durability at cryogenic temperatures. In some meters, e.g., RotaMASS sensor, the interior space must be kept free of any air and by filling the meter with a dry, inert gas [8]. As already explained, unlike other flow-measuring techniques, Coriolis meters respond directly to mass flow, eliminating the need for density compensation. Also, Coriolis mass flow meters do not have the prerequisites for inlet flow conditioning, normally applicable for other meter types. In Coriolis meters absolute accuracy of better than 0.25% and reproducibility of 0.2% are easily achievable. Modern meters have digital signal processing (DSP) for better performance, e.g., Micro motion Elite sensor with multivariable DSP. 4.6.0 Ultrasonic Flow Meter in Cryogenic Applications Noninvasive ultrasonic flow meters in cryogenic flow applications use the transit time method to determine flow through the pipe. There are two transducers at the opposite sides of the pipe line to measure the transit time, as already discussed in Section 6.1.0 of Chapter V. The arrangement is similar to that shown in Fig. V/6.1.0-2A. This transit time consists of the time taken by an ultrasound (US) signal to travel across the pipe and the time to convert an electrical signal into an acoustic signal. Temperature, especially cryogenic temperature, affects the accuracy of

Special Flow Meters, Flow Gages, and Switches Chapter | X

measurement and would cause fluctuation in fluid flow, flow cell dimension, and acoustic characteristics. In order to compensate for this, most of the measurement systems also monitor the live temperature to provide compensation. Some use a waveguides system to concentrate the US signal into the fluid. These noninvasive ultrasonic flowmetering systems are well suited for LNG and other cryogenic applications at temperatures down to 200 C. These meters are available in various sizes to cater for the highest flow rates of loading/off-loading processes, as well as very low flow rates at the start or end of operation. The major features of these (same US measurement types described in Chapter V) types of measurements include but are not limited to the following: l l l l l l l l

l l l

Noninvasive type; Highly reliable; Cost does not increase with line size; Easier to install; Cater to a wide variety of cryogenic fluids; Safe measurement; No pressure drop; No chances of leakage—no gaskets, no leakage points; Practically maintenance-free; No pipework modification; Dual beam monitoring possible.

4.7.0 Processing Electronics in Cryogenic Applications High performances of the meters have been made possible due to the use of multivariable transmitters and processing electronics. Modern high-precision digital signal processing (DSP) accurately processes field-acquired data to produce desired outputs with the help of advanced software and embedded microcontrollers. Most of the new advanced technological developments in electronics bring about good solutions in

945

cryogenic measurements. Advanced communication, graphical display, and operator interface with “soft keys” allow for easy interaction between the operator and the instrument. The operator interface has been made easy with the help of multiple easy-to-follow messages in multilingual message forms. These advanced interactions help navigate through the instrument menu which may be in hierarchical format. The advanced computation algorithms calculate the volume, mass, and density of the fluid. Extensive built-in diagnostics systems find faults immediately. Diagnostics includes but is not limited to factory setting tests and has factory test mode, troubleshooting, serial interface testing, etc. Many of the functions are password/authentication protected. The displays can be volumetric/ mass flow (either direct measured or computed) and total flow. The reset of the total flow is normally possible only by an authorized person through the use of a password and such deletion details are recorded. High-speed DSP ensures accuracy under the toughest conditions of high noise, high turndown, etc. Computation capability for concentration and net flow measurement eliminate the need for additional instruments. These features are found in micromotion MVD transmitters and DSP. Displays can be in backlit LCD or LED with an incremental rate as low as 100 ms. Most of these electronic processing units support necessary communication links and connections, such RS 485, HART, and/or various kinds of fieldbus systems for system communication and integration. In conclusion it can be argued that, behind the success of highperforming meters in cryogenic flow meters, lies the powerful processing capabilities of DSP and handling multivariable data. There are not too many independent laboratory-based cryogenic flow meter calibration facilities in the world. To the best of the author’s knowledge, only one independent laboratory for

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cryogenic flow meter calibration is in existence. Thus these flow meters are normally calibrated with water at ambient conditions. This therefore brings major uncertainty in transferring laboratory calibrations using water to other cryogenic liquids, including LNG, for which long-term custody transfer and fiscal calculations are necessary. Therefore it is recommended to get the meter tested in an independent laboratory for precise measurements. The discussions on cryogenic flow measurement thus come to an end and we now look into local flow meters or flow gages.

5.0.0 FLOW GAGES Normally, flow meters which are used for local flow monitoring are referred to as flow gages. Flow gages usually have local indications in the form of a dial or digital readout. In some cases there may not be any reading, e.g., flow/no-flow gagesesight flow indicator. Generally flow gauges work without any external power supply. However, digital flow gages, i.e., flow gages with electronic digital displays, do require power. So, with the requirement for power supply, flow gages cannot be distinguished from normal flow meters. Normally these local gages do not have remote signal transmission facilities, but some may have built-in contact with them. Another distinguishing issue could be lower performance of flow gages when compared with flow meters. There are several categories of flow gages which are discussed in this section. Different manufacturers employ different design criteria, which change from one to another, but the basic concept of flow gauge operation depends solely on the principle of dynamic pressure [9]. There are several categories of flow gages. Some use DP gages to measure the flow in the pipe. Some gages have digital displays and some have a simple sight flow indicator which indicates flow/no flow condition without any flow reading. There is another kind of flow gage which

basically is a flow meter with a local totalizer without remote transmission facility. These are mainly used as oil meters and water meters. These are also categorized here. In this section brief discussions on different flow gages have been enumerated. 5.1.0 Direct-Flow Gages From the discussion on head type flow meters it is understood that when a designed restriction is placed in the conduit, there will be a pressure drop across it. Based on Bernoulli’s theorem on energy balance, flow in the conduit can be computed or related to the pressure drop across the designed restriction. So, when DP is measured across the restriction it arrives at the flow. The dial is calibrated in terms of flow through the pipe. 5.1.1 DESCRIPTION OF DIRECT-FLOW GAGES Direct flow gages are direct reading flow meters with easy-to-read dials of different sizes. The dial is calibrated in terms of suitable flow engineering units, such as GPM/Lit/min. These gages are normally fabricated aluminum. These direct reading gages do not require external power and have a clear scale and pointer reading. These gages measure the flow based on the differentials created by a built-in calibrated Venturi/nozzle or integral orifice as shown in Fig. X/5.1.0-1A and B. These meters read the differential pressure across the built-in flow element. Since these meters are calibrated under standard atmospheric conditions, gages normally are supplied with reference flow charts. Direct flow gages are available for water, oil, and many other viscous fluids without deposition. The range of fluids covered depends on the flow gage type and associated flow element. These gages are also applicable for gases such oxygen, compressed air, nitrogen, etc. These are also used in-process steam lines.

Special Flow Meters, Flow Gages, and Switches Chapter | X

There are a variety of gage offered by the manufacturers. Some even offer provision for remote signal transmission also. Based on applications, variations in pressure drops have been

947

shown in Fig. X/5.1.0-1C. Curve B is applicable for high-pressure applications and for materials like SS. Curve A is applicable for materials like bronze.

(A)

(B) FLOW GAGE IN SCALE FOR FLOW

FLOW ELEMENTS

(C)

PSI 10

BAR

9

0.6 NET PRESSURE DROP

8

VARIATION WITH

7

0.5

APPLICATIONS 0.4

6

0.3

PRESSURE

5 4

0.2

A

3 2

B

0.1

1 0

0 0 % FULL SCALE

FIGURE X/5.1.0-1 Direct flow gage. (A) Flow gage with Venturi. (B) Flow gage with orifice. (C) Pressure drop characteristics.

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Plant Flow Measurement and Control Handbook

5.1.2 FEATURES AND APPLICATIONS OF DIRECT-FLOW GAGES Some of the noteworthy features and applications are noted here: 1. Features: The following features are normally noted in direct-flow gages: l Flow element: Flow element selections are based on the pressure loss allowed. Venturi type offers less pressure loss; l Reading: Easy to read direct flow through transparent front plastic/glass; l Indication: White background with black graduation for scale with pointer; l Housing: Durable aluminum housing; l Size: 100 mm standard size, other sizes are also available; l Scale: 270 degrees analog scale;

Accuracy and range: Accuracy of 1% FSD possible in span ration 5e6:1. 2. Application: There are a number of applications including the following: l Filter monitoring; l Limited slurries and liquids with suspended solids; l Turbine and other machine lube oil delivery monitoring; l Heat exchanger coolant/steam delivery monitoring; l Compressed air system; l Cutting oil flow in automatic machines. l

5.1.3 SPECIFICATION FOR DIRECT-FLOW GAGES A brief specification of direct-flow gages has been enumerated in Table X/5.1.0-1.

TABLE X/5.1.0-1 Specification of Direct Flow Gage SL

Specifying Point

Standard/Available Data

1

Fluid types

Various liquids like water, oil, lube oil air/gases including natural gas, compressed air, process steam, some limited: slurries, liquid with solids

2

Pressure limit

Normally within 1.5 bar

3

Over pressure

Normally >1.5 times, some are provided with over pressure protection

4

Line size

½00 (12) to 300 (80) but sizes up to 800 (200)

5

Temperature

5 to 75 C

6

Orientation

Vertical, horizontal; some with diaphragm, meant for vertical position

7

Connection

Threaded, NPT/BS of different sizes 1/200 or 3/400 sizes are common

8

Flow element

Venturi, nozzle, or integral orifice

9

Sensor material

Stainless steel SS 316

10

Casing/housing Material

Aluminum housing with plastic/glass dial

11

Seal Material

Viton/EPR/PTFE

12

IP Cl Ass

IP 65 possible

13

Accuracy

1 FSD

14

Application of interest

See Subsection 5.1.2.2 above

15

Optional

Alarm contact of rating 30 V 1 A Remote transmission (4e20 mADC) or digital readout power supply if necessary 24 VDC

User Spec.

Remarks

To specify

Continued

Special Flow Meters, Flow Gages, and Switches Chapter | X

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TABLE X/5.1.0-1 Specification of Direct Flow Gagedcont’d User Spec.

SL

Specifying Point

Standard/Available Data

16

Accessories

Mounting kits and fittings, power supply digital readout as specified

17

Special feature

To specify

We now look into another commonly used flow gage popularly known as the sight flow indicator, which is mainly used for flow/no-flow detection locally. 5.2.0 Sight Flow Indicator Matching with the name, sight flow (SF) indicators display flow or contents of pipelines. As indicated earlier SF indicators are mainly meant (A)

Remarks

for viewing flow, i.e., for detection of flow. There are several types of local flow gages that fall under this category. However, in some cases there are meter types where, in addition to viewing flow, local indications and even contacts are available for low/no flow. There are several types of sight flow indicators available and some of these are depicted in Fig. X/5.2.0-1. These are other types also but they are very similar with

WITH FLAP

(B)

TUBE TYPE

(C)

(D)

SIGHT FLOW : SF

(E)

FIGURE X/5.2.0-1 Sight flow (SF) indicator types. (A) Standard SF indicator. (B) SF indicator with ball. (C) Flow rate type. (D) Impeller type. (E) SF with spinner.

