- Email: [email protected]
Large pumps in power and desalination plants
Desalinan’on, 93 (1993) Ml-206 Elsevier Science Publishers B.V., Amsterdam
181
Large pumps in power and desalination plants Hussain Osman, A. Hanif Sultan, Adel Radif and Showki A. Aziz Water and Electricity Department, Abu Dhabi, Government of Abu Dhabi, POB 219, Abu Dhabi (UAE). Tel and Fax: 971-2-772530
SUMMARY
The main factors affecting pump selection and maintenance techniques in power and desalination plants are discussed. The paper concentrates on main pumps such as “brine recirculating pump” which is the largest pump with highest discharge head in the desalination plant, the “feed water pump” and “sea water supply pumps”. The topic includes the pump selection with respect to type, size, and material. The predictive maintenance through monitoring the operating data, plant characteristics and diagnosis increase the reliability. Emphasis has been given to pump problems at Umm Al Nar Power and Desalination Plants and its trouble shooting with respect to cavitation, vibration, bearing and shaft failures, and material defects. INTRODUCTION
Centrifugal pumps comprises a class of pumping machinery in which pumping of liquids or generating of heads is affected by rotary motion of one or more impellers. Every pump consists of three principal parts: “an impeller” which forces the liquid into a rotary motion; “the pump casing” which directs the liquid to the impeller and leads it away under a high pressure; and “a drive” to put the impeller into rotary motion. The later includes pump shaft supported by bearings and drive through a “flexible or rigid coupling” by the drive. As a result of the impeller action, liquid leaves the impeller at a higher pressure and velocity. The velocity is partly converted into pressure by the pump casing before the liquid leaves the pump through the discharge nozzle. This conversion is accomplished either in a volute casing or a set of diffusion vanes. 001 l-9164/93/$06.00
0 1993 Elsevier Science PublishersB.V. All rights reserved.
182
When the total pressure required can not be produced efficiently by one impeller, several impellers are arranged in series resulting in a “multistage pump ”.
PUMP SELECTION CRITERIA
Specific speed The basic factors to decide the shape and dimension of an impeller are the capacity, total head and speed. The term specific speed is a correlation of pump capacity, head and speed at optimum efficiency which classifies the pump impellers with respect to their geometric similarity. Specific speed is defined as the revolution per minute of impeller required for generating a head of 1 mt with the capacity of 1 m3/min and is expressed by the equation: Ns =
JJK?
-
H3t-4
N=speed rpm, Q=capacity [m3/min], and H= total head [ml. Note: the specific speed is calculated by the head / one stage in case of a multistage pump and by l/2 of the capacity in case of double suction volute pumpThe relation between the impeller design and specific speed is shown in Fig. 1. As the value of N, increases, the impeller type (shape) changed from radial type to “mixed flow” and then to “axial flow”.
ns Clatifcttio~
Volute pumps
0
0
boo
soo’ Mixed flow
Fig. 1. Relationship between impeller design and specific speed.
1400 Axlal
flow
183
Pump characteristic curve (H-Q curve)
The shape of Q-H curve of a centrifugal pump depends strongly on the specific speed. The slope of this curve normally plots
where Ho =shut off head (at zero flow) and H,, t=head at design point. As the specific speed increases, the slope oP Q-H becomes steeper. The slope is the steepest for propeller pumps (axial flow) and that it is the flattest for radial centrifugal pump with low specific speed. %Mpl - 120% radial centrifugal mixedflow 43 N?pt - 140 -200% -2OO%andbreaxialflow HJH,,
HlHopt AxialPropeller Pump nq-200
RadialPump nq-20
I 100%
0
alQopt
Fig. 2.
Piping characteristic (Istem characteristic)
The system head is plotted against Q and it is the sum of static head (static lift) and the dynamic head Static head
=
&. - h,
where h,=head at outlet and h, =head at inlet suction. Dynamichead
=
where h &=friction suction piping.
hL,+hL,
loss in discharge piping, and h L,=friction
loss in
184 A 4 hdynamic
hstatic
Q !syst&l
Fig. 3.
This dynamic part increases with the square of the capacity EZ+,,=K Q2. Note: When the pipeline consists of different diameters in series, individual characteristics are plotted and then a combined or total pipeline characteristic is plotted. Pump operating point
The centrifugal pump and the pumping plant are two system connected in series, so the operating point of the pump is the intersection between the pumps curve (Q/H) and the piping curve (system curve); see Fig. 4. So the operating point of the pump will change if the plant characteristic take up a different position this can be clearly understood from which shows a pump when it is sucking from full tank and in another case from empty tank. Fig. 5 shows for a cooling water pump when there is a high tide or a low tide for the seawater. So it is advisable that design data which are to be submitted to the pump manufacturer regarding plant installation, operating conditions, flow rate, water level etc. must be precisely correct. If there is any considerable deviation in data submission, then the pumps will not work (operate) on its design point and the further rectification of the fault can cost much (please see modification done on cooling water p/p for 180MW units at Umm Al Nar West, Abu Dhabi). Cavitation
The term “cavitation” refers to condition within the pump where owing to static pressure drop, cavities filled with vapour are formed, and as the static pressure raises again above the vapour pressure along the flow path,
185
HA
Pump head
System head
Fig. 4. Operating point of the pump.
30
25
.
“’
’
FLOW P
“”
-4 3lL
0
Fig. 5. High or low tide for seawater in a cooling water pump.
these vapour bubbles collapse quite suddenly with shock condensation causing what is called “cavitation“. The reducing of local (static) pressure to that of vapour pressure may be produced by the following:
186
1. An increase in velocity: means increased dynamic pressure at the expense of static pressure and the drop in static pressure can cause vaporization or boiling of water (thisfactor is very important in feed water pumps operating at very high speed say 6&M rpm, also when pumps operating over capacity). 2. Change in intake condition: which is related to suction behaviour, that
without the application of external heat it attains the vapour pressure associated with the temperature of liquid at that particular spot, this is due to inadequate positive suction head in front of the impeller (impeller eye) which unable the impeller to pull in the water. Signs of cavitation
The signs of cavitation are as follows: 1. Noise and vibration, the noise can be compared with the noise made by gravel in a concrete mixer. 2. Drop in head - capacity and efficiency curve, it can be noticed more clearly in radial pump than axial type. 3. Pitting of the material in impeller blades which occurs during cavitation and it has the shape of honey-combed spongy appearance). l%e net positive suction head
The net positive suction head (NPSH) value is an important concept for judging the suction behaviour of a centrifugal pump, to have a safety margin sufficient to avoid cavitation effect in the pump. The important of this parameter, NPSH is that the pressure at any point in the suction arm of pump must not be reduced to the vapour pressure of the liquid. A distinction is made between the NPSH value required by the pump NPSH,. and the existing NPSH value of the installation (plant) NPSH,.. NPSH available
The NPSH,. of the installation (pumping) plant is defined as follows -2
*=L
=
V PO + PS - Pv + -5 2g
+ ZS
where P, =atmospheric pressure (barometric press), P, = suction pressure, P, = vapour pressure, V, =velocity at suction, and Z,=static head related to datum level.
187
NPSH required
The NPSH required by the pump represents the minimum required margin between the NPSH available and the vapour pressure of the fluid. A one meter margin must be between NPSH available and NPSH required i.e.
NPSH,,
-
NPSh
lm
r
to ensure that the pump is relatively free of cavitation. The two parameters (components) which determine the NPSH required are (1) Axial velocity in the impeller eye and it is called “eye speed” (If,>; and (2) the peripheral speed at the maximum diameter of the vane entry edge (V-L The relation between these two parameters and NPSH required can be expressed as follows: NPSH,
=
(l+K)Fa2
+-
KC 2g
2g
K is the constant whole value which depends on the impeller design (shape and size) but is usually of the order of 0.25 (Fig. 6). However the curve for NPSH required for any pumps was determined at the factory’s ‘test bench by choosing approximately five different points of flow and then checking the pump behaviour when throttling the suction of the pump keeping the capacity nearly constant until the drop of total head = 3 % (breakaway point) approached that flow. That is the point of minimum NPSH required for impeller to avoid cavitation.
