A Computer-Based Automatic Flowmeter Calibration RIG

Copyright © IFAC Applied Measurements in Mineral and Metallurgical Processing, Transvaal, S. Africa 1988

A COMPUTER-BASED AUTOMATIC FLOWMETER CALIBRATION RIG B. E. White Rustenburg Section, Rustenburg Platinum Mines, p.a. Box 1, Bleskop 0292, South Africa

Abstract This paper' describes a "budget" computer-based, automatic rig for the testing and calibration of in-line Magnetic Flowmeters. The rig was designed and engineered at Rustenburg Platinum Mines out of sheer necessity for an inexpensive but accurate calibration facility where the turn-around time for ea ch test was not more than two days. In addition, the rig was required to process and analyse data and produce automatic calibration certificates. Finally, the rig also had to eliminate human error as far as possible and be relatively simple in operation with a "user-friendly" personality.

Keywords: Computer control; computer software; data handling; automatic testing; self-diagnostic programmes; accuracy statement; analog - to -di gital conversion; interfacing u ni t; flowmeter.

BACKGROUND TO THE PROBLEM

Flow

accounting

purposes Or

a

for

process

is

Metallurgical for efficient

Two Main Reasons For Regular Cal ibrat ion

This paper describes the design and operating characteristics of a "budget" computer-based flowmeter test and calibration rig, planned, engineered and installed at Rustenburg Platinum Mines - Rustenburg Section, Western Trans v aal, South Africa.

At

Rustenburg

Section,

the

two

main

requiring MagFlow calibration s on a regular basis are reasons

fOl-

a. Metallurgi cal accounting. Mass balance calculations rely hea vily on mass-flow measurement (Density X Flow)

OBJECTIVES

and

of the

in

either

control. It follows that the c alibratio n must naturally, be as accurate as possible.

INTRODUCTION

The objectives were as follows

measurement

essent i aI,

automatic

often

when

serious

dicrepencies

appear, the instrumentation becomes suspect. Density checks against pipe samples can be v erified fairly quickly, but flow is usually the questionable factor. Proof o f accurate operation is the only re al pacifier.

rig

a. To test and calibrate plant Flowmeters accurately at different flow velocities.

b~

relevant record all To particular information concerning the Flowmeter under test.

Slurries

cause

severe

wea r

on

MagFlow linings. The first indication of erratic behaviour or complete failure is usually the late evidence of this fact. An early warning system to combat this problem can sav e expensIve shutdowns and production losses.

b.

To incorporate built-in c. checks for determining the diagnostic calibration accuracy limits of the rig itself.

There d. To issue calibration certificates automatically and analyse statistical/historical data at the "touch of a button".

are

Flowmeters

man y in

different ser v ice,

each

types

of

selected

for its own individual advantages. Table I shows a typical selection guide to flow measurement.

45

46

B. E. White In mini 'ng applications, where abrasive slurries are common, th e mo st suitable type of flowmeter is the Electromagnetic type, which has the distinct ad v antage of having no mo v ing parts, no flow obstructions, no head loss and possessing high accura cy and repeatabilit y . The only operational requirement being that the fluid must be conductive, which is the case with mine slurrie s .

ADVANTAGES OF AN "IN-HOUSE CALIBRATION RIG Aside from the fact that all the disad v antages mentioned above are overcome, additional ad v antages are a. Record keep ing becomes a relati v el y simple and manageable task. b. Staff are activel y involved in the process of scientifi c calibration.

This paper will deal only with the Electra-magnetic t y pe fl o wmet er , although the principles ou tli n ed co uld easil y be adapted for oth er ty pe s as well.

c. History tre n ds and analysis of plant MagFlows can contri bute to import ant in f orm at ion b eing obtained about the p roc ess itself.

Th e main probl em with c alibrati on of these "Mag Fl ows ", (as th e y a re known in th e trade, a l though this is d

d. Fault-finding is simplified and the rig is particularly use ful on stubborn i ntermitt e nt faults.

misnomer

it

because

describes

the

working prin c iple r a ther than a "v elocit y flowmeter"), is that short of opening the pipe somewhere do wnstream and filling a known-capacity container

of

time,

the

basically

in

"x"

volumetric flow

an

unknown

quantifiable physical

amount

rate

parameter

e . apprent i ces

Hands - on and

training for newly-quali fie d

I nstrunl ent

Mech a nicians

enhanced with practical

is

greatly

demonstration.

is in

ACCURACY STA TEMENT

terms.