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Plant Flow Measurement and Control Handbook

little modification and addition of various options, such as output contacts, etc. Also, it is worth noting that sight flow indicators are available also with liners for corrosive fluids. In this section a few types are discussed one by one. The discussions start with the tube type. 5.2.1 TUBE TYPE SIGHT FLOW INDICATORS This type is simply a piece of tube of transparent material through which the fluid passes and the flow is visible from outside, purely for a confirmatory flow or no-flow signal. A typical straight type has been shown in Fig. X/5.2.0-1A (left). This can be used for lube oil lines but may not be suitable for transparent liquids like water. The tube diameter should be the same as the main pipeline, so that there is no pressure loss. There are two options: one straight type and one angular (viewing at right angles to the flow direction). The main disadvantage is that it has joints made by exerting on it. A brief specification is enumerated below: 1. Sizes available: 12 mm (1/200 ) up to 150 mm (600 ); 2. Body material (angular): Cast iron (200 C)/ CS or SS (250 C); 3. Cover (angular): MS; 4. Viewing glass: Borosilicate glass; 5. Fastener: MS; 6. End connection: Flanges (RF/FF) of CS or SS (250 C): ANSI 150 lb or equivalent; 7. Gasket: PTFE; 8. Pressure rating: Normally 3 barg for straight type and about 20 barg for angular type. Very similar to this is the flat type discussed below. 5.2.2 SIGHT FLOW INDICATOR WITH FLAP These are low-cost sight flow indicators, these are also a straight through indicator with a sprout. They are available with a flap and scale to indicate flow. A typical SF indicator has been depicted in

Fig. X/5.2.0-1A (right). A short specification of the same has been enumerated below: 1. 2. 3. 4. 5. 6. 7. 8. 9.

10.

Available sizes: 12 up to 80 mm; Pressure: Up to 10 barg; Temperature: Around 200 C; Scale: Scale when provided, show flap position which is calibrated in terms of flow; Body: Stainless steel (casting quality CF8M); Flap: SS316; Cover (angular): MS; Viewing glass: Borosilicate glass; End connection: Screwed of different sizes (NPT/BSP) or flanges (RF/FF) of CS or SS (250 C): ANSI 150 lb or equivalent; Gasket: PTFE.

5.2.3 SIGHT FLOW INDICATOR WITH BALL/SPINNER A typical SF indicator of this type is shown in Fig. X/5.2.0-1B. The flow is indicated by a ball or spinner. They are used for plant protection applications with local indication of flow, such as coolant/oil to plant machinery. A short specification of the same has been enumerated below: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Available sizes: 10 up to 80 mm; Pressure: Up to 16 barg; Temperature: Around 150 C; Body: Stainless steel (casting quality CF8M)/gun metal; Ball/spinner: Nylon/PTFE; Cover: MS; Viewing glass: Borosilicate glass; Gasket: Nitril O ring; End connection: Screwed of different sizes (NPT/BSP); Gasket: PTFE.

5.2.4 SIGHT FLOW INDICATOR WITH SPINNER A typical SF indicator of this type is shown in Fig. X/5.2.0-1E. The flow is indicated by a spinner. This is a double-sided indicator [10] and

Special Flow Meters, Flow Gages, and Switches Chapter | X

is practically a modified version of the one discussed above. This type can be used for vacuum services also. A short specification of the same has been enumerated here: 1. 2. 3. 4.

5. 6. 7. 8. 9.

Available sizes: 10 up to 150 mm; Pressure: Vacuum to 16 barg [10]; Temperature: Up to 250 C possible; Body: Gun metal/cast iron/carbon steel (A216 WCB)/stainless steel (casting quality CF8M)/ gun metal; Spinner: SS, other material also possible; Cover: MS; Viewing glass: Borosilicate glass; Gasket: PTFE; End connection: Screwed of different sizes (NPT/BSP), flange ANSI 150 lb or equivalent.

5.2.5 SIGHT FLOW INDICATOR— IMPELLER TYPE A typical SF indicator of this type is shown in Fig. X/5.2.0-1D. These are available in double or single viewing glasses. A short specification of the same has been enumerated here: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Services: Gas/liquid [11]; Available sizes: 10 up to 80 mm; Pressure: Vacuum up to 10 barg [11]; Temperature: Up to 100 C possible; Body: Bronze/carbon steel/stainless steel (casting quality CF8M)/others; Spinner: SS, other material also possible; Viewing glass: Borosilicate glass; Gasket: PTFE; End connection: Screwed of different sizes (NPT/BSP), flange ANSI 150 lb or equivalent.

5.2.6 FLOW RATE TYPE INDICATOR This type really is not commensurate with its name, but is discussed here because this one also make use of a similar technique as used in the other types discussed. These types of gages have a mechanical indicator on a dial. Optionally they are provided with suitable contact output. They are available with a wide range of flow and a wide range of meter specifications. These are

951

available from sizes ½00 to 800 . There are varieties of sensing possible using swing vane, flow piston, and variable orifice types. Some of these are supplied with a power supply and can give remote transmission facility. Since this section is mainly on local indications, this has been discussed with other mechanical flow meters in Section 6.2.2 of this chapter. With this the discussions on sight flow indication come to an end and we now investigate digital display type local flow gages. 5.3.0 Digital Local Flow Indicator These types of meters are electronic type small flow meters. They are installed either in-line or at the end of a hose. The large easy-to-read display and compact lightweight design makes it easy to handle. These meters work on paddle wheel flowmetering principles with a suitable flow sensor. Most are microprocessor-based meters with provisions for battery backup. The majority of these meters can display both the rate flow and the totalized volumetric flow locally. Totalized flows can be resettable/nonresettable types. Some also offer the facility to control batch operations. They are mainly used for oil and water metering. In oil metering it is very handy, and is installed in a fuel transfer pump for oil delivery system. They are mainly made up of aluminum housing and are quite durable. Such water meters find their uses in domestic as well as industrial applications, such as greenhouses, small plant process water supply, cement mixing machines, and pond water supply lines. Some of these meters also offer remote transmission facility. 5.3.1 FEATURES OF DIGITAL LOCAL FLOW METERS Some typical features of this type of flow meter are enumerated below: 1. 2. 3. 4. 5.

Ranges available: 0e>750 L/s; Construction: Rugged construction; Application: Indoor/outdoor; Battery Backup: provided; Totalizer reset: Possible;

952

6. 7. 8. 9.

Plant Flow Measurement and Control Handbook

Batch control: Possible [12]; Display: Rate and/or totalized; Display size: Tall display; Batch size: Six digit up to 999,999.

5.3.2 SHORT SPECIFICATION OF DIGITAL LOCAL FLOW METERS Local digital flow meter specification details have been enumerated here: 1. Flow range: Depending on size up to 800 Lpm; 2. Sizes: Available in various sizes from 12 to 50 mm /80 mm; 3. Maximum pressure: Up to 20 barg; 4. Maximum temperature: 60 C; 5. Housing material: Aluminum/cast steel; 6. Wetted part materials: Aluminum, Nitril, steel [12]; 7. Electronics housing materials: ABS; 8. End connection: Threaded normally, NPT/ BSP; 9. Display: Rate flow and/or totalized; 10. Display type and size: Backlit LCD/LED; 11. Display size: 10e12 mm [12]; 12. Reset: Possible in some cases; 13. Control: Batch control possible; 14. Accuracy: Around 1% AR. There are a few other kinds of flow meter which are basically mechanical meters with local mechanical readings for flow rate as well as totalized flow. Many of these meters also have provision for electrical transmission facilities. Therefore, these cannot be treated as simple local flow gages. The main use of these meters is as a water meter. They have several built-in working principles such as Woltmann metering and rotary piston principles. Some fuel meters based on PD metering also come under this heading. These are treated separately in the next section.

6.0.0 MECHANICAL TYPE FLOW METERS In this section discussions are presented on a few mechanical type flow meters which are used for

mechanical totalizing fluid flows. Some of these meters have provisions for electrical remote transmissions and can also show the flow rate. They are mainly used as water flow meters and fuel flow meters. Woltmann type flow meters discussed in Section 2.9.2 of Chapter V are mainly used as water meters, and are used as irrigation and agriculture water meters, and fertilizer meters used for fertilizer and chemical solutions. On the other hand, rotary piston principles discussed in Section 5.1.1 of Chapter IV are used for domestic water meters and some industrial water meters. The other category is fuel meters, which are mainly based on PD meter principles. These meters are used for measuring oil receipt and oil consumption. 6.1.0 Mechanical Water Meters As indicated above, most of these flow meters are Woltmann flow meters (discussed in Section 2.9.2 of Chapter V). There are several categories of these meters, these are domestic water flow meters, industrial water flow meters, irrigation and agricultural water flow meters, and fertilizer flow meters. Many data and details presented here are based on data from Kent Water Meters [courtesy of Kent Water Meters]. The discussions start with general design details of these water meters. 6.1.1 GENERAL DESIGN DETAILS FOR WATER METERS Some of the standard features and design details available for water meters are briefly discussed below. These are generalized in nature. 1. Standard construction features: For domestic and industrial water flow, meter working based on a rotary piston has a grooved piston to reduce stoppages and to ensure durability. Good engineering practices are adapted to make the system leak-proof. For irrigation, agricultural, and fertilizer meters Woltmann flow metering principles are used. Advanced engineering plastics are often used for

Special Flow Meters, Flow Gages, and Switches Chapter | X

2.

3.

4.

5.

6.

7.

measuring chambers, e.g., the V100 Kent meter. Suitable seals are used between the measuring chamber and the meter body as discussed in Chapter IV. Woltmann meters used for irrigation and agriculture, as well as for fertilizer water meter designs, should be such that there will be negligible loss of head and they should be simple to maintain. These meters are now available with the possibility for a field-replaceable measuring unit. Registers are available as hermetically sealed units. Mechanical registers are offered with a totalizer, three pointers, and a leakage detector. In fertilizer water meters, plastic constructions are used to avoid corrosion. Register: The register is fully sealed and vacuum-filled [13]. A number of rollers are immersed in a nontoxic liquid to act as lubricant. The register is placed in a window to give clear readings, and in some cases these are provided with a window lens. Tamperproof: Since the meters are used for billing purposes, features are included in the meter to resist illegal tampering by not allowing disassembling during operation. Remote reading: These meters have provision for remote signaling/transmission. Some have a built-in remote transmission facility and also facilities for remote reading. It is even possible in some meters to be interrogated by a PLC or a computer. As applicable, pulse signals are the normal outputs from the meter. Reverse flow: In most of cases reverse flow metering is possible to facilitate information and network management and revenue billing applications. Depending on applications there can be an internal disc-type reverse-flow restrictor to eliminate water from flowing back illegally. High performance: For better meter performance, systems incorporate necessary design, e.g., grooved rotary piston in water meters. Management tool: In order to carry out effective management, such as consumption and flow, etc., many meters are available with the necessary management information tool supports.