. fT -
rev/ min “n”
P
"P "P ,.-,
t
I
D (inches) d
eye dkmtttr
hub dhmtttr
“f (f(/s)
eye nbcity
”
peripheral V&city
M/s)
Cl (gallmin) A, (in’)
=
Pump capacity
area
a f
(D’-
f VL I
0.385-Q A,
Fig. 6.
I
(inches)
ft Is hclucks rtxrvc)
d’).
188
Suction number This is a characteristic factor for the evaluation of the NPSH value of a centrifugal pump (also called “suction specific speed”). Nq
8=
Nl@ (NPSIzl.b9>3’”
Cavitation coeflcient
The suction requirement to be met by a pump as defined by the cavitation coefficient (r (thoma factor) as determined by the specific field condition (I =
NP!‘H_ H
where H = total head/stage. See Fig;. 7 for the relationship between N, and
Fig. 7.
How to minimize the cavitation e#ect
1. Provision of increase NPSH from the feed suction for booster pump (feed water pumps), also improvement of suction intakefor vertkal cooling
189
waterpumps and seawaterpumps. Model tests have to be made for big sea water pumps (higher capacity) to study the suction behaviour at impeller inlet to ensure vortex free. 2. Selection of cavitation resistant impeller material. 3. Change in impeller hydraulic design. Material selection
Corrosion, cavitation, erosion, welding and foundry practice are the main factors for selecting the material of different parts of the pump (impeller, diffuser, column pipe, . . .). Material resistance of cavitation are those with high fatique strength combined with a high corrosion resistance (see sheet of material). The stress corrosion cracks have caused failure of different parts at UAN sea water pumps and brine recirculating pumps (refer to UAN pump problems). The material of these parts is Ni-Resist ductile GGG NiCr NB 202. The similar parts in other pumps at UAN, which are manufactured by stainless steel casting no sign of cracks or corrosion is noticed although these pumps pass more running hours than the defective one. 1. Brine recirculating pumps 4 mgd distiller Total head Q Speed
Vertical mixed flow impeller 61.5 / 67.5 mt 184.79 / 203.27 m3/min 550 / 585 rpm 2265 I 2735 kW
Impeller Pump shaft Diffuser Bell mouth Column pipe
Stainless steel casting SC 16 Stainless steel SUS 316 L Stainless steel casting SC 16 Stainless steel casting SC 16 Ni - Resist Ductile D2
Type
output Material
2.
Sea water
pumps4 mgd distiller
Type Total head Q Motor power Speed
Vertical mixed flow 31.5 I33.5 mt 185.711204.28 m3/min. 1630 kW 585 rpm
Material Impeller Pump shaft Diffuser Bell mouth Column pipe
Stainless steel casting SC 16 Stainless steel SUS 316 L Stainless steel casting SC 16 Stainless steel casting SC 16 Ni - Resist Ductile D2
190 3.
Sea water pump 5 mgd distiller Type Total head
Q Motor power speed
Vertical mixed flow 32 I 27.6 mt 1612 / 1555 m3/min 1612 kW 594 rpm
Material Impeller pump shaft Diffuser BeII mouth Column pipe 4.
l-4406 l-4404 NI - Resist Ductile l-4404 Ni - Resist Ductile
Brine recirculating pumps 5 mgd distiller Type Total head
Q Motor power Speed
Vertical mixed flow 60 / 57 mt 11,824 I 13,007 m3/min 2369 kW 496 t-pm
Material Impeller pump shaft Diffuser BeU mouth Column pipe
5.
l-4406 l-4404 Ni - Resist Ductile l-4404 Ni - Resist Ductile
Cooling eater pumps 60 MW units Type Total head
Q speed Power
Vertical mixed flow 13.2 mt 15,500 m3/min 425 rpm 860 kW
Material Impeller pump shaft Diffuser Column pipe 6.
l-4347 l-4460 Ni - Resist Ductile Ni - Resist Ductile
Cooling water pumps 75 MW units Total head
Q speed Power (motor)
14.2 mt 11,373 m3/min 495 rpm 1060 kW
Material Impeller pump shaft Diffuser Column pipe
l-4347 l-4460 Ni - Resist Ductile Ni - Resist Ductile
191 7.
Cooling water pumps 180 hfW units 5~ Total head
Q Motor power Speed
Vertical mixed flow 11 mt 33,250 m3/min 370 I 2551 kW 1519 ‘pm
Material Impeller Pump shaft Diffuser Column pipe 8.
l-4347 l-4580 Ni - Resist Ductile Ni - Resist Ductile
Feed water pumps 40 A4W units Type Total head
Q Speed Power
Multistage (9) centrifugal pump 1120 mt 200 m3/min 2980 rpm 1065 kW
Material Impeller Pump shaft Shaft wearing ring 9.
GX 20 Cr 14 GX 12 Cr 14 1.4571 GX 170 Cr 18
Feed water pumps 75 MV units Type
Total head
Q speed Material Impeller Pump shaft Shaft wearing ring
Multistage (9) centrifugal pump 1107 mt 174 m3/min 2980 rpm
I-4027.92 l-4027.92 l-2714.90 B 48
10. Feed water pumps 180 MW units
Type Booster Pump Total head Booster pump pressure
Q Booster pump speed Main pump speed Motor power Material
Main pump 5 stages centrifugal double suction, single stage 191/210bar 7.856 bar 412 I330 tlh 1490 rpm 5375 I4357 rpm 353311411kW
of main pump
Stage casing Diffuser Impeller Shaft
l-4008 l-4027 l-4027 l-4027
192 Impeller wearing ring Diffuser wearing rings Booster
pump (double suction)
Volute casing pump shaft Impeller Casing wearing ring
MAIN
GX 170 C 18 CK 45
G-X 12Cr14 X 20 Cr 13 G-X20Cr14 G-X170Cr18
PUMPS
Boiler feed pumps
The importance of optimizing boiler feed pump arrangement and drive type is obvious when it is appreciated that feed pump power consumption accounts for between one-third and one-half of a total generating plant auxiliary load. The construction of BFP’s in respect of shaft power - construction material, pump types and drive is largely governed by the developments which have taken place in power station technology. Boiler feed pumps with drive rating of 30MW and with some exceptions up to 50MW (in large units). Mass flow rate were in the region of 350 t/h and they have risen today up to 2500 - 4000 t/h. Pressure was raised up to 400 bar. Feed water pump drive
1. For small units normal asynchronous motor. 2. For higher capacity units speed control electrical driven boiler feed pumps is affected via a fluid coupling or by electric regulation for the motor speed which is controlled by thyristors (used today up to 18 MW electric motor). 3. In case of conventional plants above 400 MW the drive of full load feed pump is usually via a steam turbines. In most cases condensing turbine running at 5000 - 6000 rpm are used. Function of booster pump
The booster pumps has the duty to generate the necessary NPSH value of the installation (NPSH available) for the high speed boiler feed pump connected down stream. The slow speed booster pump is usually driven off the extension stub shaft of the boiler feed pump via a speed reducing. gear.
193 FEED WATER PUMP CONSTRUCTION
AND DESIGN
Rotor construction BFP are usually fitted with pump shaft which have as small distance between bearing as possible, combined with a relatively large shaft diameter; the impellers are shrunk on the shaft. Due to the ample size of the shaft, the static sag is very small, the shaft is largely insensitive to vibrations, the critical speed will be brought to high level and at normal running the conditions are smooth, without any undesirable radial contact with the casing. The shaft is balanced statically and dynamically with all rotating parts. Rubbing Pump rotor can suffer from rubbing at the annular seal clearance under some critical conditions such as low flow operation. Rubbing can cause unexpectedly high stress in the shaft and may introduce fracture of the shaft. Therefore the impeller split rings are design in such a way that a static hydraulic restoring force will be produced (LOMAKIN effect). During operation the critical speed (wet critical speed) is far from the operating speed. Impeller material The development of high strength corrosion resistance martensitic chrome steels paved the way for the present day boiler feed pumps with speeds of 5000-6000 rpm and today’s full load BFP for conventional 750 MW power blocks are constructed with 4-5 stages with stage pressure up to 80 bar. The high quality precision cast stainless steel impeller, impeller casing are subjected to extreme radiographic inspection.