The only real practic a l solution is to remove the Magflow from the pipe, replace it with a "spool pie ce" so that production is least affected, and transport the "to-be - tested" MagFlow to a calibration rig for proper scientific calibration, usually at a ve ndor's premises .

In order to understand the accu racy of a flowmete ring system , it i s important that the statement of accuracy should specify percent of rate or percent of ma>
fl ow .

making

up

rated

fdct ,

system

ac cording

to

the

components

should also

the same

be

accu r acy

statement. shows a graphical comparison numerlca ll y equal (IX)

Figure I between

DISADVANTAGES OF USING A VENDOR'S CALI BR AT I ON RIG

In

the

accuracy

statements.

Percent of Maximum Fl ow A

The most serio us problems encountered in using a v endor's calibr a tio11 rig ,

are

b .

Turn- aroun d

c.

Cost .

minute),

has an

At full flow minute) At half f low At 20 llm

tim~ .

Transporting a 250mm or 300mm MagFlow can be difficult . These MagFlows weigh between 60 and 100kgs a nd are to

intrinsic

d a mag~

if

mis ha ndled. the time a MagFlow on the plant until

leaves its it i s again

on -line, can sometimes be l ong as 3 months, in a mining environment , due to stores , administration and transport constraints.

Cos t of transportation and skilled l abou r are already very expensive, not to mention tha t, understandably, the ve ndor

scale capacity pel-

Transportation .

From site

is +-I'l. of has a full Ilm (litres of say 100

whose accura cy flow a nd which

uncertainty

range

as fol lows

a.

prone

meter

ma ~i mum

has

to

hope fu lly make calibr ati on .

recover

a

his costs

p rofi t

on

and

ea ch

The amount l/m

=

99-101

I l m

49 - 5 1

l/m .

19-21

l /m.

(li tres per

of uncertainty is

in e very case

and

thus +-1

is const ant

over

the entire range of flow. This means that the uncertainty is a greater percent of the actual flow at ~ flows than at higher fl ow rate s . This is why percent of maximum flow is usually suit able for primary meters such as linear fl ow recorders etc.

rotameters ,

Percent of Rate The Magflow fal l s lnto this cat eg ory and is defined a s Percent of rate accurac y states that throug h out a glven rang e, the uncertainty of flow in llm ( litres pe r minute ) decreases as the flow rate decreases.

47

Automatic Flowmeter Calibration Rig For example, a +-1% of rate allow uncertainty as follows At full flow (100 l/m) At half flow At 20 l /m

would

99-101 I/m 49.5-50.5 l/m 19.8-20.2 l/m

The amount of uncertainty is thus decreased as the flow is decreased. An important fact now emerges, if a proper scientific calibration facility is installed, then flow control over the entire range is essential while the "test" Magflow is being calibrated. Also, another fact becomes clear there must be a minimum "cut-off" flow rate, below which the induced voltage pick-up is so small and erratic as to become unreliable and unrepeatable. In fact, during initial calibration of the rig itself, much time was consumed in establishing individual optimum oper ating range characterisitics for each MagF low pipe -s ize. Table 2 shows the tabu la tion f or 1% of maximum flow and the correspo nding rate percentages

for

a

given example .

BASIC COMPONENTS OF THE COMPUTER-BASED CALI BRATION RIG The

Rig

distinct

be di vid ed

could

sections ,

a. The piping , va riabl e speed motor . The b. interfacing .

int o

pump

tank,

and

electrical/electronic

c.

The

d.

The co mputer and software.

PIPING,

fou r

n am ely

the

inter Tanks 1 and 2 were later connected to act as a buffer and I imi t turbulence in the pipes.

Water is 8 X 6 (8 pump

pumped f rom the tanks vi a a n inch inlet and 6 inch outlet)

which

is

driven

by

a

var iable

speed motor which In turn is controlled through a Thyristor Drive Unit. Depending on the pos itions of the valve r in the inlet and outlet manifolds,

water

is

pre-sel ected size of the tanks in

pumped

through

connec to rs.