953

Now the discussions move on to standard domestic portable water and industrial water meters. 6.1.2 DOMESTIC WATER FLOW METERS Domestic water flow meters offer working on rotary piston normally offering the desired accuracy with tamper-proof operation. It is also possible to get remote readings. These are available in various sizes. A typical domestic water flow meter based on the Kent meter is depicted in Fig. X/6.1.0-1. Normally these are available with a cover lid as shown in the left-hand side of this figure. Details of the top dial of the meter are illustrated on the right-hand side of the figure. The dial shows flow directions, a pointer for bidirectional communication, 6e7 digit register, and unit selection with bidirectional pulse communication facility. The discussion starts with some features of the meter. 1. Features: The following are a few features worth noting: l Principles of operation: Rotary piston principles (see Section 5.1.1 of Chapter IV); l Accuracy: Desired accuracy and performance in any position; l Durability: Grooved piston for better durability; l Tamperproof: Tamperproof construction and operation; l Sizes: Available in sizes up to 250 mm size; l Pressure rating: Pressure up to 16 barg at temperature up to 50 C; l Output: Inductive pulse output with multiple pulses to support management information; l Reverse flow: Bidirectional flow measurement possible; l Register: Built-in mechanical register; l Readability: Clear readability; l Calibration: Some meters do not require calibration throughout life span [13]; l Special feature: Exceeds Class B specification in forward direction and for sizes up to 150 mm in reverse direction [14].

954

Plant Flow Measurement and Control Handbook

PRIMARY PULSE

999999 TOTAL FLOW IN FLOW UNIT SECONDARY PULSE (1/10)

FIGURE X/6.1.0-1 Domestic water meter. Developed based on Kent meter. Courtesy: George Kent.

2. Specification: A brief specification for a domestic/industrial water meter is given in Table X/6.1.2-1. It is always recommended to use a net strainer for protection of the meter. There is a wide variety of this meter available

in the market. The specification given covers the general technical data and the bestknown technical data. Data furnished here have been taken from various models of reputed makes.

TABLE X/6.1.2-1 Specification for Domestic Water Meter SL

Specifying Point

Standard/Available Data

1

Meter size

Various models with sizes from 12 to up to 300 mm

2

Flow range

1.5e1000 m3/h

3

Overload flow range

Generally twice the flow range

4

Pressure limit

Normally within 16 barg

5

Temperature

Normally 50 C

6

Register

Normally million m3. In 6e7 digits

7

Head loss at overload flow

84e21 KPa, as the meter size increases head loss decreases

8

Body material

Copper alloys are common

9

Measuring chamber

Polystyrene/plastic

User Spec.

Remarks

Continued

Special Flow Meters, Flow Gages, and Switches Chapter | X

955

TABLE X/6.1.2-1 Specification for Domestic Water Meterdcont’d User Spec.

SL

Specifying Point

Standard/Available Data

10

Rotor

Polyamide

11

O ring

Elastomer

12

Orientation

Vertical, horizontal; some with diaphragm, meant for vertical position

13

Connection

Threaded, NPT/BS of different suitable sizes. Flange connections for higher sizes are also possible

14

Output

Pulse output

15

Pulse rate

Variable normally litter per pulse

16

Primary pulse

For bidirectional flow; two wire connection; one wire carries pulse and the other for direction flag

For Kent meter Courtesy: Kent meter

17

Secondary pulse

Two-wire connection; one-wire pulse compensation; other flag to indicate compensation process

For Kent meter Courtesy: Kent meter

18

Accuracy

2% average

6.1.3 IRRIGATION, AGRICULTURE AND FERTILIZER WATER FLOW METER Woltmann type meters used for irrigation, agriculture, and fertilizer purposes are discussed in this section. It is to be noted that here fertilizer meter means a kind of water meter for fertilizer/ chemical dosing purposes, i.e., a fertilizer for

Remarks

agricultural applications. A typical mechanical water meter (fertilizer) is shown in Fig. X/6.1.0-2. The display part is similar to that in the domestic/industrial water flow meter shown above. The discussion starts with features of a Woltmann water meter.

TOTAL FLOW 999999

FIGURE X/6.1.0-2 Mechanical water meter (fertilizer). Developed based on Kent meter. Courtesy: George Kent.

956

Plant Flow Measurement and Control Handbook

1. Features: The typical features of this type of water meter are enumerated here: l Loss: Low-loss meter; l Replacement: Field-replaceable measuring chamber; l High-flow condition: Possible to cater to high flow rate; l Harsh environment: Ability to cater to harsh environmental conditions with high humidity and vibration; l Corrosion protection: Corrosion-resistant plastic components when necessary; l Register: Mechanical register with totalizer, three pointers, and leakage detector;

Hermetical seal: Hermetically sealed register; l Readability: Easy readable register with lens; l Rate flow: Rate flow indication possible; l Wide range and accuracy: Possible for accuracy curve to cover wide range based on ISO standard. 2. Specification: A brief specification for Irrigation agriculture and fertilizer water meter is given in Table X/6.1.3-1. It is always recommended to use an in-net strainer for the protection of the meter. There is a wide variety of meter available in the market. l

TABLE X/6.1.3-1 Specification for Irrigation Agriculture and Fertilizer Water Meter SL

Specifying Point

1

Meter Size

Various models with sizes: Plastic fertilizer meters: 12e25 mm. Water meters: 40e250 mm

2

Flow range

Water meter: 50e7500 m3/h Fertilizer: 0.6e6 m3/h

3

Overload flow range

Generally nearly (less than) twice the flow range

4

Pressure limit

Normally within 13 barg

5

Temperature

Normally 60 C

6

Register

Normally million m3. In 6e7 digits

7

Register connection

Magnetic coupling for register

8

Head loss at overload flow

Varies with meter type; data sheet to be consulted

9

Orientation

Any position: Horizontal/vertical

10

Body material

Fertilizer meter: Organic polymer; polyphenylene sulfide (PPS) Water meter: Polyester-coated cast iron/brass

11

O ring

Elastomer

12

Standard

ISO4064/EEC

13

Connection

Threaded, NPT/BS of different suitable sizes. Flange connections ISO standard

14

Output

Optional electrical outputs possible

15

Electrical output options

Rate flow and volume flow as well as electrical contact output

16

Accuracy

2% AR (average)

17

Accessories

Filter at upstream

Standard/Available Data

User Spec.

Remarks

Special Flow Meters, Flow Gages, and Switches Chapter | X

The specification given covers general technical data and the best-known technical data. The data furnished here have been taken from various models of reputed makes. 6.2.0 Mechanical Oil and Other Flow Meter Mechanical flow meters with a local totalizer and/ or rate indicators find their applications in oil flow measurement. There are a few other mechanical flow meters with local-scale pointer indicators available for various applications. In this section these shall be discussed.

957

Oil consumption and oil receipts are quite important in this respect. The discussion starts with oil meters. 6.2.1 MECHANICAL OIL FLOW METERS In the modern world, on account of more and more energy consumption, fuel price is in a rising trend. Therefore, it is needless to say that monitoring of fuel at each stage is always crucial. A typical oil flow meter and its application are depicted in Fig. X/6.2.1-1. 1. Oil receipt monitoring: Petroleum products, such as diesel, LSHS, and furnace oil are all

(A)

(B) OIL STORAGE TANK

OIL TANKER CONSUMPTION METERING

PIPE (Typ) RECEIVING METERING' ONLY RELEVANT PARTS SHOWN; NOT TO SCALE

DIESEL GENERATOR

FIGURE X/6.2.1-1 Oil flow meter. (A) Oil meter details. (B) Application of oil meter.

958

Plant Flow Measurement and Control Handbook

received in plants or oil depots by tankers or by railways and tanks that are filled by pipeline. In most earlier cases oil receipt was measured by dipsticks in the tank. This is not only a very crude way of measuring the quantity but there is the possibility of pilferage by unscrupulous personnel. In modern times people use an oil flow meter to monitor the quantity of oil delivered. Therefore, the actual oil receipt can be accounted for. In this connection, Fig. X/6.2.1-1 may be referenced. 2. Oil consumption: As petroleum products are very costly it is important to account for the quantity of oil consumed by various users, such as diesel generators, boilers, and hot air generators. In such applications people have

started deploying these kinds of oil meter. Boilers, etc. mentioned here are small units, for larger units or for utility boilers regular electronic PD meters are used. In this connection, Fig. X/6.2.1-1 may be referenced. 3. Features: The major features of these meters are as follows: l Principles: Simple rotary piston principle; l Components: Least possible number of components; l Register: Register magnetically coupled; l Pressure loss: Low line pressure loss; l Quality: Reliability and accuracy; l Output: Electrical output. 4. Specification: A brief specification of an oil meter is given in Table X/6.2.1-1

TABLE X/6.2.1-1 Specification for Oil Flow Meter SL

Specifying Point

Standard/Available Data

1

Meter size

15e80 mm

2

Flow range

0.01e50 m3/h

3

Fluid types

Petroleum products, such as MS, HSD, LDO, LSHS, etc.

4

Viscosity

<1000 cP

5

Pressure limit

Normally within 40 barg

6

Temperature

Normally 150 C

7

Register

Normally million m3. In 6e7 digits

8

Register connection

Magnetic coupling for register

9

Head loss at overload flow

Actual data sheet and loss chart to be consulted

10

Orientation

Any position: Horizontal/vertical

11

Body and working chamber material

Brass/bronze, cast iron

12

Piston material

Aluminum

13

O ring/gasket

Viton/special compound

14

Standard

ISO4064/EEC

15

Connection

Threaded, NPT/BS of different suitable sizes. Flange connections ISO standard

16

Output

Optional electrical outputs possible

User Spec.

Remarks

Continued

Special Flow Meters, Flow Gages, and Switches Chapter | X

959

TABLE X/6.2.1-1 Specification for Oil Flow Meterdcont’d SL

Specifying Point

Standard/Available Data

17

Accuracy

0.5% AR average

18

Accessories

Filter/air release at upstream

6.2.2 OTHER MECHANICAL FLOW METERS There are a few other mechanical flow meters, as shown in Fig. X/5.2.0-1C. These meters are basically rate flow indicators. In these meters flow rates are indicated in a dial with the help of a mechanical pointer. These are mechanical flow meters with a wide range of sizes, from 12 to 150 mm size. These meters also provide contact output and can act as a flow switch to be operated at a specified set point. Optionally these meters are provided with an electrical supply to give electrical outputs for remote transmissions and for batch and other controls. These meters operate in vane displacement mode. A tapered needle passing through an orifice in the face of a piston completely seals the port. When there is flow, the piston is displaced against a differential pressure/ spring load and moves over the tapered section of the needle. Flow is allowed through the orifice. The tapered section is meant to cater to the changes in viscosity and flow variations. On account of the vane (spring-loaded) displacement there will be a variable orifice for variable flow for a specified viscosity value. Therefore, vane displacement is proportional to flow rate. These meters do not demand any upstream straight length for their operation. They can operate in pressure as high as 140 barg. Any kind of meter orientation is allowed. These are available in a wide range of body and vane materials to cater to a wide range of fluids. They are available with screwed end connections and are used in water treatment plants, synthetic base oils, corrosive fluids, paints, and solvents.

User Spec.

Remarks Refer to Chapter IV

With this, the discussions on local flow gages/ meters come to an end and we now investigate the flow switch types available and their applications.