BRINE RECIRCULATING
PUMP
This is the largest pump in the desalination system. This pump is a vertical mounted in a can (shell) which forms the inlet intake well. It draws water from a low pressure chamber (last stage of heat rejection section) and not from an open sump where atmospheric pressure would contribute a large proportion of the NPSH required by the pump. Pump failure causes serious problems in a MSF desalination plant, for instance no water can be produced
194
if the brine can not be circulated because of a recirculating pump or drive fault. The following points have to be considered in pump choice and design: 1. The pump characteristic will be determined according to service condition of the plant, so the designer should obtain accurate information about the system characteristic. Design of the pump as a single or two stage mixed flow type depends upon different factors: a. Shape of the curve (Q-H) according to Ns for a single stage or two stage in the point of adaptability to discharge capacity fluctuation and head variation. b. Flow control, if it is by adjusting discharge valve opening (throttle control) or by changing the speed of the pump. c. Cavitation performance. d. Comparison of velocity with erosion corrosion for both single stage and two stage. (Velocity at bell mouth, impeller, diffuser casing). e. The cost of spare parts in addition to pump weight and pump length from the economic point of view. 2. Material selection (see section on material selection). 3. NPSH - Provision of adequate NPSH to avoid cavitation in the impeller, safe margin to be considered between the NPSH required for the pump and the NPSH of the system. 4. The vibration characteristic of the drive unit (motor - flange support and intermediate gear) to be matched with the pump that means the fundamental frequency and read critical frequency (critical speed) to be analysed to ensure compatibility between the pump and the motor. (Refer to problems at UAN pumps). In case of turbine drive a particular attention must be paid to reaching an agreement on the critical pump speed because it can lie in the permissible over speed range of the steam turbine.
SEAWATER
PUMPS AND COOLING WATER PUMPS
The seawater pumps and cooling water pumps both are vertical mounted type, deliver seawater to the distillers or to the condenser of steam turbines for cooling. Because of the continuous increase in the size of power stations the cooling water pumps are now built with capacity of 100,000 m3/h at delivery head up to 30 mt. The pumps can be mixed flow or axial type.
195
The production faults represents higher percentage of pump damage. The production fault can be due to planning and design error, erection fault, manufacture fault, incorrect stress calculation or material defect. The following points have to be considered in pump choice and design: 1. The pump can be mixed flow or axial type, as known mixed flow have a steep curve and max efficiency at high discharge, axial flow can cause problems if it is running at low flow. 2. The pump would be matched to the system so that it operates at its maximum efficiency. If due to planning error quote too high delivery head, the pump will then operate continuously in the range of excessive and sometimes unallowable discharge flow and hence always have a poor efficiency. 3. The choice of minimum submergence from MWL (minimum water level) to bell mouth edge is usually very important for preventing the air draw vortex and the cavitation of the pump. S = min submergence from MWL to bell mouth = 1.5 D C = distance or clearance between bell mouth and sump flow = 0.5 D. If an inlet cone is provided, the clearance C will be greater. The inlet cone is an antivortex device to improve sump performance (Fig. 8). In general the submergence of an intake should be large enough, to reduce possible occurrence of air entraining vortex and swirling flow. On the other hand the shortest pump length under the ground level is the best for maintenance, vibration and pump rigidity and of course cost of the civil work. 4. Front wall dimensions and length of approach channel to be chosen to keep the approach velocity low. Length of approach channel = 10 D. Typical mean velocity in a basic sump: Pump inlet = 4 m/s Bell mouth = 1.3 m/s Approach vel . = 0.3 m/s If no empirical figures are available from existing installation for use during the design phase model test is strongly recommended, in order to optimise the intake conditions and reduce cavitation effect. 5. The designer should obtain accurate information of the chlorination of the seawater and to be taken in consideration when selecting the material. 6. Material selection - material has to be chosen to resist corrosion, cavitation, erosion (see part of material selection). 7. Vibration characteristic (see section on BRP).
196
-7.30
i”j f c! (3
-9.00
-1030 --
m
LET CONE
CENTER Of
Fig. 8. An inlet cone.
PUMP
D = 1.5-1.8 /d
197 PREDICTIVE MAINTENANCE
Predictive maintenance is a systematic method of monitoring and trending rotating equipment to determine the condition of the machine. Diagnostic monitoring can reduce down time by detecting abnormal mechanical behaviour at an early stage. It also allows better outage scheduling and should minimize forced outage. Shutdown for repair can be scheduled for convenient time which allow maintenance staff to prepare the work schedule, together with the requirements for manpower, tools and spare parts. Monitoringfor boilerfeed pumps 1. Vibration monitoring for detecting of abnormal operation, this can be on-line vibration monitoring by installing sensor transducers at bearing house in addition phase detection which is important in evaluating source of trouble or it can be periodic system to monitor vibration with small computer, reading can be taken every two weeks and plot the data for comparison and trending. 2. Monitoring of noise level. 3. Lubricating oil analysis is used for trending which we can monitor, the wear particles in the oil samples on a monthly basis. The good quality of oil will increase the life time of bearings. 4. Monitoring of main pump parameters specially at min flow and design point, also monitoring of thrust balance water pressure and axial displacement (wear of thrust bearing), such monitoring is very essential to know the condition of the pump. In addition a periodic checking for the balancing device (balancing disc and counter balance, throttle bush, balancing piston) can give clear indication about the status of the pump. For example, according to monitoring and diagnostic at UAN power station, we have one of the feed water pumps worked about 60,000 hrs without any major overhaul and still in good condition, another feedwater pumps of other manufacturer, of the same capacity and head as the first one worked only 25,000 hrs and it is planned to be overhauled due to excess clearance found during our monitoring and checking. 5. Monitoring and control of pressure and temperature of feed water tank and deaerator: To avoid steaming in the inlet system which can cause damage to the feed water pump, a various pressure gradient limitation control to be applied. Attention to be given to temperature variation in the feed water tank in case of turbo set fall-out and cold condensation flows into the dearator.
198
6. Reverse running monitoring and protection is very essential for feed water pumps, a lack of reverse running monitoring can cause damage for different parts of the pump, the reverse running monitoring must initiate the following: a. Close the discharge valve of the pump, b. Switch on the auxiliary oil pump. In addition, the starting cycle of the electric motor should never be initiated because running the motor at that condition certainly causes extensive damage to the pump. Monitoring of seawater pumps and brine recycle pumps
1. Vibration monitoring by using a portable type vibrometer with small computer, reading can be taken every two weeks and plot the data for comparison and trending. A recent development is the application of submerssible proximity transducer to measure the vibration behaviour of the vertical pumps but this application still not widely applied, to have continuous monitoring. 2. Noise monitoring - It is very essential for BRP and water pumps to monitor noise level at different flow specially at low flow and also at over capacity. The noise generated falls as follows: a. Interference noise resulting from the interaction between flow at entry and exit of impeller. b. Turbulence noise resulting from vortices setup in the flow. c. Cavitation noise. d. Noise due to mechanical sources. 3. Level monitoring to shut down the pump if the level drop below the minimum, to avoid cavitation. 4. Bearing temperature monitoring for thrust bearing and flow control for lubricating water to guide bearing.
BEARINGS
The main two forces affecting on pump shaft are the radial force (FR) and the axial force (FA), the bearing system is designed to absorb these two forces. The bearing can be either Anti Friction Bearing or a plain bearings (sleeve bearing) with a separate thrust bearing (tilting pad thrust bearing).