RIG INTERFACE TO COM PUTER handled All electronic signals are PCI-3000 through a Burr Brown Interface unit. Communication between is via the Burr Brown and the IBM PC an RS232/422 serial link. Pump speed ramping is controlled by the computer through this PCI-3000 sy stem in normal 4-20 mA signals. Likewise, the outputs from both the 11test"

and

"master"

gauges

received by the PCI-3000 and to the IBM PC in serial format.

are

passed

The Burr Brown handles analog and digIta l inputs/outputs. Valve micro-switch es deliver positional information to the computer, via the interface unit . on/off level detector switch A simple on the tan~ similiarly reports when the tank is heading for overfill. This eliminates excessive spillage.

THE

PUMP AND VARIABLE SPEED MOTOR

Figure 2 shows the layout of piping configuration of the Rig.

Electrical /e lectronic hook-up between the gauges and their electronic instrumentation is made via quick-snap

The Bur r- Brown is controlled by a dedicated ZB0 microprocessor chip and a block diagram of the system architecture is shown in Figure 3.

instrumentation.

TANK,

This "Master" gauge is an extremely accurate MagFlow costing twice the normal price of an equivalent size. It's components are miltary-specification rated and is microprocessor-based. It is reputedly rated at better than 0.01% accuracy.

Saftronics DC-6 full A stand-alone bridge converter was used to control the DC motor and pump. A 4-20mA Signal adjusts the speed of the motor when the unit is switched to automatic mode . TWTX VC signal converter was A Conlog used to conv ert the 0-10 volt signal from the Burr Brown unit to a 4-20mA signal for the Thyristor Drive Unit. The specification on this Conlog Unit claims better than 0,2% lin ear accuracy.

THE COMPUTER SOFTWARE

a

pi pe an d back to a ring main

configuration .

the manifolds, the "test" Between is placed in series with a MagFlow gauge MagFlow of the same "Master" diameter pipe size .

INSTRUMENTATION

The

PCI-3000

communicates with

interface the computer

RS-232 / 422 serial line. The here is a slight time delay.

unit via

dn

overhead

The computer language used was BASICA as it wa s found that this lent it itself admirably to the various aspects of port technical communication and engineering unit conversions etc. Although the simple filing routine written works v er y well,

48

B. E. White

it was oecided, rather than re-inventing the wheel, standard software packages would be used, such as LABVIEW and the fairl y recently announced LOTUS MEASURE package. This includes a set of software dri v ers for LOTUS 1-2-3 and SYMPHONY that allows data to be captured, formatted and stored dire ct l y into a spreadsheet.

Electromagnetic flowmeters. it was discovered, are able to produce accuracies as high as 0.2% with a 10:1 turn-down ratio in some cases and generally a repeatibility factor of 0.1% .

PROBLEMS ENCOUNTERED Figure 4 shows a software structure.

flow

ch art of

the

An interesting feature of this programme is the "Diagnostic or Fault-Finding" routine. This is a question/answer trouble-shooting aid for the Technician. Often, a MagFlow has been sent back to the suppliers as "faulty" when in real ity the caus e was only loose or wet connections or an elec tri cal leak between electrodes etc .

On the la rger pipe sizes. it was discovered that the accuracy of the MagFlow was detrimentally affected at high velocities. On investigation. it was found that this was due mostly to the design of the rig configuration which produced turbulence. Errors of g r eater than 6% were being measured. By leaving the 50mm and 100mm pipes open, it was found that the samples obtained by the 200mm and 250mm MagFlows were more consistent and the err or fell to an acceptable 1%.

THE MODUS OPERANDI OF THE F.C .R. The

logistics involved

the

operator~s

as seen through

eyes are

Test Remo ve from ser vice .

MagFlow

Head

(TMH)

2 Remove Test MagFlow Ele c tro nics (TME) from service. 3 Replace space with spool piece or another MagFlow if a v ailable. 4 Transport TM to F.C.R. site. 5 On ar riv al, mar k TMH and TM E with coded number . Inse rt TMH in series with 6 Master MagFlowmeter Head (MM H) in pipe system. 7 Temp orarily connect TME to TMH next to Master MagFlowmeter Electronics (MME) . 8 Run computer programme and follow screen

instructions .