7.0.0 FLOW SWITCH In order to monitor flow in the conduit, flow switches are used. Here the word “monitor” actually refers to a broad spectrum in the sense that such monitoring could be local and remote. The local monitoring function could be met by the simple flow gages discussed above. Remote monitoring again could be continuous monitoring by indication, recording, etc. Alternatively, it could be when the flow in the conduit goes beyond a set value that some action is initiated. Most plants now operate remotely, therefore, such flow-monitoring devices should be fieldmounted for initiating a remote action. Here, the word beyond has been used intentionally to mean that when flow goes below the set point action is to be initiated or when flow goes above a set point, action is to be initiated. Also, there are applications where flow is to be kept within a band (as seen in a batch process) using two such monitoring devices. The word “action” mentioned above could be a simple alarm, interlock, and/or tripping of equipment and/or a (sub) system. In such actions all the time flow trends are not always necessary. Therefore, a costly continuous monitoring (indicating/transmitting) type flow meter may not be necessary. From the above discussions it transpires that all instruments which can measure flow can also be used as flow switches, e.g., initiating contact for such action

960

Plant Flow Measurement and Control Handbook

discussed above could be obtained by using a simple limit value monitor (which could be a hardware device or could be software action from DCS/PLC). However, at times this is a costly proposition, e.g., providing a flow transmitter and limit value monitor in a lube oil line for a big pump/fan is too costly and there could be space constraint in the lube oil skid also. So, if only flow monitoring (that it is within the set point) is required for a particular application, the deployment of indicating or transmitting devices cannot be economically justified and at times due to space constraints cannot be accommodated also. This is where a flow switch is useful. Flow switches are used to monitor the line flow to determine if the flow rate is beyond a certain value. This specific certain value is referred to as the set point. Depending on the application, this set value could be fixed or adjustable. Most of the process switches offer adjustable set points. On reaching the set point (in rising/falling mode) like other process switches, the flow switch needs to respond by actuating an electric or pneumatic circuit. Electric flow switches (of interest here) need to actuate a set of contact(s). On reaching the set point when the flow switch is actuated, contact(s) configuration will stay in that state until the flow rate moves back from the set point. Let us take a specific case; when the flow is above x set point it will actuate. So when flow goes above x set point, e.g., NO contact closes. It remains closed as long as flow >x. However, it will be seen that it stays in that condition even if flow goes below x set point up to a certain value. This difference between the set point and the reactivation point is called the switch differential [15]. Next we define a few terms which will be frequently referred to and are applicable not only for flow switches but for process switches in general.

7.0.1 DEFINITIONS AND TERMINOLOGIES WITH EXPLANATIONS The following terminologies are frequently used and it is important that these terms are well understood. It is recommended to refer to Section 1.2.1 of Chapter I also, for further understanding. These terms have been explained in Fig. X/7.0.0-1.

1. Accuracy and repeatability: Accuracy measurement is possible only when there is continuous output from the device and it is indicating type. However, flow switches can be the blind type also, so accuracy as defined and discussed in Section 1.2.1 of Chapter I may not be applicable for flow switches (or for that matter for any process switches). Flow switch may be indicating type or non indicating type. However, repeatability as defined in Section 1.2.1 of Chapter I, is applicable for flow switches. 2. Actuation and deactuation point: The actuation point refers to the set point. Actuation is that point exactly where the state of the electrical contact associated with the flow (process) switch changes state depending on the configuration of electrical contact(s), i.e., on reaching the set point, NC and NO contacts change states to become NO and NC, respectively. As long as flow is beyond the set point the change of state will not change. When the flow changes and returns to a point which is within the set point the electrical contact(s) will revert back to their original state. This point where the electrical contact(s) reverts back to the original state is referred to as the deactuation point. Theoretically both points should coincide but, in reality, these two points are never the same. The difference is called the differential or dead band. Refer to Fig. X/7.0.0-1B for detailed explanation. 3. Adjustable set point: As indicated above, the actuating point is referred to as the set point. When such a set point is adjustable it is referred to as an adjustable set point. Normally the set is adjustable within the measuring span of the instrument (refer to Subsection 1.2.1.7 of Chapter I). Normally the set point facility never exceeds measuring span. 4. Dead band: The dead band for process switches refers to the “differential” between actuation and deactuation in the flow (process) scale is called the dead band. Dead band is an important issue in connection with any process switch. There two types of dead band possible: fixed dead band and adjustable dead band. As the name signifies, when dead band can

Special Flow Meters, Flow Gages, and Switches Chapter | X

(B)

SPDT OR

TWO INDEPENDENT SETS

B

ON

NO

DPDT

OFF0

NO OR NC CONTACTS SHOWN AT NO FLOW NO POWER CONDITION TO IDENTIFY NO/NC CONTACT FOR PROCESS SWITCH

OFF0

A

DEACTUATION POINT

A

ON

SET POINT ACTUATION

INCREASING

SPST

NC

< SET POINT

DECRESING

NC

DECRESING

NO

> SET POINT

COC

INCREASING

COC

HYSTERESIS

CONDITIONS AT 0 OR A (B) IN HYSTERESIS CURVE

HYSTERESIS

(A)

961

B

SET POINT ACTUATION DEACTUATION POINT

(C) COC

SHOWN WITH SPDT; BUT EQUALLY APPLICABLE FOR OTHER CONFIGURATIONS ALSO FLOW GREATER THAN SET POINT FOR INCREASING (>) SET POINT REMAIN IN THIS STATE

NC

NO

COC NC SPDT

EVEN WHEN FLOW LOWER THAN SET POINT BUT > DEACTUATION POINT (say) POINT B. IN FIG X/7.0.0-1B.

SPDT

SHOWN WITH SPDT; BUT EQUALLY APPLICABLE FOR OTHER CONFIGURATIONS ALSO FLOW LESS THAN SET POINT FOR DECREASING (<) SET POINT REMAIN IN THIS STATE

NO

EVEN WHEN FLOW MORE THAN SET POINT BUT < DEACTUATION POINT (say) POINT A. IN FIG X/7.0.0-1B.

FIGURE X/7.0.0-1 Flow switch general terms explanations. (A) Contact configuration. (B) Hysteresis explained. (C) Contact actuation/deactuation conditions.

be adjusted (externally or by opening the switching unit) in a process switch it is adjustable dead band. When there is no adjustment facility in a switch it is fixed dead band. Dead band again can be two of types: narrow and wide dead band. Often dead band is used for, e.g., actuation of pump/fan, etc. Refer to Fig. X/7.0.0-1B 5. Hysteresis: Refer to Subsection 1.2.1.11 of Chapter I. 6. Contact configuration: As shown in Fig. X/ 7.0.0-1A, there are many contact configurations available, these are listed here: l SPDT (COC): Single-pole double-throw, i.e., single changeover element which at different conditions makes and breaks two separate terminals, i.e., one normally open, one normally closed, and one common terminal. This is also referred to as changeover contact (COC).

DPDT: DPDT stands for double-pole double-throw (DPDT). Same as SPDT only in this case there will be two independent changeover elements instead of one. l SPST: SPST is single-pole single-throw, i.e., when there is a single switch element and two terminals it is either NO/NC contact depending on initial configuration as elaborated in NO/NC contact decision discussed below. 7. NO/NC contact decision: Often we use the terms NO/NC contacts in connection with process switch. Therefore, how are NO or NC decided for a switch? Generally, the contact configuration is decided based on their condition at NO FLOW NO POWER CONDITION. Any switch when bought from market has no flow nor it is energized, so closed contact at this condition is NC and contact at open l

962

Plant Flow Measurement and Control Handbook

condition is NO contact. The same philosophy applies for electrical switches and relays [9]. 8. Actuation or switching types: Mercury actuated, microswitch, and magnetically actuated Reed switches are commonly used with flow switches. Of these, microswitches and snapacting switches are mostly used because in many cases snap-acting switches meet or exceed industrial standards for reliability, electrical capacity, and longer life. Handling of a mercury switch is not easy and in many cases the help of hermetically sealed contacts may be needed for its operation. As seen above, mainly Reed switches and microswitches are deployed in flow and other process switches. We not us look at these issues in depth. l Snap action (microswitch): In microswitches the operation depends on working of a plunger and set of contacts between a common contact and, e.g., the NO/NC contact point. In a microswitch, when the plunger is completely released, i.e., in the free position, the common contact is against the normally closed contact to complete the circuit through them. In this condition, the normally closed circuit of the switch can carry current, and the common terminal is electrically insulated from the normally open terminal/contact. With depression of the plunger, the switch reaches the operating point. The distance from the free position to the operating point is called the pretravel. At the operating point, without further movement of the plunger, the common contact accelerates away from the normally closed contact [16]. Within a short time (a few milliseconds), the common contact strikes, bounces, and comes to rest against the normally open contact. This is snap action, and because of this action common contact cannot stop part way between the normally closed and normally open contacts. The distance the plunger travels past the operating point in the same direction is called over travel. Past full over travel, further depression of the plunger is prevented by the switch

l

mechanism. The distance from the free position to the point of full over travel is called the total travel. When the plunger is released from the point of full over travel by further force, it goes past the operating point without any change in contact position till the plunger reaches the release point. Only at the release point, without further movement of the plunger, the common contact accelerates away from the normally open contact and within a few milliseconds the common contact strikes, bounces, and comes to rest, against the normally closed contact. The distance between the operating and release points is called the differential travel. The force and travel characteristics of the snapaction switch can be represented by graphical means, which is of interest to the designer. Reed switch: Reed switches are available in a small glass bulb which is either a vacuum or filled with an inert gas like argon. The bulb is always sealed to prevent oxidation effect. Reed switches in process switch applications are operated with a magnet so that switches are made up of ferromagnetic material. However, nonferromagnetic material Reed switches are also available. Within the bulb, there is a flexible Reed strip contact. Operating force for the Reed switch, expressed in Ampere turns, is the minimum force necessary to close the Reed switch and this force is referred to as just-operate force. As the force between the poles increases as the gap decreases, a force of approximately half the just-operate force will maintain the operated state. Speed of operation of the Reed switch is determined by the excess of operating force over the just-operate force. Reed switch contact set has a comparatively lower breaking capacity of around 10e70 VA max. Therefore, this is suitable for low-power devices, such as DCS/PLC, solid state relay, etc. It is not suitable for circuits with more load requirements, such as motor circuits, solenoid valves, etc. For this reason, many flow

Special Flow Meters, Flow Gages, and Switches Chapter | X

switches use Reed switches, along with builtin relay, e.g., SOR flap type flow switch. l Comparison between switch types: The advantages and disadvantages between the two types of switches discussed above are been presented in Table X/7.0.1-1, so that the instrument designer can choose the best-suited type for the application in hand. 9. Manual reset: Set and reset of flow switches have been discussed above, however, there are some applications, where the manual reset feature is adapted to ensure that precautionary measures are taken before resting the switch manually. In such a way the operator is well aware of the situation, and necessary action is initiated.