199
Anti-fiction bearing
It can be divided into three categories: 1. Axial bearing (deep groove bearing) (FA). 2. Cylindrical bearing (roller bearing) (FR). 3. Radial deep groove (FR, FA). The friction coefficient of the anti friction bearings is same, 2550% lower than for plain bearing, other advantage of this type is its more accurate running because of the closer clearances that can be achieved. The disadvantage of the anti friction bearing that it is sensitive to shock loads, noise running, also misal,ignment can effect much on life time of anti friction bearing. As a general roller contact bearing can be accepted in small size pumps but has not proved successful in large high energy pumps, stresses in rolling contact elements are high so as to make fatigue acknowledge factor. When this type of bearings began service in pumps, selected according to life test data, the life of the bearings in many pump applications fell below expectations. The empirical solution for the problem was a load multiplication factor. In some cases even this did not prevent failure. Plain bearing
This type of bearings is to bear the load imposed on the main shaft i.e. the radial load which acts at a right angle to the axis of the shaft. The structure of the bearings differs by the pumps type. In a horizontal pump, the radial load is usually borne by two bearings, the material used is a white metal bearings or lead bronze bearing. These types of bearings are provided outside the casing. The moving part is the shaft or journal, the stationary part which can be in different forms is the bearing shell, such as feed water pumps bearing. There are also the submerged bearing type where the bearings are provided in the pump casing such as the vertical pumps as seawater pumps and brine recirculating pumps. There are different types of submerged plain bearing used in vertical pumps and can be summarized as follows: I. Rubber bearing Rubber bearing have been successfully used up to pheripheral velocities of 6 m/s and bearing load of 20 N/cm*.
200
The lubricating water must pass through a fine strainer to remove sandy impurities. Water lubrication has the advantage that no grease or oil can find its way into the cooling water circuit, but when dry running occurs rubber bearing destroyed on few revolution. It is estimated about O-2 s (refer to problems at UAN pumps). So a separate flow control must be provided before each bearing down stream of the fine strainer, also to the standby pumps, the lubricating water must be continuously supplied for flushing and to avoid blockage or corrosion of the lubricating water pipe lines feeding the bearings. 2. Grease lubricated lead bronze It is necessary to accept bearing loads which exceed the permissible figure of rubber bearing, a grease lubricated lead bronze are often installed, they are suitable for peripheral velocity up to 12 m/s. Such bearings are continuously lubricated by a grease pump drive from main pump shaft or separate motor. The bearings must be fed with grease 60 seconds before the pump started. 3. Ceramic bearings Development in recent years has proceeded in direction of fully maintenance free bearings, for this reason hard metal ceramic bearings used specially in pumps for continuous operation (cooling water pumps). Ceramic bearings are continuously lubricated from the discharge of seawater pumps, cooling water pumps. there is no hazard if small particles penetrated into bearing gap, also during starting operation it is not necessary to pre lubricate upper bearing which has been left dry. 4. thrust bearing
a. Vertical pumps (seawater pumps and brine recirculating pumps) An external thrust bearing on the vertical pumps have to sustain the entire weight of all the revolving parts that lie below. The bearing is of tilting pad oil bath lubricated and water cooled due to combination of loading and high ambient temperature. b. Feed water pumps - Balancing device and compensation of axial thrust in feed water pumps. The high total head of multi-stage feed water pumps creates axial thrust (force) on the pump rotor. This axial force results from pressure difference acting on unbalance impeller surface area (between suction and delivery of the impeller) acting toward suction side. The bal-
201
ancing device creates an axial thrust in the opposite direction to that generated at the impellers (see Fig. 9). A segmental pad thrust bearing arranged at the discharge end and capable of absorbing thrusts in both directions serves to absorb any small residual thrust and at the same time locates the shaft axially (Fig. 10).
Fie. 9.
Fig. 10.
202 PROBLEMS FOR PUMPS AT UMM AL NAR
Distillate pump damage (4 mgd distiller)
Incident: The distillate pump tripped during its start up, while changing over from working pump to standby. The state of the damage to the pump are as follows: Twisting of main shaft, distortion and shearing of coupling key. The intermediate bearing surfaces were burned and the lower bearing was more severely burned. Shearing of the impeller key, shearing of flat heat screw for impeller wear ring. Cause of failure
The failure was caused by the shortage of water at cutless bearing, this has caused the dry rotation at the start up, burning the cutless bearing and creating a large clearance. The big clearance has caused the impeller to touch the casing and cause pump damage. The cause of water shortage at the cutless bearing, that the distillate pump suction barrel is connected to the evaporator which is under vacuum, the minor air leak which occur through the gland packing of the standby pump creates a positive pressure inside the casing which pushes down the water level in this casing to a level below that of the cutless bearing which become dry. Action
A balancing line 15 mm diameter have been installed connecting pump casing to pump barrel to equalize the pressure in both and in order to keep water level in pump casing to avoid drying of the cutless bearing. Problem of dry running has occurred twice in a 5 mgd desalination plant. Cooling waterpump for 65 mw unitsfailure of muflcoupling nuts and studs
During the major overhaul of the cooling water pump in 1984, the muff coupling nuts were found seriously corroded and in brittle form which could result in a serious breakdown of the pump. The material of muff coupling nuts and studs is l-4460 DIN standard. Sample of the defective parts were sent to the manufacturer. The examination was carried out by him, it was found that damages were caused by a selective corrosion attack. The definite cause of this could not be clarified,
203
but there is no doubt that a considerable chlorination of the flow medium aggravates corrosion due to the fact that chlorination procedure was modified, and to be injected at the upstream of the band screen instead of in front of pump chamber. In view of operational safety the screw and nuts of muff coupling material are replaced to DIN l-4462, which is more resistant to corrosion attack. Seawater recirculating pump (4 mgd distiller)failure of journul bearing of the pump
The pump is double suction volute casing, the two radial bearings were white metal journal bearing with oil ring lubricated system, the thrust bearing is an angular contact ball bearing. In the field operation, the coupling side bearing metal have been damaged. On investigation by the manufacturer, the condition of damage were found as follows: a. Damage to the metal is concentrated at coupling side bearing. The anti coupling side is free from any damage. b. The place and area of damage metal at the upper metal side (upper shell of bearing). From the above condition, the radial load is applied into upward direction, which is unexpected. The oil ring is attached at the upper bearing metal and when upward direction radial load is applied, radial load receiving area of the metal is reduced and the oil film became insufficient, to the load thus the damage happened. Because of the reasons as above described upward direction load is applied to the pump it is difficult to protect this damage. So, the metal type bearing has been replaced by a spherical roller bearing which can withstand any directional load. Cavitation on brine recirculating pump impeller (4 mgd) distiller
The impeller of this pump is a mixed flow shrouded type with five impeller blades. By inspection of this impeller after = 20,000 hr it was found a cavitation effect appeared as erosion corrosion near the inlet of impeller blades. The following measurements have been taken: nominal thickness of impeller blade = 13 mm nominal length of impeller blade = 1075 mm
204
max. erosion 3 .I3thickness min. erosion 1.8 thickness
x x
200 mm length 130 mm
Partial load operation can cause such erosion corrosion by cavitation phenomenon also suction strainer blockage and in this condition continuously operating the pump at its nominal capacity can create cavitation effect on the impeller. A new impeller was installed and the old one was repaired by welding after receiving the manufacturer’s advice. High vibrationfor brine recirculating pump (5 mgd) distiller
When starting the brine recirculating pump it was noticed that the vibration value of BRP and its motor was extremely high. After investigation by the manufacturer and many trials to decrease the level of vibration by balancing of motor pump set at site and by improving the alignment. The vibration problem was solved by achieving a big enough difference between the fundamental natural frequency of the whole system and the rotating frequency of the pump. The technical solution is found in the connection of motor lantern and pump to be tight fit between motor stool and pump, which leads to the increase of fundamental natural frequency. This has been achieved by inserting thin plates between guide ring of motor lantern and the stationary ring at the thrust bearing lantern. After the above solution, the vibration data records confirmed the solution. Severe cracks in dl@erentparts of the seawater ing pumps
pumps
and brine recirculat-
Stress corrosion cracking has occurred in different parts of seawater
pumps and brine recirculating pumps at UAN causing a serious threat. The material of these parts is “Ni-Resist Ductile”. The stress cracking is probably internal stress resulting from lack of stress relief heat treatment or incorrectly performed stress relief. The cracks were found in the following pumps in the UAN area: a. Seawater pumps (4 mgd) distiller -
A big crack was developed in upper and lower column pipes, to put the pump back in service a new column pipes manufactured, locally by carbon steel coated. b. Seawater pumps (7 mgd) distiller - The set of guide vanes of the diffiser casing had cracks on every vane, the cracks appeared at the fillet between the van and outer casing, it appeared to be initiated at the top corner of this fillet and propogated down wards along the filter in the direction of the impeller.