9 Remove Calibration Certificate (CC) from printer . 10 Return TMH and TME site to together with CC. 11 Remove spool piece. 12 Install TMH. 13 Install TME . hand to 14 Commission and CC Metallurgi c al Dept.

with

the

Master,

then

At this stage, a decision was taken to have differe nt r a mping speed curves for the different pipe sizes . The rule being that the larger the pipe size. the lower the maximum speed. In practice, this philosophy worked well. The "Master" MagFlows have an error of 0.S F at 0.3 m/s flow velocity and this decreases to 0.2F abo v e 1 m/s flow v elocit y . ("F" is the error as a percentage o f the flow rate) . At first, only one sample from the "test" and "maste,'" MagFlows was taken at each different pump speed. It was found howe ver , that by taking the average of se v eral samples at each step,

If, during operation 8, the TMH has a gross of error of greater than 5% in co mparison

It was also found that a volu me of approximately 350 cubic metres per hour could be pumped through the 250mm MagFlow at max ixmum pump speed. However, at this speed, a vol ume of only 30 cubic metres per hour could be pumped through the 50mm MagFlow. This tends to indicate that a back pressure is built up and a pump speed is reached, above which the pump is unable to move a larger volume of fluid.

a

diagnostic flowmeter fault-finding rout,ne will automatically be summoned by the computer, which ass umes that the MagFlow under test is faulty or at the very least, suspect . Normal testing will be temporarily suspen de d until the TMH produces below 5% accuracy. If a real fault is detected, the computer will indicate this and terminate further testing and calibrating on that particular TMH. Final calibration must be better than 1%.

much

better

accuracy

was

obtained . The pump i ncre ases in speed from QI~. to 100% in 10 steps. I t takes the computer about 5 second s to obtain 20 readings from the PCI 3000 and t o a v erage them. Another problem encountered, was that the flow per c entage samples increased as the motor warmed up. The motor speed was controlled by an armature feedback signal. By installing a tachometer and using this signal in place of the armature feedback signal. this problem was solved. In addition, the repeatability of the tests improved from 2% to 1%. Trapped air in the system before a test commences is also a problem. By installing relief valves at critical points however, this problem too was overcome .

Automatic Flowmeter Calibration Rig Number of times MagFlows should be re-calibrated in anyone year twice

WATER VERSUS SLURRY Although the piping components of the F.C.R. are geared for slurry, i.e. rubber-lined pipes etc., the rig uses only water for calibration to limit wear

on

MagFlows build-up

the

expensive

IIMaster"

and eliminate sludge when the rig is not in use.

A test was conducted in the plant using a 150mm "Master" MagFlow from the rig and a previously calibrated Fo~boro "test" MagFlow. The units were mechanically coupled in series on a C.C.O (Crus her Classifier Overflow) pipe line whose pump is driven by a variable speed thyristor drive. The maln advantage of this particular pipeline was that water or slurry could be pumped on demand. The following parameters were recorded simultaneously a. b. c. d. e.

Variable speed voltage. Variable speed current. Density. "Master" MagFlow 4-20mA output. "Test" MagFlow 4-20mA output.

to slurry.

some slurries

USIng

water

as

40 X 2 cost of

80 months 40 X 500

6.6 years R20000.00

-*Allowing 3 vital at the same time cost of

MagFlows to be away 13 X 2 26 months 2,1 years 39 X 500 R19500.00

an

awesome

and

InCidently, it should be noted that in some cases, the ma~imum permissible time for the plant to run without the MagFlow input may not be more than one day at the most. In these cases, it would be necessary to consider purchasing additional MagFlows. The current cost of a 50 mm MagFlow with its associated electronics is +R10000.00 The turn around time on the Calibration Flow Rig is one day. Costs are hidden in the guise of normal salaries and no additional e~penditure is required.

a

higher

standard

though, will tend to average out this error between MagFlows spread out over the entire plant, especially, as is the case at Rustenburg, where the total number of MagFlows in servlce is 122.