963

7.0.2 FLOW SWITCH TYPES There are two types of flow switches, one is the direct type and the other is the indirect type. Flow switches, such as the vane type and flap type, are direct flow switches. In contrast, flow switches with a flow element along with DP switches are an example of indirect flow switches. LVM in conjunction with a flow transmitter is another example of an indirect flow switch. Flow switches can be meant for fluids and others for solid flow. From a technology point of view flow switches can be divided into the following: 1. Paddle type; 2. In-line flow switch (piston); 3. DP type/bypass type;

TABLE X/7.0.1-1 Comparison Between Microswitch and Reed Switch Important Issues

Snap Acting Switch

Reed Switch

Operating force

Larger force to operate

Lower force to operate

Volt, watt and capacity

Capable of handling larger voltages like 230/110 AVC and higher current-handling capacity

Much lower load-handling capacity and normally operates at lower voltages

Contact configuration

Possible wide range of contact configurations, like SPNO/NC, SPST, SPDT

This is not possible single contact

Time of operation

About 1 m/s, so time to make break is very nominal (1/1000) no radio frequency effect

Comparatively slower. May requires contact protection circuitry

Environmental effect

These are not hermetically sealed and necessary precaution/certification may be necessary for hazardous applications

Hermetically sealed in glass environment, free from contamination, safe to use in harsh industrial and explosive environments. Very high contact isolation resistance with very low contact resistance

Process switch application

Use with magnet is not applicable and needs force to operate

In-process switch applications can be used in combination with magnets and coils, to assist the operation. They can be used to form many different types of relays reed relay

Cost

Comparatively costlier

Much cheaper

Differential

Larger differential travels hence large process differential

Lower differential

Life expectancy

Good

Higher

964

Plant Flow Measurement and Control Handbook

4. Disc type switch (with push valve); 5. Reluctance type, such as capacitance/inductance type flow meters; 6. Thermal type (also hot wire anemometer); 7. Variable orifice area (vane/piston) type; 8. Ultrasonic (Doppler also); 9. Microwave type (solid flow switch); 10. Solid flow switch (US type). We now us look into the details of flow switches starting with some general requirements. 7.1.0 General Requirements of Flow Switches With Explanations Based on applicability and available flow switches the following are general requirements that can be specified. These are general requirements that may vary with a particular flow switch. However, these technical requirements should be specified for procurement. They are specified to facilitate the designer to draw up specifications. 1. Sensing material: Stainless steel or better for corrosion resistance. It is important to specify suitable material for the application. 2. End connections: Normally screwed of suitable size and style. However, in certain cases flange connections of suitable size, pressure rating, and standard are specified. Typical flanges are PN10, ANSI 150/300 lb class size depends on monitor size. 3. Repeatability: 5%e0.5% AR or better. There will be huge variations of repeatability of the flow switch depending on the technology and type of switch. In some cases mounting and improper installation can affect the value. 4. Contacts: Typical configuration: 2nos. SPDT snap acting dry contact. For usage in applications, potential free dry type contacts are preferred. Several combinations of contacts are specified with variations in ratings. The designer should choose the most suitable for the intended application. 5. Contact rating: To specify the maximum current at the maximum AC and DC voltages

6. 7.

8. 9.

separately. Also, the maximum/minimum wattage should be specified. Normally manufacturers also specify these ratings, as while selecting it is important to note the minimum of the possible combination. This will be clarified from a rating, e.g., it may be specified 1 A at 110 VAC; 2 A at 30 VDC and power 30 W (VA). This means the maximum AC/DC voltage could be applied as 110 V and 30 V for AC and DC, respectively. However, the current should be limited so that 30 VA power is not exceeded. (e.g., for 110 VAC current < 0.27 A). The maximum currents that could be applied are 1 and 2 A but NOT at maximum voltage as it wouldbeat lowervoltage,e.g., 1 A ACis allowed with 30 VAC as wattage is limiting—hence voltage allowed to be selected accordingly. Set point: Adjustable range to be specified. Dead band: Application-dependent; to specify adjustable/fixed dead band. For fixed dead band narrow or wide range to be specified. Enclosure class: Generally IP65. Minimum velocity and response time: Another important issue is the minimum velocity of fluid that could be detected. Minimum velocity for detection varies widely with: l Type of fluid, i.e., gas (air) and liquid; l Type of sensing.

In gas this may vary between 0.1 and 70 m/s in liquid and gas. These variations given are based on various types of sensing element, i.e., thermal sensors have the capability to sense lower velocities, e.g., a thermal switch can sense 0.06 m/s velocity. On the other hand, the response time of the thermal sensor is not good enough as these have a higher time constant of around 6e8 s. We now investigate the details of a few flow switches normally encountered in industrial applications. There could be a number of choices available, however only a few common types are covered here; specifications enumerated are generalized in nature—for specific details manufacturers need to be consulted.

Special Flow Meters, Flow Gages, and Switches Chapter | X

7.2.0 Flow/No-Flow Switch: Paddle(/Vane) Type Paddle/vane-operated flow switches are basically flow/no-flow switches mounted vertically in a pipe. Depending on the pipe size, the paddle length varies to give flow actuation. 7.2.1 DESCRIPTIVE DETAILS OF PADDLE TYPE FLOW/NO-FLOW SWITCHES This flow switch utilizes the force of the liquid flow to propel the paddle (vane) for detection of flow and no flow conditions. The switch consists of a body, O ring, paddle pivot, central extension rod, magnet, spring, and hermetically sealed Reed switch. As shown in Fig. X/7.2.0-1, the paddle is connected to the switching unit through a pivot connection. Flow and no-flow detection is as discussed here: 1. No flow: At static condition, i.e., at no flow, the spring is expanded, and pressing the magnet vertically downward, so that the hermetically sealed contact is in the NO condition. 2. Flow condition: When flow occurs, due to flow, the paddle is thrust and raised to about

20e30 degrees, i.e., with flow the paddle moves (swings) about the pivot, to move out of the liquid path creating very low pressure loss of around 3 psi irrespective of flow rate. On account of this paddle movement, the vane extension arm moves against the spring, to give upward motion to the magnet that actuates (deactuates) a hermetically sealed Reed switch, where open or closed contacts are required to signal flow or no-flow conditions [Note that magnetic movements have not been shown since the movement is very small]. 3. Design variations: l Length: There are two kinds of paddle available: short paddles for pipe sizes up to <40 mm and long paddles for pipe sizes between >40 up to 100 mm. Cut-off paddle lengths for different pipe lengths are marked on the paddle/vane of the long paddles. The paddle needs to be trimmed during installation to permit switch actuation during desired flow. l Switching mechanism: In the discussions above, we have considered magnet and

LEAD WIRE OUTPUT ELECTRICAL HOUSING LEAD WIRE REED SWITCH SPRING REED SWITCH SCREWED

MAGNET

SCREWED

CONNECTION FLANGE POSSIBLE

PIVOTED PADDLE

CONNECTION FLANGE POSSIBLE

NO FLOW

965

FLOW

PADDLE DEFLECTED

FIGURE X/7.2.0-1 Paddle type flow switch (flow/no flow).

966

l

Plant Flow Measurement and Control Handbook

Reed switches. There are variations in those also. Paddle type flow switches are also available with microswitches. In this type, in place of a magnet and Reed switch, microswitches are used. The paddle is pushed by liquid (water) flow, which actuates the microswitch. When the flow is decreased, it is deactuated. This simple microswitch is used for vertical mounting. There is another version where a flap is provided with a magnet for horizontal mounting for flow from top to bottom or bottom to top. This is a gravity pullback paddle switch, e.g., R1Y(R1E) for PN10 DN63 of JPC France (www.jpcfrance.fr). When flow occurs, the paddle swings away, causing the Reed switch to operate, i.e., the contact closes. When flow decreases the paddle returns to its original position, and the Reed switch deactuates. To assist the swinging of the paddle the device is operated with a set of magnets and a repelling force of magnets (one in the paddle) assists swinging. Mounting: Normally paddle switches are meant for vertical mounting. A gravity pullback paddle switch is meant for horizontal mounting.

7.2.2 INSTALLATION REQUIREMENTS OF PADDLE TYPE FLOW SWITCHES The following installation points need to be accounted for during installing of the switch: 1. Orientation: Vertical;

2. Connection: Screwed through 100 or 1½00 NPT/ BSP connections; 3. Flow direction: To make sure that the marked flow direction is parallel to the pipe run; 4. Pivot length: Since the pivot length is directly related to the flow actuation and deactuation point so the paddle length (duly marked on the paddle for various pipe diameters) is to be trimmed at the site based on the pipe diameter; 5. Straight length: Minimum 3D (internal diameter of pipe) horizontal straight length to be kept on both sides of the flow switch; 6. Operating conditions: The operating pressure temperature specified by the manufacturer should not be exceeded. Such temperature does not have much effect, yet, with a change of temperature if there is a chance of changes in liquid density it may affect the thrust on the paddle like a pressure change affecting the thrust. In case of the possibility of sudden changes in operating conditions the manufacturer’s recommendations are to be followed. Installation should be made at a place with the least possible shock and vibration. 7.2.3 SPECIFICATIONS OF PADDLE (VANE) TYPE FLOW SWITCHES Based on reputed manufacturers’ data, a brief generalized specification of a paddle (vane) type flow/no-flow switch is given in Table X/7.2.0-1. The table shows the maximum possible data, to the best of author’s knowledge, and hence they may not be possible in any single switch. Based

TABLE X/7.2.0-1 Specification of Paddle (Vane) Type Flow/No Flow Switch SL

Specifying Point

Standard/Available Data

1

Orientation

Vertical

2

Connection Type

Screwed/flanges also possible

3

Connection size and ratings (as applicable)

100 NPT through 600 Thread: 1e200 NPT/BSP Flange: 2½00 to 600 ANSI 150/300 lb class

User Spec.

Remarks

Continued

Special Flow Meters, Flow Gages, and Switches Chapter | X

967

TABLE X/7.2.0-1 Specification of Paddle (Vane) Type Flow/No Flow Switchdcont’d SL

Specifying Point

Standard/Available Data

4

Approx. pressure drop

207 millibar (3 psi)

5

Pressure limit

<340 barg

6

Temperature

40 to 200 C

7

Min. velocity

0.3 m/s (1 FPS)

8

Pipe sizes

Short 1¼00 long In 1½00 to 400 To trim the paddle accordingly

9

Min./max. actuation (deactuation) flow in L/min

Short: 18 (11.3) to 109 (83) Long: 57 (42) to 147 (94)

10

Flap/spring material

304 SS or 316 SS/316SS

11

Body material

Brass/316SS

12

Piston material

Aluminum

13

Other parts

Teflon/ceramic

14

Electrical housing

IP65 with explosion certification from appropriate authority as necessary

15

Output

Contact output with relay

16

Contact configuration

SPST/SPDT with relay or low-power SPST and SPDT

17

Contact rating

Refer to Subsection 7.1.0.5 for explanation. Available in AC: V(max) 110 V I max: 0.9 A DC: 30 V I 2 A Max Watt: 30 VA

18

Repeatability

5% AR average

19

Hazardous application

Necessary certificate from authorized agencies

on the application, the required data may be modified after consulting the manufacturer.

User Spec.

Remarks

7.3.1 IN-LINE (PISTON) FLOW SWITCH

7.3.0 In-Line and DP Type Flow Switches

A typical in-line piston flow switch is depicted in Fig. X/7.3.0-1.

In this section two types of flow switches are discussed and both use a piston, magnet, and Reed switch for their operation. These two types are the in-line (piston) flow switch and the DP type flow switch. The discussions start with the in-line flow switch.