205
These cracked vanes repaired by the manufacturer by welding and stress relieving. c. Brine recirculating pumps (5 mgd) - Two discharge elbows a big crack due to stress corrosion was noticed. The elbows have been replaced, recently big cracks were noticed in the diffuser vanes, some of these vanes already broken and separated from the diffuser. The crack of the diffiser vanes appeared on two pumps. d. Brine recirculating pumps (7 mgd) - On these pumps, cracks have developed on the diffuser vanes also, the diffuser vanes had been repaired by Belzona material. Finally, it is needed to mention that we have seawater pump and brine recirculating pumps some of them passed more than 85,000 h, its diffuser casing material is “stainless steel casting” and during inspection no sign of cracks or corrosion exist. Cooling water pumps 180 MW
Two modifications have been, done the first modification is by trimming the impeller to decrease pump head and flow, the second modification was in pump entry to decrease the high noise level. a. Trimming ofpump impeller
According to the specificationcooling water pump shall operate under the worst condition, low water level (-5.1 mt) and cleaning factor of condenser tubes 0.85. During the test period it was found that the results of the theoretical pressure loss calculation are higher as the actual pressure loss of cooling water pipes, also the low water level (-5.1 mt) was not reached, max. LWL was (-3.8 mt) during this period. For correction the delivery head a reduction in impeller of the cooling water pump by machining from 1442/904 to 1405/904 after that the impeller was balanced.
Q, m3/h H, mt
With modified impeller
Before
33,250 11
36,000 14
206
b. Modi@cation ofpump entry
When starting the cooling water pump it has been noticed that pump cannot be operated safely in continuous operation due to constant noise running and pressure pulsation. The manufacture studied this problem and stated that running noise and pressure pulsation can be reduced by removing the rib parts of the inlet rectifier (inlet cone). A complete study have been done by the manufacturer including model test to have complete information concerning the flow condition to the pump, an evaluation of the velocity profile in front of impeller has been done. Based on the model test, the modification has been done on the inlet cone (rectifier) and it gave positive response. 1. Modification of the inlet rectifier with 5 antivortex ribs to 3 shortened anti vortex ribs. 2. Arrangement of central longitudinal fin (splitter) in the pump chamber.
REFERENCES
1 Centrifugal Pumps for General Refinery Services (API Standard GlO) American Petrolium Institute. 2 The Hydraulic Design Pump Sumps and Intakes (M.J. Prosser Mami Mech. E.) Bhra Fluid Engineering (1977). 3 Allianz Hand Book of Loss Prevention (May 1987). 4 Kent’s Mechanical Engineers Hand Book. 5 Kubota Pump Hand Book. 6 Centrifugal Pump Lexicon (K.S.B.).
181
Large pumps in power and desalination plants Hussain Osman, A. Hanif Sultan, Adel Radif and Showki A. Aziz Water and Electricity Department, Abu Dhabi, Government of Abu Dhabi, POB 219, Abu Dhabi (UAE). Tel and Fax: 971-2-772530
SUMMARY
The main factors affecting pump selection and maintenance techniques in power and desalination plants are discussed. The paper concentrates on main pumps such as “brine recirculating pump” which is the largest pump with highest discharge head in the desalination plant, the “feed water pump” and “sea water supply pumps”. The topic includes the pump selection with respect to type, size, and material. The predictive maintenance through monitoring the operating data, plant characteristics and diagnosis increase the reliability. Emphasis has been given to pump problems at Umm Al Nar Power and Desalination Plants and its trouble shooting with respect to cavitation, vibration, bearing and shaft failures, and material defects. INTRODUCTION
Centrifugal pumps comprises a class of pumping machinery in which pumping of liquids or generating of heads is affected by rotary motion of one or more impellers. Every pump consists of three principal parts: “an impeller” which forces the liquid into a rotary motion; “the pump casing” which directs the liquid to the impeller and leads it away under a high pressure; and “a drive” to put the impeller into rotary motion. The later includes pump shaft supported by bearings and drive through a “flexible or rigid coupling” by the drive. As a result of the impeller action, liquid leaves the impeller at a higher pressure and velocity. The velocity is partly converted into pressure by the pump casing before the liquid leaves the pump through the discharge nozzle. This conversion is accomplished either in a volute casing or a set of diffusion vanes. 001 l-9164/93/$06.00
0 1993 Elsevier Science PublishersB.V. All rights reserved.
182
When the total pressure required can not be produced efficiently by one impeller, several impellers are arranged in series resulting in a “multistage pump ”.
PUMP SELECTION CRITERIA
Specific speed The basic factors to decide the shape and dimension of an impeller are the capacity, total head and speed. The term specific speed is a correlation of pump capacity, head and speed at optimum efficiency which classifies the pump impellers with respect to their geometric similarity. Specific speed is defined as the revolution per minute of impeller required for generating a head of 1 mt with the capacity of 1 m3/min and is expressed by the equation: Ns =
JJK?
-
H3t-4
N=speed rpm, Q=capacity [m3/min], and H= total head [ml. Note: the specific speed is calculated by the head / one stage in case of a multistage pump and by l/2 of the capacity in case of double suction volute pumpThe relation between the impeller design and specific speed is shown in Fig. 1. As the value of N, increases, the impeller type (shape) changed from radial type to “mixed flow” and then to “axial flow”.
ns Clatifcttio~
Volute pumps
0
0
boo
soo’ Mixed flow
Fig. 1. Relationship between impeller design and specific speed.
1400 Axlal
flow
183
Pump characteristic curve (H-Q curve)
The shape of Q-H curve of a centrifugal pump depends strongly on the specific speed. The slope of this curve normally plots
where Ho =shut off head (at zero flow) and H,, t=head at design point. As the specific speed increases, the slope oP Q-H becomes steeper. The slope is the steepest for propeller pumps (axial flow) and that it is the flattest for radial centrifugal pump with low specific speed. %Mpl - 120% radial centrifugal mixedflow 43 N?pt - 140 -200% -2OO%andbreaxialflow HJH,,
HlHopt AxialPropeller Pump nq-200
RadialPump nq-20
I 100%
0
alQopt
Fig. 2.
Piping characteristic (Istem characteristic)
The system head is plotted against Q and it is the sum of static head (static lift) and the dynamic head Static head
=
&. - h,
where h,=head at outlet and h, =head at inlet suction. Dynamichead
=
where h &=friction suction piping.
hL,+hL,
loss in discharge piping, and h L,=friction
loss in
184 A 4 hdynamic
hstatic
Q !syst&l
Fig. 3.