COST SAVINGS The time and cost savings, in the case of Rustenburg Platinum Mines have been considerable, as the following calculations will show Number of R.P.M.- R

at a

that carry

noble metals will display a conductivity rate than water. In general,

R61000.00

Consolidating, and only sending away the Absolut.ly •••• nti.l units (accounting purposes), then

remains the same expensive venture

Howe v er, it should be noted that depending on the site location of the MagFlow, there will definately be a difference which will depend primarily on temperature and con ducti v ity. example,

At a cost of 122 X 500

Similiar figures can be juggled and manouvered, but the bottom line still

Using the recor ded charts of the data obtained during the test and back-calculating, it was established that apart f rom the obvious density effect on the pump speed when pumping slurry as opposed to water, the "test" MagFlow showed an average error of 0.417\1. for the same flow rate of

For

per MagFlow for including transport = R500.00 To send all the MagFlows away one at a time, will take 122 X 2 = 244 months = 20,3 years

-*-

Results

compdred

Average cost re-calibration,

at a

Periodic v erification of the density was established by manually sampling the water/slu rry into the Flotation Feed Sump.

water,

49

MagFlowmeters installed

on 122

Average turn-around time to remove MagFlow, package and send to vendor by road or rail and return = 2 months

ENHANCED EFFICIENCY Overall enhanced 1.

MagFlow

performance

is

for a number of reasons :Accurate "automatic" records

are kept of each and every MagFlow, so calibration due date can be timeously arranged. 2. Errors due to drift etc., are detected much earlier. 3. "Trouble" areas are identified, where MagFlows seem to operate less accurately than others. 4. History analysis over a long term begin to reveal certain inconsistencies with regard to plant operation. Metallurgical "Trouble-shooters" can then home in on ne~t

incongruous phenomena.

5. MagFlow reliability is more widely accepted within the Metallurgical department in particular, especially if they are present during a "physical" calibration.

50

B. E. White

As the MagFlow is the primary 6. element in Mass Flow data (Ma ss Flow = reading), the total S.G X MagFlow " tons /h our"

also

becomes more accurate

and r eliable. 7. Unnecessary repair eliminated . 8. Overall r edu ced co s t s . ADDITIONAL US ES OF The Calibration of v ery useful well, namel y

costs

THE CALIBRATION RIG

Flow Rig has future uses

a

number

as

1. By only including a mean s to add zinc- c hloride in pow der form to the ci rculation water, a potential s ys tem f or calibrating Densit y Gauges becomes ver y

viable.

2. Similiarly, a ddi ng a simple h ea ting devi ce would pave the way for c alibr a tin g or t es ting l ow - range thermo cou ples an d R .T. D 's (Resistance Tem p e rature De tectors) which are use d un d er

flo w co n d iti on environm e nts.

3. It could also be u se d as a "pi l ot plant " wh e n test i ng the "pu mpabi lit y" o f Cf' rt ain Metallu r gical new products . or feasiblity studies invol vi ng pump.ng and flow veloc it y me a surem e n t .

4. diffi cult MagFlows ,

Identifying int er mitt ent that

extre me ly fa ults on

sometime s

the

v e n dor

h i mself is unable to re c tify . CONCLUSION Th e

F. C . R

mainly

from

at

Rustenbu rg

wa s

built

s alv a g e d co mpon en t s , so wele minimal . The advent

i nitia l co s t s of IB M compat ibl es also prov ides e xtremel y e cono mical and yet r eliable co mputer powe,. exp e nsive Als o , the r e l ati v ely ess en tidl. Bu rr - Brown P C I -30 00 is not In fact, plug - in i nter f ace boar d s are now a va ilabl e that plug dir ec tly into th e IBM port b us-str u cture . thu s elimil'at i rlg tt,e associated de l ay

ser i a l t ime s.

interface

and

Considf'r lng the totdl return on the i nvestment , th e proje c t h as proved to be a very usefu l tool In en han ci ng quality co ntrol a nd pro vidin g increased aCCUraCIes o f in - lin e flow mea sur ements f or the Metallurgi c al Depar t men t at Rustenburg Platinum Mi n e s .

Automatic Flowm e ter Ca li bration Ri~ TA BLE 1

Type

Cori oli s

Dall Tube

Select ion Gui de to Flo w Measurement Instrumentati o n

Advantages

Disadvantages

Direct measure of Mass Flow. difficul t material.

Can handle

Expensive .

High head loss f or loop c o nfigura tion .

Higher cost than o rifice plate .