1. Principle of operation: An in-line flow switch consists of a moving piston, a magnet in the piston, and a hermetically sealed Reed switch unit, as shown in Fig. X/7.3.0-1. Major parts of the flow switch have been marked in this figure. The Reed switch is magnetically

968

Plant Flow Measurement and Control Handbook

SPRING

OVER PRESSURE VALVE REED SWITCH PISTON MAGNET

FIGURE X/7.3.0-1 In-line (piston) flow switch.

coupled with the piston and magnet. In this type of construction, as shown, the wetted area is completely separated from the electrical area, hence there is no sealing problem. These are basically meant to be used in vertical mounting. With flow of liquid from bottom to top, due to flow/increase in flow of fluid, the piston is displaced by the differential pressure from fluid flow. The piston has a magnet which, due to displacement of the piston, also moves up. On account of the movement of the magnet, the Reed switch is actuated. On the other hand, when the flow stops or decreases, due its own weight the piston and magnet comes down and the Reed switch is naturally deactuated. Then what is the purpose of the spring? Return of the piston is carried out by the spring. When the flow of fluid is from the top to bottom, then the spring is more

necessary to retract the piston. Also, it helps in regulating the movement of the piston. In this design the piston is placed directly in the 100% flow path. There is another version also which uses a flap with a magnet. The flap with a magnet is also placed directly in the 100% flow path. In-line flow switches are often provided with overpressure protection, as shown in Fig. X/7.3.0-1. In the case of a piston there is linear movement of the piston, in the case of a flap, it swings around a pivot in the main body. The operation is similar to those discussed in Section 7.2.0 above. The action and construction are the same. These meters are available in various sizes and normally have screw type end connections. 2. Specification: Specification of an in-line flow switch has been enumerated in Table X/7.3.1-1

Special Flow Meters, Flow Gages, and Switches Chapter | X

969

TABLE X/7.3.1-1 Short Specification of In-Line (Piston) Flow Switch SL

Specifying Point

Standard/Available Data

1

Fluid type

Liquid (mainly water)

2

Orientation

Vertical

3

Connection type

Screwed

4

Connection size

½00 BSP/NPT (M) at inlet and female at outlet

5

Pressure limit

1 MPa (PN10)

6

Temperature

0e100 C

7

Ambient temperature

0e50 C

8

Pipe sizes

Small up to 15 mm

9

Set adjustability

Adjustable through piston

10

Body/piston material

Polyphenylene oxide (PPO)/stainless steel

11

Spring material

304 SS

12

Electrical housing

IP65

13

Output

Potential free dry contact

14

Contact configuration

NO

15

Contact rating

Refer to Subsection 7.1.0.5 for explanation. Available in: AC: V(max) 230 V Current: 1 A Max Watt: 70 W.

16

Repeatability

2%e3% AR Average

17

Hazardous application

Necessary certificate from authorized agencies

7.3.2 DP TYPE FLOW SWITCH From the discussion on the DP type flow meter and DP type flow metering it has been seen that flow through the flow element is proportional to the square root of DP across a flow element. DP type flow switches basically work on the principle of measurement of DP across a flow element and generating a contact at the desired DP and hence flow point. Description: DP flow switches operate on the differential pressure principle without any bearing or sliding surfaces to corrode and stick. DP type flow switches can be used for both gas and liquids, as well as for dirty fluids. Fig. X/ 7.3.0-2 shows a typical DP type flow switch.

User Spec.

Remarks

This basically is a flow element with a DP gage with suitable contact. These flow switches can be blind type also. Use of both a snap-acting switch with a DP gage is quite common. Some also use a Reed switch and magnet placed either on the lever mechanism of the indicator or to sensor such sensing bellow so that when the desired DP is reached the Reed switch closes contact. Some use a Reed relay in place of a Reed contact to facilitate the switching action, as already discussed above. Normally there are different versions for liquid and gas applications. Many of these flow switches have an indication for flow also and it is possible to adjust the flow set points. There can be one or two field-adjustable

970

Plant Flow Measurement and Control Handbook

switch set points. Some of these flow switches have a built-in terminal box. Most of these flow switches are suitable for hazardous applications. These are reliable but may not offer good repeatability and accuracy (applicable for flow switches with indication). Specification: Table X/7.3.2-1 presents brief specification of the flow switch type. 7.4.0 Variable Orifice Type Flow Switches A typical variable orifice type flow monitor is shown in Fig. X/7.4.0-1. There are two types of such variable orifice type flow monitors: vane type and piston (valve) type. Operating the vane against the spring is the major component of the flow switch. Functioning of the system has been explained in two sets of figures shown on the right hand side of Fig. X/7.4.0-1. As shown in the figure, this is a flow monitor with flow indication and flow switch, e.g., FLW series of flow monitors from Omega Engineering USA. 7.4.1 WORKING PRINCIPLE OF VARIABLE ORIFICE FLOW SWITCH

FIGURE X/7.3.0-2 DP type flow switch.

The kinetic energy of a flowing liquid/gas is utilized to move a spring-biased swing vane. On account of flow, a vane is swung against a spring in a vane type instrument or piston/disc (valve) seat against the spring in a piston (valve) type flow monitor. When the vane (left side figure in Fig. X/ 7.4.0-1)/seat moves there will be variations in the

TABLE X/7.3.2-1 Brief Specification of DP Type Flow Switch SL

Specifying Point

Standard/Available Data

1

Fluid type

Liquid and gases/air

2

Orientation

Horizontal

3

Connection type

Screwed/flange

4

Connection size

BSP/NPT (M) based on meter size. Flanges 150/300 lb ANSI flange (wafer type as shown available)

5

Pressure limit

30 barg

6

Pressure drop

Depends on element around 0.7 Kg/cm2.*

User Spec.

Remarks

*Ref: manufacturer Continued

Special Flow Meters, Flow Gages, and Switches Chapter | X

971

TABLE X/7.3.2-1 Brief Specification of DP Type Flow Switchdcont’d SL

Specifying Point

User Spec.

Standard/Available Data

Remarks



7

Temperature

30 to 180 C

8

Ambient temperature

0e50 C

9

Pipe sizes

15e200 mm

10

Flow range

Liquid: 8e12 KL/min Gas: 15e2000 Nm3/h

11

Set adjustability

Adjustable max. two set points settable

12

Body

Bronze, stainless steel, also Monel possible

13

Housing

Aluminum/SS

14

Sensor materials

Bronze, stainless steel, Ionel/Inconel

15

Electrical housing

IP65/IP66

16

Output

Potential free dry contact

17

Contact configuration

One or two SPDT

18

Contact rating

Refer to Subsection 7.1.0.5 for explanation. Available in: AC/DC: V(max) 230 V Current: 1 A Max Watt: 10 W

19

Repeatability

1% AR average

20

Hazardous application

Necessary certificate from authorized agencies

Data given here are based on reputed manufacturer and maximum possible conditions have been considered so, may not match any particular model.

flow passage, i.e., variation in orifice size proportion to the flow rate of the fluid. The vane is mechanically linked to a pointer to indicate the flow rate on a scale. This linkage to the indicator pointer is also used to actuate the switching unit as shown in Fig. X/7.4.0-1. During no-flow condition, the vane closes the flow path due to spring action. There could be another piston (valve) type design shown in the RHS of Fig. X/7.4.0-1, here the piston rests on the seat. When flow starts, the kinetic energy of the fluid overcomes the spring force to admit and establish fluid flow. In the case of a vane type design, the vane is forced to swing, creating an opening for fluid flow. Similarly, in the piston (valve) type design shown in on the RHS of

Fig. X/7.4.0-1, trim is forced against the spring, allowing flow to take place. In instruments, switches are available with magnetic coupling, e.g., the disc type flow switch of Magnetrol. In both cases, on account of the design, the flow admitted is proportional to the orifice opening. The movement of vane or piston arrangement is linked with a flow indicator and switching unit. When flow increases the linkage compresses the switching unit, so that when flow exceeds the desired set point the switch actuates. Similarly, when the flow decreases, the switch is deactuated. For vertical upward flow, a Rotameter with contact can be used for flow switching also. This is also a variable orifice flow switch used in industrial applications.

972

Plant Flow Measurement and Control Handbook

INDICATOR

SPRING OUT

IN

SWITCHING UNIT

(ACTUATED)

VANE TO CREATE VARIAB;LE ORIFICE

FIGURE X/7.4.0-1 Vane type flow switch.

7.4.2 SPECIFICATION OF VARIABLE ORIFICE FLOW SWITCHES

7.5.0 Thermal Dispersion Type Flow Switch (Monitor)

The specification of a variable orifice flow monitor has been enumerated in Table X/7.4.2-1.

Solid-state designs of thermal dispersion flow switches are quite popular in industries. This is a

TABLE X/7.4.2-1 Specification of Variable Orifice Flow Switch SL

Specifying Point

Standard/Available Data

1

Fluid type

Liquid and gases/air

2

Orientation

Horizontal/vertical

3

Connection Type

Screwed/flange

4

Connection size

BSP/NPT (M) based on meter size. Flanges 150/300 lb ANSI flange

5

Pressure limit

Around 25 barg

6

Pressure drop

Type dependent around 0.2 barg*

7

Temperature

18 to around 100 C

User Spec.

Remarks

*Refer to manufacturer

Continued

Special Flow Meters, Flow Gages, and Switches Chapter | X

973

TABLE X/7.4.2-1 Specification of Variable Orifice Flow Switchdcont’d SL

Specifying Point

Standard/Available Data

User Spec.

Remarks



8

Ambient temperature

0e50 C

9

Pipe sizes

15e100 mm

10

Flow range

3e350 Lpm

11

Set adjustability

Adjustable set point settable

12

Wetted parts

Stainless steel

13

Seal

PTFE

14

Electrical Housing

IP65

15

Output

Potential free dry contact

16

Contact configuration

One or two SPDT

17

Contact rating

Refer to Subsection 7.1.0.5 for explanation. Available in: AC/(DC): V(max) 110/230 V Current (max): 10 A Max watt: 120 W

18

Accuracy

3% FSD for indication

19

Repeatability

1% AR Average

20

Hazardous application

Necessary certificate from authorized agencies

mass flow detection type monitor and is available in different designs. Since they measure mass they are immune to changes in viscosity and density. These can be used in slurry and fluid with particle applications. It is very sensitive and capable of measuring small changes in flow. 7.5.1 THEORETICAL BACKGROUND OF THERMAL DISPERSION FLOW MONITORS Thermal dispersion type flow switches sense the stoppage or movement of the process stream, i.e., the mass flow of fluid by detecting the temperature change, i.e., the cooling effect of the sensing probes as detailed, with the theoretical background of thermal dispersion type flow metering,

in Section 4.0.2 of Chapter VI. These are available as a probe or a flow through device (with transmitter). So, basically the system should consists of one heating element, one reference sensor, and another sensing probe, as shown in leftmost figure of Fig. X/7.5.0-1. As has been noted in Chapter VI, this is a mass flow detection, hence it is independent of density and is able to detect very small changes. On the other hand, as this is thermal sensing, like any other thermal/temperature sensing the method has a higher time constant, therefore instantaneous changes may not be detected properly. Based on the fluid type, i.e., heat transfer capability and device adjustment, the response can be in seconds or minutes [15].