This dynamic part increases with the square of the capacity EZ+,,=K Q2. Note: When the pipeline consists of different diameters in series, individual characteristics are plotted and then a combined or total pipeline characteristic is plotted. Pump operating point
The centrifugal pump and the pumping plant are two system connected in series, so the operating point of the pump is the intersection between the pumps curve (Q/H) and the piping curve (system curve); see Fig. 4. So the operating point of the pump will change if the plant characteristic take up a different position this can be clearly understood from which shows a pump when it is sucking from full tank and in another case from empty tank. Fig. 5 shows for a cooling water pump when there is a high tide or a low tide for the seawater. So it is advisable that design data which are to be submitted to the pump manufacturer regarding plant installation, operating conditions, flow rate, water level etc. must be precisely correct. If there is any considerable deviation in data submission, then the pumps will not work (operate) on its design point and the further rectification of the fault can cost much (please see modification done on cooling water p/p for 180MW units at Umm Al Nar West, Abu Dhabi). Cavitation
The term “cavitation” refers to condition within the pump where owing to static pressure drop, cavities filled with vapour are formed, and as the static pressure raises again above the vapour pressure along the flow path,
185
HA
Pump head
System head
Fig. 4. Operating point of the pump.
30
25
.
“’
’
FLOW P
“”
-4 3lL
0
Fig. 5. High or low tide for seawater in a cooling water pump.
these vapour bubbles collapse quite suddenly with shock condensation causing what is called “cavitation“. The reducing of local (static) pressure to that of vapour pressure may be produced by the following:
186
1. An increase in velocity: means increased dynamic pressure at the expense of static pressure and the drop in static pressure can cause vaporization or boiling of water (thisfactor is very important in feed water pumps operating at very high speed say 6&M rpm, also when pumps operating over capacity). 2. Change in intake condition: which is related to suction behaviour, that
without the application of external heat it attains the vapour pressure associated with the temperature of liquid at that particular spot, this is due to inadequate positive suction head in front of the impeller (impeller eye) which unable the impeller to pull in the water. Signs of cavitation
The signs of cavitation are as follows: 1. Noise and vibration, the noise can be compared with the noise made by gravel in a concrete mixer. 2. Drop in head - capacity and efficiency curve, it can be noticed more clearly in radial pump than axial type. 3. Pitting of the material in impeller blades which occurs during cavitation and it has the shape of honey-combed spongy appearance). l%e net positive suction head
The net positive suction head (NPSH) value is an important concept for judging the suction behaviour of a centrifugal pump, to have a safety margin sufficient to avoid cavitation effect in the pump. The important of this parameter, NPSH is that the pressure at any point in the suction arm of pump must not be reduced to the vapour pressure of the liquid. A distinction is made between the NPSH value required by the pump NPSH,. and the existing NPSH value of the installation (plant) NPSH,.. NPSH available
The NPSH,. of the installation (pumping) plant is defined as follows -2
*=L
=
V PO + PS - Pv + -5 2g
+ ZS
where P, =atmospheric pressure (barometric press), P, = suction pressure, P, = vapour pressure, V, =velocity at suction, and Z,=static head related to datum level.
187
NPSH required
The NPSH required by the pump represents the minimum required margin between the NPSH available and the vapour pressure of the fluid. A one meter margin must be between NPSH available and NPSH required i.e.
NPSH,,
-
NPSh
lm
r
to ensure that the pump is relatively free of cavitation. The two parameters (components) which determine the NPSH required are (1) Axial velocity in the impeller eye and it is called “eye speed” (If,>; and (2) the peripheral speed at the maximum diameter of the vane entry edge (V-L The relation between these two parameters and NPSH required can be expressed as follows: NPSH,
=
(l+K)Fa2
+-
KC 2g
2g
K is the constant whole value which depends on the impeller design (shape and size) but is usually of the order of 0.25 (Fig. 6). However the curve for NPSH required for any pumps was determined at the factory’s ‘test bench by choosing approximately five different points of flow and then checking the pump behaviour when throttling the suction of the pump keeping the capacity nearly constant until the drop of total head = 3 % (breakaway point) approached that flow. That is the point of minimum NPSH required for impeller to avoid cavitation.
. fT -
rev/ min “n”
P
"P "P ,.-,
t
I
D (inches) d
eye dkmtttr
hub dhmtttr
“f (f(/s)
eye nbcity
”
peripheral V&city
M/s)
Cl (gallmin) A, (in’)
=
Pump capacity
area
a f
(D’-
f VL I
0.385-Q A,
Fig. 6.
I
(inches)
ft Is hclucks rtxrvc)
d’).
188
Suction number This is a characteristic factor for the evaluation of the NPSH value of a centrifugal pump (also called “suction specific speed”). Nq
8=
Nl@ (NPSIzl.b9>3’”
Cavitation coeflcient
The suction requirement to be met by a pump as defined by the cavitation coefficient (r (thoma factor) as determined by the specific field condition (I =
NP!‘H_ H
where H = total head/stage. See Fig;. 7 for the relationship between N, and
Fig. 7.
How to minimize the cavitation e#ect
1. Provision of increase NPSH from the feed suction for booster pump (feed water pumps), also improvement of suction intakefor vertkal cooling
189
waterpumps and seawaterpumps. Model tests have to be made for big sea water pumps (higher capacity) to study the suction behaviour at impeller inlet to ensure vortex free. 2. Selection of cavitation resistant impeller material. 3. Change in impeller hydraulic design. Material selection
Corrosion, cavitation, erosion, welding and foundry practice are the main factors for selecting the material of different parts of the pump (impeller, diffuser, column pipe, . . .). Material resistance of cavitation are those with high fatique strength combined with a high corrosion resistance (see sheet of material). The stress corrosion cracks have caused failure of different parts at UAN sea water pumps and brine recirculating pumps (refer to UAN pump problems). The material of these parts is Ni-Resist ductile GGG NiCr NB 202. The similar parts in other pumps at UAN, which are manufactured by stainless steel casting no sign of cracks or corrosion is noticed although these pumps pass more running hours than the defective one. 1. Brine recirculating pumps 4 mgd distiller Total head Q Speed
Vertical mixed flow impeller 61.5 / 67.5 mt 184.79 / 203.27 m3/min 550 / 585 rpm 2265 I 2735 kW
Impeller Pump shaft Diffuser Bell mouth Column pipe
Stainless steel casting SC 16 Stainless steel SUS 316 L Stainless steel casting SC 16 Stainless steel casting SC 16 Ni - Resist Ductile D2
Type
output Material
2.
Sea water
pumps4 mgd distiller
Type Total head Q Motor power Speed
Vertical mixed flow 31.5 I33.5 mt 185.711204.28 m3/min. 1630 kW 585 rpm
Material Impeller Pump shaft Diffuser Bell mouth Column pipe
Stainless steel casting SC 16 Stainless steel SUS 316 L Stainless steel casting SC 16 Stainless steel casting SC 16 Ni - Resist Ductile D2
190 3.
Sea water pump 5 mgd distiller Type Total head
Q Motor power speed
Vertical mixed flow 32 I 27.6 mt 1612 / 1555 m3/min 1612 kW 594 rpm
Material Impeller pump shaft Diffuser BeII mouth Column pipe 4.
l-4406 l-4404 NI - Resist Ductile l-4404 Ni - Resist Ductile
Brine recirculating pumps 5 mgd distiller Type Total head
Q Motor power Speed
Vertical mixed flow 60 / 57 mt 11,824 I 13,007 m3/min 2369 kW 496 t-pm
Material Impeller pump shaft Diffuser BeU mouth Column pipe
5.
l-4406 l-4404 Ni - Resist Ductile l-4404 Ni - Resist Ductile
Cooling eater pumps 60 MW units Type Total head
Q speed Power
Vertical mixed flow 13.2 mt 15,500 m3/min 425 rpm 860 kW
Material Impeller pump shaft Diffuser Column pipe 6.
l-4347 l-4460 Ni - Resist Ductile Ni - Resist Ductile
Cooling water pumps 75 MW units Total head
Q speed Power (motor)
14.2 mt 11,373 m3/min 495 rpm 1060 kW
Material Impeller pump shaft Diffuser Column pipe
l-4347 l-4460 Ni - Resist Ductile Ni - Resist Ductile
191 7.