Lower pressure loss than Venturi . Lowe r cost than Ve nt uri .

Elect r o ,"agnetic

No mov i ng parts . Unobstructed bore . Suitable for slurr ies and abrasive liquids. No viscosity limits. Bi - directional.

Can be use d o n ly o n elec trica l co nduct i. \' e liquids . Po\\'er required at ::Ieasupeme nt

Orifice Plate

Simple. No moving parts. Wide variety & types .

High pressure loss . 'I : 1 F 1 (l\\' range. \' i s cos i ty effects 101-; r ange . r\Ccuracy effected \;y h ea l' , density a nd flo"" profU e .

Pit ot Tube

Can be used to determine flow profile . Low c ost .

Only for cl ea n f 1ui{;s .

Venturi

Use with fl uids that car ry solids . Low pressure loss . Relatively high accuracy .

Not suitable f o ), ab T' asl\'cs . Ex pL'ns i \'12 Large size . Difficult tu :nn(\ if'y Iln("(' i nstalled .

Vortex

No moving parts. Good accuracy . Wide flow Good cost/performance. Not aff ected by viscosity. range. density or pressure.

No t suitable f or '; iscuus fluids .

pOint .

Low cost .

Expensi\'e .

\'101i teJ accuracy .

.

--

- -~~--"- -- ----~---

TABLE 2

Flow

Tabulat i on f o r

1~

o f Maximum Plo w

Limits of Actual Flow ( 1 % of maxi

%

Hate Accuracy

100

101 99

90

91 89

1 . 11 %

80

81 79

1 . 25%

71

1 . 43%

70

+- 1% --

69 60

61 59

1 . 07 %

50

51 "9

2 . 00~

40

41 39

2 · 50';

31

3 . 33%

30

29 20

21 19

5 . 00%

10

11 9

10 . 00%

B. E. White

52 8

!5 4

3 2

>.

u 0 ......

:::l U U

«

0


~-1 ~

---

-2

...,...-

-3

...,...-

...,...-

,./

/

-4

/ /

-!5

-8

10

Pig. 1.

~

-~~ --- - ------------------

20

30

40

50 Flow

70

60

100

90

80

Rate

Graphic al comparis on of two "numeri cally equal " (U) accuracy statemen ts.

LEGE ND +

1% Max Flow 1% Max Flow

+ 1%

Rate

1% Rate

Outlet Manifo ld

Test MagFlow

Ref. MagFlow Variab le Speed Motor

Pump

Inlet Manifo ld

Pig. 2.

Drive Belts

Calibra tio n Rig - piping layout.

53

Automatic Flowmeter Calibration Rig

__- - - - - - -.. RS - 232C /RS -422 ~--------~RS -232C

IEEE-488

A

B

C

Communications

Power

Channels

Supply Z-80 A

Timing Contro l

Read-Only Memory

Memeory

I / O Board PCI - 3000 MASTER EN CLOSUR E

I/O Board No . q

I/O Board No.3

I

L-

---,

- - - - - - -r - - - _ - l L L - _..... PCI - 3000

To/From

Expansion

T er mina t ion

Enclosure(s)

P ig .

3.

Bl oc k diag ram of peI - 30DD sys t em

Panel(s)

architectur~

54

B. E. White

( Yes

START

irst time operation today? No elect Option

Test rig for accuracx afYin stored DA l' ca!. information (Ref.Standard)

•

Call Check Valve Routine

,

Get stored data

•

Do test and display

~" Specs?

Call 'MASTER' Diagnostic Routine

~Y" Specs ?

No

Call Option 2

Option 1 calibrate a MagFlow

~ Option 2 Re-calibrate the test rig

Option 3 Obtain records records & data

Data acquisition Get stored

Check for valid operator

1. Specific

Verify data Operator mput

Calibrate a line

,

~ith

Select:

,

,

,

Call Check valve Routine

Select Option

•

Display chccking sequence

I

'-

~

Call 'TEST' Diagnostic Routine

Yes Print Calibration Certificate

J

Select Option

Fig. 4

•

Print data

,

( Select OjJtlon

No

End

reco rds 2. History TrendlOg 3. Analysis 4. Performance charts 5. Synopsis

No test and display results

Specs?

Option 4 Quit

Flow Chart of Software Structure

I