974

Plant Flow Measurement and Control Handbook

ACTIVE SENSOR HEATER REFERENCE SENSOR

SENSING ELEMENT

SENSING ELEMENTS

FIGURE X/7.5.0-1 Thermal type flow switch (monitor).

7.5.2 DESCRIPTION OF THERMAL TYPE FLOW SWITCHES As stated above, a thermal flow switch/monitor utilizes thermal dispersion principles, i.e., cooling effect of flowing fluid is used to measure the mass flow rate of fluid (liquid/gas). The sensor consists of two RTD (or transistors) sensors and a heating element. One sensor is located at the sensor tip very close to the flowing medium, while the other one is a reference sensor meant to sense the ambient condition of the fluid. In order to create the temperature difference one of the sensors is heated through a heater by applying power. The differential is greatest when no liquid (dry condition) is present when there is fluid (i.e., liquid) on account of flow and quenching action, the

temperature differential would decrease. When flow is in the conduit, the heated sensor will be cooled more. The cooling effect is a function of how fast heat is conducted through the flowing fluid. Therefore, there will be a difference of temperature between the two sensors. When the velocity of fluid flow is slow, then the temperature difference would be greater, while for faster velocity the difference will be less. This is because temperature sensing has a higher time constant. From this it is clear that the differential temperature between the two sensors is a function of the fluid velocity and hence flow. Normally these types of instruments are available in the form of monitoring instruments, i.e., for continuous measurement and switching is just a part of it. Some of

Special Flow Meters, Flow Gages, and Switches Chapter | X

these instruments are provided with built-in LED indicators to indicate normal and abnormal conditions. Many of these instruments are often used for two fluid (hydrocarbon and water) interface detection, and level detection also, e.g., the T21 thermal differential instrument of SOR or the TS thermal dispersion switch. In continuous casting machines these types of flow switches are often used, usually for granular solids. 7.5.3 SPECIFICATION OF THERMAL FLOW SWITCHES The specification of a thermal flow switch/ monitor has been enumerated in Table X/7.5.3-1. The values/data provided here are from reputed manufacturers. The maximum possible data have been indicated as far as possible; naturally all may not be available with a single device.

975

7.6.0 Discussions on Miscellaneous Flow Switches There are a few other types of flow switches, such as ultrasonic flow monitoring based on transit time and Doppler type, as already covered in Chapter V. There are a few other types also, these are capacitance type flow/no-flow switches and bypass type switches. 1. Ultrasonic flow monitor: Both transit type and Doppler effect flow monitors are used to monitor flow. This is very useful in retrofitting applications as these types of flow monitors can be mounted external to the pipe and no shutdown is needed. Basically these are flow meters with variations as described in Section 6.0.0 of Chapter V. In view of this these are not repeated again here. For switching action necessary contact outputs are generated at

TABLE X/7.5.3-1 Specification of Thermal Flow Monitor SL

Specifying Point

Standard/Available Data

1

Fluid type

Liquid and gases/air

2

Mounting and insertion length

Mounted from the top, in the conduit with variable insertion lengths

3

Sensor length

From 40 up to 3000 mm

4

Connection type

Screwed male or female (Tee type fitting) or with tri-clamp sanitary fitting

5

Connection size

½00 , ¾00 to 20 NPT/G/BSP threads

6

Pressure limit

Vacuum to 300 barg

7

Temperature range

70 to 200 C

8

Ambient condition

40 to 60 C with high humidity (>95%)

9

Pipe sizes

15e500 mm

10

Flow range

3e100 Lpm

11

Set adjustability

Liquid: 3 mm/s to 2 m/s Gas: 0.1e150 m/s

12

Body and wetted parts

Stainless steel 316

13

Housing

Aluminum powder coated

14

Electrical housing

IP65/66

15

Output

Potential free dry contact

User Spec.

Remarks

Continued

976

Plant Flow Measurement and Control Handbook

TABLE X/7.5.3-1 Specification of Thermal Flow Monitordcont’d User Spec.

SL

Specifying Point

Standard/Available Data

16

Contact configuration

One or two DPDTs with relay

17

Contact rating

Refer to Subsection 7.1.0.5 for explanation. Available in: Direct: 30 VAC at 50 mA and 42 VDC at 65 mA. With relay: 8 A at 250 VDC

18

Accuracy

3% FSD for indication; set point about 10%

19

Repeatability

1% AR Average

20

Differential adjustment

15%e20%

21

Power supply

24 VDC/AC or 240 VAC

22

Hazardous application

Necessary certificate from authorized agencies are possible

the desired set point from the secondary electronics. 2. Capacitance type flow switch: These are mainly used for liquid flow sensing. These are flow/no-flow sensing switches. This capacitance switch type is fitted between two flanges in standard pipe sizes between 50 and 200 mm [15]. It is known that air has a dielectric constant of 1, therefore when there is liquid flow, the dielectric constant of the flowing fluid will be greater than 1. Therefore, when there is no flow (empty), the dielectric will be low radio frequency (RF) current. When flow of liquid is established, the dielectric will be higher, giving rise to higher RF current. In secondary electronics the higher current will be detected to generate contact output. These contacts can be used for protection of the pump. An adjustable time delay of 0e20 s is provided to protect from premature shutdowns [15]. 3. Bypass type flow switch: There is another kind of flow switching popularly known as

Remarks

the bypass type flow switch. In the main line there will be a differential pressure-producing vane. On account of flow in the main line a proportionate part of the flow will be diverted to the bypass line where there will be a magnet. On account of flow in the bypass line the magnet will be deflected and the position of the magnet and Reed switch can be externally adjusted. Therefore, based on the deflection of the magnet the Reed switch will be operated. With this, the discussions on fluid flow switches come to an end and we now investigate flow switches for solid and bulk materials. 7.7.0 Discussions on Solid (Bulk) Flow Monitors In solids handling systems, it is necessary to quickly detect any abnormal conditions such as blockage/feed loss, leakage of bag filter, etc. These are extremely important for cement plant fly

Special Flow Meters, Flow Gages, and Switches Chapter | X

ash handling in power plants and alumina plants, etc. Flow monitor utilizing microwave and electric charges is often used for not only detection of flow/no-flow conditions but also for continuous flow monitoring. Electric charge type devices, such as the “Triboflow,” collect, on their probe surface, the static charges of the solid particles passing over their surface. The resulting current is related to the flow rate of solids [15]. Microwave type flow measurement for solid flow has been discussed in detail in Section 5.0.0 of Chapter VIII. The discussion starts with microwave type solid flow monitors. 7.7.1 MICROWAVE TYPE SOLID FLOW MONITORS

2.

3.

By now, the reader will be well aware of the microwave type flow monitor discussed in Section 5.0.0 of Chapter VIII. However, discussions now start with a short recap of the measuring principles. 1. Principle of operation discussions: Microwave-type solid flow measurement is used mainly for granular solid materials and dusts. Microwave-type solid flow measurement utilizes the Doppler effect for monitoring of solid flow. These instruments consist of a transmitter and receiver. The transmitter emits a microwave signal around 24 GHz towards the flowing solids. The signal is reflected by the flowing solid particles. This reflected signal is received by the receiver. On analyzing the reflected frequency (Doppler effect), the velocity of the flowing solid particles is determined. On account of the material motion there will be a change in the frequency of the reflected signal. Thus when the material is in motion, the returned signal will have a frequency different from the emitted signal. On the other hand, when there is no motion, the returned signal

4.

5.

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will be the same frequency as the emitted signal. Normally a microwave type instrument is not affected by material build up. As stated earlier, since these are flow monitors (i.e., capable of continuous measurement) they offer adjustable set points. It is possible to detect a wide range of flow velocities of solids. A typical flow monitor with application details is depicted in Fig. X/7.7.1-1. Penetration: With very little attenuation microwaves can penetrate nonconductive materials, e.g., plastic, glass, and wood. Nonconducting material build up on the wall has hardly any effect. Table X/7.7.1-1 gives the wall thickness values for various materials [17]. Functional application area: As shown in Fig. X/7.7.1-1, some of the functional application areas include but are not limited to: l Mechanical conveyor; l Silo discharge; l Feeders; l Dryers; l Mixing unit; l Grinding. Industry application areas: Industry-wise application includes but is not limited to the following: l Cement; l Gypsum; l Woodchip; l Fertilizer; l Powder; l Food items/products; l Animal feed; l Coffee and granular materials. Features: The features of microwave solid flow switches include but are not limited to the following: l Highly reliable solid mass flow measurement; l Noninvasive measurement; l External mounting hence easy for retrofitting;

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Plant Flow Measurement and Control Handbook

INSTRUMENT

APPLICATION AREAS

DRYING

SCREENING

GRINDING

MIXING

FIGURE X/7.7.1-1 Microwave solid flow switch. Developed based on Measuring Systems for Solids, Mutec instruments (Catalog). http://www.muetec-instruments.de/wp-content/uploads/2016/04/LC510M-brochure.pdf. Courtesy: Mutec instruments; Developed based on Thermo Scientific Granuflow GTR 130 Flow/No-Flow Detector; Technical catalogdproduct Specification, Thermoscientific; Thermo Fisher. https://assets. thermofisher.com/TFS-Assets/CAD/Specification-Sheets/D10578w.pdf. Courtesy: Thermo Fisher.

Special Flow Meters, Flow Gages, and Switches Chapter | X

TABLE X/7.7.1-1 Microwave Signal Penetration Wall Materials

Wall Thickness (mm)

Glass

25e50

Wood (dry chip board)

13e25

Plastic (PVC PE PTFE)

<100

Thermo Scientific Granuflow GTR 130 Flow/No-Flow Detector; Technical catalogdproduct Specification, Thermoscientific; Thermo Fisher. https://assets.thermofisher.com/TFSAssets/CAD/Specification-Sheets/D10578w.pdf. Courtesy: Thermofisher.

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Suitable for all bulk material granular material; l Adjustable sensitivity, damping, hysteresis, and filter time [18]; l Very easy installation. 6. Specification: A brief specification of a microwave solid flow monitor has been enumerated in Table X/7.1.1-2. The technical data enlisted here are from reputed manufacturer and the best possible data have been presented, so naturally these may not be available in single instruments. l

TABLE X/7.1.1-2 Specification of Microwave Solid Flow Monitor SL

Specifying Point

Standard/Available Data

1

Material type

Bulk solid/granular material

2

Mounting

Mounted externally in any orientation as shown

3

Connection type and size

Screwed 100 or 1 ½00

4

Sensor surface

Teflon

5

Pressure limit

25 barg

6

Temperature range

20 to 80 C

7

Ambient condition

20 to 60 C

8

Operating frequency

w24 GHz

9

Set and dead band

Yes Adjustable

10

Body and wetted parts

Stainless steel 316

11

Electrical Housing

IP65/66

12

Power supply

30 VDC/AC

13

Detection range

0e1.8 m

14

Output

Potential free dry contact

15

Contact configuration

One or two SPDT with relay

16

Contact rating

Refer to Subsection 7.1.0.5 for explanation. Available in: 250 Vmax, 4 Amax 500 VA (Max)

17

Damping

0e10 s

18

Repeatability

1% AR average

19

Indication

LED

20

Hazardous application

Necessary certificates from authorized agencies are possible

User Spec.