Cooling water pumps 180 hfW units 5~ Total head
Q Motor power Speed
Vertical mixed flow 11 mt 33,250 m3/min 370 I 2551 kW 1519 ‘pm
Material Impeller Pump shaft Diffuser Column pipe 8.
l-4347 l-4580 Ni - Resist Ductile Ni - Resist Ductile
Feed water pumps 40 A4W units Type Total head
Q Speed Power
Multistage (9) centrifugal pump 1120 mt 200 m3/min 2980 rpm 1065 kW
Material Impeller Pump shaft Shaft wearing ring 9.
GX 20 Cr 14 GX 12 Cr 14 1.4571 GX 170 Cr 18
Feed water pumps 75 MV units Type
Total head
Q speed Material Impeller Pump shaft Shaft wearing ring
Multistage (9) centrifugal pump 1107 mt 174 m3/min 2980 rpm
I-4027.92 l-4027.92 l-2714.90 B 48
10. Feed water pumps 180 MW units
Type Booster Pump Total head Booster pump pressure
Q Booster pump speed Main pump speed Motor power Material
Main pump 5 stages centrifugal double suction, single stage 191/210bar 7.856 bar 412 I330 tlh 1490 rpm 5375 I4357 rpm 353311411kW
of main pump
Stage casing Diffuser Impeller Shaft
l-4008 l-4027 l-4027 l-4027
192 Impeller wearing ring Diffuser wearing rings Booster
pump (double suction)
Volute casing pump shaft Impeller Casing wearing ring
MAIN
GX 170 C 18 CK 45
G-X 12Cr14 X 20 Cr 13 G-X20Cr14 G-X170Cr18
PUMPS
Boiler feed pumps
The importance of optimizing boiler feed pump arrangement and drive type is obvious when it is appreciated that feed pump power consumption accounts for between one-third and one-half of a total generating plant auxiliary load. The construction of BFP’s in respect of shaft power - construction material, pump types and drive is largely governed by the developments which have taken place in power station technology. Boiler feed pumps with drive rating of 30MW and with some exceptions up to 50MW (in large units). Mass flow rate were in the region of 350 t/h and they have risen today up to 2500 - 4000 t/h. Pressure was raised up to 400 bar. Feed water pump drive
1. For small units normal asynchronous motor. 2. For higher capacity units speed control electrical driven boiler feed pumps is affected via a fluid coupling or by electric regulation for the motor speed which is controlled by thyristors (used today up to 18 MW electric motor). 3. In case of conventional plants above 400 MW the drive of full load feed pump is usually via a steam turbines. In most cases condensing turbine running at 5000 - 6000 rpm are used. Function of booster pump
The booster pumps has the duty to generate the necessary NPSH value of the installation (NPSH available) for the high speed boiler feed pump connected down stream. The slow speed booster pump is usually driven off the extension stub shaft of the boiler feed pump via a speed reducing. gear.
193 FEED WATER PUMP CONSTRUCTION
AND DESIGN
Rotor construction BFP are usually fitted with pump shaft which have as small distance between bearing as possible, combined with a relatively large shaft diameter; the impellers are shrunk on the shaft. Due to the ample size of the shaft, the static sag is very small, the shaft is largely insensitive to vibrations, the critical speed will be brought to high level and at normal running the conditions are smooth, without any undesirable radial contact with the casing. The shaft is balanced statically and dynamically with all rotating parts. Rubbing Pump rotor can suffer from rubbing at the annular seal clearance under some critical conditions such as low flow operation. Rubbing can cause unexpectedly high stress in the shaft and may introduce fracture of the shaft. Therefore the impeller split rings are design in such a way that a static hydraulic restoring force will be produced (LOMAKIN effect). During operation the critical speed (wet critical speed) is far from the operating speed. Impeller material The development of high strength corrosion resistance martensitic chrome steels paved the way for the present day boiler feed pumps with speeds of 5000-6000 rpm and today’s full load BFP for conventional 750 MW power blocks are constructed with 4-5 stages with stage pressure up to 80 bar. The high quality precision cast stainless steel impeller, impeller casing are subjected to extreme radiographic inspection.
BRINE RECIRCULATING
PUMP
This is the largest pump in the desalination system. This pump is a vertical mounted in a can (shell) which forms the inlet intake well. It draws water from a low pressure chamber (last stage of heat rejection section) and not from an open sump where atmospheric pressure would contribute a large proportion of the NPSH required by the pump. Pump failure causes serious problems in a MSF desalination plant, for instance no water can be produced
194
if the brine can not be circulated because of a recirculating pump or drive fault. The following points have to be considered in pump choice and design: 1. The pump characteristic will be determined according to service condition of the plant, so the designer should obtain accurate information about the system characteristic. Design of the pump as a single or two stage mixed flow type depends upon different factors: a. Shape of the curve (Q-H) according to Ns for a single stage or two stage in the point of adaptability to discharge capacity fluctuation and head variation. b. Flow control, if it is by adjusting discharge valve opening (throttle control) or by changing the speed of the pump. c. Cavitation performance. d. Comparison of velocity with erosion corrosion for both single stage and two stage. (Velocity at bell mouth, impeller, diffuser casing). e. The cost of spare parts in addition to pump weight and pump length from the economic point of view. 2. Material selection (see section on material selection). 3. NPSH - Provision of adequate NPSH to avoid cavitation in the impeller, safe margin to be considered between the NPSH required for the pump and the NPSH of the system. 4. The vibration characteristic of the drive unit (motor - flange support and intermediate gear) to be matched with the pump that means the fundamental frequency and read critical frequency (critical speed) to be analysed to ensure compatibility between the pump and the motor. (Refer to problems at UAN pumps). In case of turbine drive a particular attention must be paid to reaching an agreement on the critical pump speed because it can lie in the permissible over speed range of the steam turbine.
SEAWATER
PUMPS AND COOLING WATER PUMPS
The seawater pumps and cooling water pumps both are vertical mounted type, deliver seawater to the distillers or to the condenser of steam turbines for cooling. Because of the continuous increase in the size of power stations the cooling water pumps are now built with capacity of 100,000 m3/h at delivery head up to 30 mt. The pumps can be mixed flow or axial type.
195
The production faults represents higher percentage of pump damage. The production fault can be due to planning and design error, erection fault, manufacture fault, incorrect stress calculation or material defect. The following points have to be considered in pump choice and design: 1. The pump can be mixed flow or axial type, as known mixed flow have a steep curve and max efficiency at high discharge, axial flow can cause problems if it is running at low flow. 2. The pump would be matched to the system so that it operates at its maximum efficiency. If due to planning error quote too high delivery head, the pump will then operate continuously in the range of excessive and sometimes unallowable discharge flow and hence always have a poor efficiency. 3. The choice of minimum submergence from MWL (minimum water level) to bell mouth edge is usually very important for preventing the air draw vortex and the cavitation of the pump. S = min submergence from MWL to bell mouth = 1.5 D C = distance or clearance between bell mouth and sump flow = 0.5 D. If an inlet cone is provided, the clearance C will be greater. The inlet cone is an antivortex device to improve sump performance (Fig. 8). In general the submergence of an intake should be large enough, to reduce possible occurrence of air entraining vortex and swirling flow. On the other hand the shortest pump length under the ground level is the best for maintenance, vibration and pump rigidity and of course cost of the civil work. 4. Front wall dimensions and length of approach channel to be chosen to keep the approach velocity low. Length of approach channel = 10 D. Typical mean velocity in a basic sump: Pump inlet = 4 m/s Bell mouth = 1.3 m/s Approach vel . = 0.3 m/s If no empirical figures are available from existing installation for use during the design phase model test is strongly recommended, in order to optimise the intake conditions and reduce cavitation effect. 5. The designer should obtain accurate information of the chlorination of the seawater and to be taken in consideration when selecting the material. 6. Material selection - material has to be chosen to resist corrosion, cavitation, erosion (see part of material selection). 7. Vibration characteristic (see section on BRP).
196
-7.30
i”j f c! (3
-9.00
-1030 --
m
LET CONE
CENTER Of
Fig. 8. An inlet cone.