Remarks

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Plant Flow Measurement and Control Handbook

7.7.2 ELECTRIC CHARGE TYPE FLOW MONITORS An electric charge-based flow switch is known as a “Triboflow.” When solid particles pass over the probe surface static electric charges are collected on the surface of the probe. These charges give rise to current, which is related to the flow rate of solids. These probes are sensitive to small changes in solid flow. A typical electrical charge flow monitor is shown in Fig. X/7.7.2-1. This type of flow monitor can be used for continuous bulk solid flow metering, as well as as a flow switch. This is a ring sensor, the measurements are taken integrally and without contact over the pipe cross-section [18]. As stated earlier, on account of the flow of solid particles, charges are developed and these electrically charged particles produce a charge signal against the grounded conveyor duct. On the basis of statistical fluctuations in the particle flow, a current noise is produced which depends on the solids concentration and also on the solids velocity [18]. Stationary particles, such as sediments, do not contribute to the results. A brief specification has been enumerated in Table X/7.7.2-1. On account of a few limitations of this kind of measurement and nonapplicability for all kinds of materials as well as installation issues, continuous

FIGURE X/7.7.2-1 Electric charge type flow switch.

TABLE X/7.7.2-1 Specification for Electric Charge Type Solid Flow Monitor SL

Specifying Point

Standard/Available Data

1

Material type

Bulk solid/granular material

2

Mounting

Between flanges or in flanged pipe

3

Connection type and size

Flange DIN/ANSI or screwed version also available

4

Sensor surface

Teflon

5

Pressure limit

40 barg

6

Temperature range

20 to 90 C

7

Ambient condition

20 to 70 C

User Spec.

Remarks

Continued

Special Flow Meters, Flow Gages, and Switches Chapter | X

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TABLE X/7.7.2-1 Specification for Electric Charge Type Solid Flow Monitordcont’d SL

Specifying Point

Standard/Available Data

8

Set and dead band

Yes adjustable

9

Body and wetted parts

Stainless steel 316

10

Electrical housing

IP65/66

11

Power supply

17e30 VDC

12

Build up effect

Generally no effect

13

Damping

0e10 s

14

Output

Potential free dry contact continuous measurement possible

15

Contact configuration

One or two SPDTs with relay

16

Contact rating

Refer to Subsection 7.1.0.5 for explanation. Available in: 50 Vmax, 1 Amax 500 VA (Max)

17

Current consumption

Nominal

18

Repeatability

1% AR average

19

Indication

LED

20

Hazardous application

Necessary certificates from authorized agencies are possible

measurement of solid flow with this type of flow metering is not very popular in solid flow measurement. Flow conditioning computation and controls are important aspects of flow measurement. We now conclude the discussions on special

User Spec.

Remarks

instrumentation chapter with a special call out below (Fig. X/7.7.2-2). In the next chapter let us investigate other important aspects on flow measurement to cover the discussions on flow conditioning, computation and controls in next chapter.

Do you know there are disposable flow sensors? Yes, there are low-cost baery-operated disposable sensors cum monitors for biomedical applicaons such as advanced infusion therapy, drug delivery devices, urine catheters, surgical instruments and bioreactors. These flow sensors are reversible type.

FIGURE X/7.7.2-2 Disposable flow sensor.

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Plant Flow Measurement and Control Handbook

LIST OF ABBREVIATIONS ABS Absolute AC Alternating current ADC Analog to digital converter AI Analog input AO Analog output AR Actual reading (in connection with accuracy) CCW(/CW) Counterclockwise (/clockwise) CMRR Common mode rejection ratio CMV Common mode voltage COC Change over contact CS Carbon steel DAS Data acquisition system DC Direct current DCS Digital control system DI Ductile iron/digital input DO Digital output DP Differential pressure DPDT Double-pole double-throw DPT Differential pressure transmitter/transducer DSP Digital signal processing EMC Electromagnetic compatibility EMI Electromagnetic interference FAT Factory acceptance test FC Fail to close (for valve) FO Fail to open (for valve) FSD Full-scale division (in connection with accuracy) HVAC Heating ventilation and air conditioning IC Integrated chip/internal combustion (engine) ID Internal diameter I/O Input/output IS Intrinsic safety LCD Liquid crystal display

LDA Laser Doppler anemometry LDV Laser Doppler velocimetry LED Light-emitting diode LHS Left-hand side LVM Limit value monitor MS Mild steel (main steam) MUX Multiplexer MVT Multivariable transmitter NB Nominal bore NIST National Institute of Standards and Technology OD Outer diameter PLC Programmable logic controller PTFE Polytetrafluoroethylene PU Processing unit PVC Polyvinyl chloride PVT Pressure volume temperature RF Raised face/radio frequency RHS Right-hand side RPM Revolutions per minute RTD Resistance temperature detector SIL Safety integrity level SPDT Single-pole double-throw SPST Single-pole single-throw SS Stainless steel STP Standard temperature and pressure (Fig. I/1.1.2-3) T/C Thermocouple US Ultrasonic/United States VDU Visual display unit VFD Variable-frequency drive VM Valve manifold W/O Without/water in oil (emulsion) WRT With respect to

Special Flow Meters, Flow Gages, and Switches Chapter | X

REFERENCES [1] Hall Effect Sensing and Application, Honeywell (Technical internet document). https://sensing.honeywell.com/hall book.pdf. [2] Model 60 Meter-Master Flow Sensor Operating Instructions, F.S. Brainard & Company, June 2011. Rev 3.1, http://www. meter-master.com/library/pdf_library/MM60_(11.6).pdf. [3] K. Nugent, The Value of an Integrated Cryogenic Metering Solution, CryoGas International, July 2013 (Internet document), http://www.turbinesincorporated.com/images/stories/ TIdownloads/0713_value%20of%20metering%20solution_ nugent.pdf. [4] R.H. Ashmore, Two-Phase Cryogenic Flow Meter: A Proof of Concept, Florida State University; College of Engineering, 2006 (Internet document), http://diginole.lib.fsu.edu/ islandora/object/fsu:168365/datastream/PDF/view. [5] T. de Jonge, T. Patten, A. Rivetti, L. Serio, Development of a Mass Flow Meter Based on the Coriolis Acceleration for Liquid, Supercritical and Super Fluid Helium (Internet document). http://citeseerx.ist.psu.edu/viewdoc/download? doi¼10.1.1.459.3318&rep¼rep1&type¼pdf. [6] K. Peter, Advances in Cryogenic Engineering, vol. 39, Spinger, June 1994. [7] Cryogenic Turbine Flow Meter, Turbines Inc. (Data sheet; TMC series; Catalog). http://www.turbinesincorporated. com/images/stories/TIdownloads/TI_datasheet_Cryogenic_ Turbine_Flow_Meter.pdf. [8] Metering Cryogenic Fluids Using the RotaMASS Coriolis Flow Meter, March 2004. APPLICATION NOTE; Yokogawa AN 01R4B4-E-A, https://web-material3.yokogawa. com/AN01R4B4-E-A.us.pdf. [9] S. Basu, A. Kumar, Debnath, Power Plant Instrumentation and Control Handbook, Elsevier, November 2014. http:// store.elsevier.com/Power-Plant-Instrumentation-andControl-Handbook/Swapan-Basu/isbn-9780128011737/. [10] RHODES Sight Glasses and Flow Indicators, Delta fluid Product Limited (Technical Catalog). http://www.rhodesflow. com/resources/db/Rhodes%20Full%20Catalogue%202008. pdf. [11] Midwest Sight Flow Indicators, Dwyer Instruments, Inc. (Technical Catalog). http://www.dwyer-inst.com/PDF_files/ SFI_100_300_300F_400_700.i.pdf. [12] Digital Flow Meter Selection Guide, Assured Automation (Technical catalog). https://assuredautomation.com/digitalflow-meters/. [13] Volumetric Cold Portable Water Meter (GKM KSM), George Kent (Technical catalog). http://www.georgekent.net/wpcontent/uploads/2017/05/GKM-KSM-V1-250517.pdf. [14] Elster Kent Helix H4000 Bulk Water Meter Range, Elster Kent metering Pvt Ltd; George Kent (Technical catalog).

[15]

[16]

[17]

[18]

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http://www.incledon.co.za/catalogues/20%20Watermeters/ 3b%20Elster%20Kent%20H4000%20Bulk%20Water% 20Meter%20Brochure.pdf. Instrument engineers’ handbook, vol. 1: in: Process Measurement and Analysis,; vol. 1, CRC Press (Chapter 2 Flow measurement). Applying Precision Switches; Micro Switch General Technical Bulletien No.14, Honeywell. https://sensing.honeywell. com/honeywell-sensing-basic-switches-general-technicalbulletin-001017-2-en2.pdf. Thermo Scientific Granuflow GTR 130 Flow/No-Flow Detector; Technical catalog—product Specification, Thermoscientific; Thermo Fisher. https://assets.thermofisher.com/ TFS-Assets/CAD/Specification-Sheets/D10578w.pdf. Measuring Systems for Solids, Mutec instruments (Catalog). http://www.muetec-instruments.de/wp-content/uploads/ 2016/04/LC510M-brochure.pdf.

FURTHER READING [1] M.A. Crabtree, Industrial Flow Measurement, The University of Huddersfield, June 2009. http://eprints.hud.ac.uk/5098/1/ macrabtreefinalthesis.pdf&sa¼u&ei¼v66ttp_ccojmialag7mnb q&ved¼0cdiqfjat&usg¼afqjcngao5vc1jsrrbjucjvkxotjjoah6q. [2] F. Frenzel, H. Grothey, C. Habersetzer, M. Hiatt, W. Hogrefe, M. Kirchner, G. Lütkepohl, W. Marchewka, U. Mecke, M. Ohm, F. Otto, K.-H. Rackebrandt, D. Sievert, A. Thöne, H.-J. Wegener, F. Buhl, C. Koch, Deppe, E. Horlebein, A. Schüssler, U. Pohl, B. Jung, H. Lawrence, F. Lohrengel, G. Rasche, S. Pagano, A. Kaiser, T. Mutongo, Industrial Flow Measurement Basics and Practice, ABB Automation Products GmbH. http:// nfogm.no/wp-content/uploads/2015/04/Industrial-Flow-Measu rement_Basics-and-Practice.pdf. [3] S. Basu, Plant Hazard Analysis and Safety Instrumentation Systems, Elsevier; IChemE, 2016. http://store.elsevier.com/ Plant-Hazard-Analysis-and-Safety-Instrumentation-Systems/ Swapan-Basu/isbn-9780128037638/. https://icheme.myshop ify.com/products/plant-hazard-analysis-and-safety-instrumen tation-systems-1st-edition. [4] R.C. Baker, Flow Measurement Handbook, second ed., Cambridge University Press. [5] K. Dunphy, T. Patten, T. O’Banion, Cryogenic Services with Micro Motion Flow Meter, Micro motion white paper. http:// www2.emersonprocess.com/siteadmincenter/PM%20Micro% 20Motion%20Documents/Best-Practices-Cryogenic-WP00515.pdf. [6] FLW Series Water Monitor, Omega Engineering eAn Omega Technologies company, November 2006 (Catalog; OMEGA Manual; M-0373), http://www.omega.com/green/pdf/FL-X. pdf.