PUMP
D = 1.5-1.8 /d
197 PREDICTIVE MAINTENANCE
Predictive maintenance is a systematic method of monitoring and trending rotating equipment to determine the condition of the machine. Diagnostic monitoring can reduce down time by detecting abnormal mechanical behaviour at an early stage. It also allows better outage scheduling and should minimize forced outage. Shutdown for repair can be scheduled for convenient time which allow maintenance staff to prepare the work schedule, together with the requirements for manpower, tools and spare parts. Monitoringfor boilerfeed pumps 1. Vibration monitoring for detecting of abnormal operation, this can be on-line vibration monitoring by installing sensor transducers at bearing house in addition phase detection which is important in evaluating source of trouble or it can be periodic system to monitor vibration with small computer, reading can be taken every two weeks and plot the data for comparison and trending. 2. Monitoring of noise level. 3. Lubricating oil analysis is used for trending which we can monitor, the wear particles in the oil samples on a monthly basis. The good quality of oil will increase the life time of bearings. 4. Monitoring of main pump parameters specially at min flow and design point, also monitoring of thrust balance water pressure and axial displacement (wear of thrust bearing), such monitoring is very essential to know the condition of the pump. In addition a periodic checking for the balancing device (balancing disc and counter balance, throttle bush, balancing piston) can give clear indication about the status of the pump. For example, according to monitoring and diagnostic at UAN power station, we have one of the feed water pumps worked about 60,000 hrs without any major overhaul and still in good condition, another feedwater pumps of other manufacturer, of the same capacity and head as the first one worked only 25,000 hrs and it is planned to be overhauled due to excess clearance found during our monitoring and checking. 5. Monitoring and control of pressure and temperature of feed water tank and deaerator: To avoid steaming in the inlet system which can cause damage to the feed water pump, a various pressure gradient limitation control to be applied. Attention to be given to temperature variation in the feed water tank in case of turbo set fall-out and cold condensation flows into the dearator.
198
6. Reverse running monitoring and protection is very essential for feed water pumps, a lack of reverse running monitoring can cause damage for different parts of the pump, the reverse running monitoring must initiate the following: a. Close the discharge valve of the pump, b. Switch on the auxiliary oil pump. In addition, the starting cycle of the electric motor should never be initiated because running the motor at that condition certainly causes extensive damage to the pump. Monitoring of seawater pumps and brine recycle pumps
1. Vibration monitoring by using a portable type vibrometer with small computer, reading can be taken every two weeks and plot the data for comparison and trending. A recent development is the application of submerssible proximity transducer to measure the vibration behaviour of the vertical pumps but this application still not widely applied, to have continuous monitoring. 2. Noise monitoring - It is very essential for BRP and water pumps to monitor noise level at different flow specially at low flow and also at over capacity. The noise generated falls as follows: a. Interference noise resulting from the interaction between flow at entry and exit of impeller. b. Turbulence noise resulting from vortices setup in the flow. c. Cavitation noise. d. Noise due to mechanical sources. 3. Level monitoring to shut down the pump if the level drop below the minimum, to avoid cavitation. 4. Bearing temperature monitoring for thrust bearing and flow control for lubricating water to guide bearing.
BEARINGS
The main two forces affecting on pump shaft are the radial force (FR) and the axial force (FA), the bearing system is designed to absorb these two forces. The bearing can be either Anti Friction Bearing or a plain bearings (sleeve bearing) with a separate thrust bearing (tilting pad thrust bearing).
199
Anti-fiction bearing
It can be divided into three categories: 1. Axial bearing (deep groove bearing) (FA). 2. Cylindrical bearing (roller bearing) (FR). 3. Radial deep groove (FR, FA). The friction coefficient of the anti friction bearings is same, 2550% lower than for plain bearing, other advantage of this type is its more accurate running because of the closer clearances that can be achieved. The disadvantage of the anti friction bearing that it is sensitive to shock loads, noise running, also misal,ignment can effect much on life time of anti friction bearing. As a general roller contact bearing can be accepted in small size pumps but has not proved successful in large high energy pumps, stresses in rolling contact elements are high so as to make fatigue acknowledge factor. When this type of bearings began service in pumps, selected according to life test data, the life of the bearings in many pump applications fell below expectations. The empirical solution for the problem was a load multiplication factor. In some cases even this did not prevent failure. Plain bearing
This type of bearings is to bear the load imposed on the main shaft i.e. the radial load which acts at a right angle to the axis of the shaft. The structure of the bearings differs by the pumps type. In a horizontal pump, the radial load is usually borne by two bearings, the material used is a white metal bearings or lead bronze bearing. These types of bearings are provided outside the casing. The moving part is the shaft or journal, the stationary part which can be in different forms is the bearing shell, such as feed water pumps bearing. There are also the submerged bearing type where the bearings are provided in the pump casing such as the vertical pumps as seawater pumps and brine recirculating pumps. There are different types of submerged plain bearing used in vertical pumps and can be summarized as follows: I. Rubber bearing Rubber bearing have been successfully used up to pheripheral velocities of 6 m/s and bearing load of 20 N/cm*.
200
The lubricating water must pass through a fine strainer to remove sandy impurities. Water lubrication has the advantage that no grease or oil can find its way into the cooling water circuit, but when dry running occurs rubber bearing destroyed on few revolution. It is estimated about O-2 s (refer to problems at UAN pumps). So a separate flow control must be provided before each bearing down stream of the fine strainer, also to the standby pumps, the lubricating water must be continuously supplied for flushing and to avoid blockage or corrosion of the lubricating water pipe lines feeding the bearings. 2. Grease lubricated lead bronze It is necessary to accept bearing loads which exceed the permissible figure of rubber bearing, a grease lubricated lead bronze are often installed, they are suitable for peripheral velocity up to 12 m/s. Such bearings are continuously lubricated by a grease pump drive from main pump shaft or separate motor. The bearings must be fed with grease 60 seconds before the pump started. 3. Ceramic bearings Development in recent years has proceeded in direction of fully maintenance free bearings, for this reason hard metal ceramic bearings used specially in pumps for continuous operation (cooling water pumps). Ceramic bearings are continuously lubricated from the discharge of seawater pumps, cooling water pumps. there is no hazard if small particles penetrated into bearing gap, also during starting operation it is not necessary to pre lubricate upper bearing which has been left dry. 4. thrust bearing
a. Vertical pumps (seawater pumps and brine recirculating pumps) An external thrust bearing on the vertical pumps have to sustain the entire weight of all the revolving parts that lie below. The bearing is of tilting pad oil bath lubricated and water cooled due to combination of loading and high ambient temperature. b. Feed water pumps - Balancing device and compensation of axial thrust in feed water pumps. The high total head of multi-stage feed water pumps creates axial thrust (force) on the pump rotor. This axial force results from pressure difference acting on unbalance impeller surface area (between suction and delivery of the impeller) acting toward suction side. The bal-
201
ancing device creates an axial thrust in the opposite direction to that generated at the impellers (see Fig. 9). A segmental pad thrust bearing arranged at the discharge end and capable of absorbing thrusts in both directions serves to absorb any small residual thrust and at the same time locates the shaft axially (Fig. 10).
Fie. 9.
Fig. 10.
202 PROBLEMS FOR PUMPS AT UMM AL NAR
Distillate pump damage (4 mgd distiller)
Incident: The distillate pump tripped during its start up, while changing over from working pump to standby. The state of the damage to the pump are as follows: Twisting of main shaft, distortion and shearing of coupling key. The intermediate bearing surfaces were burned and the lower bearing was more severely burned. Shearing of the impeller key, shearing of flat heat screw for impeller wear ring. Cause of failure
The failure was caused by the shortage of water at cutless bearing, this has caused the dry rotation at the start up, burning the cutless bearing and creating a large clearance. The big clearance has caused the impeller to touch the casing and cause pump damage. The cause of water shortage at the cutless bearing, that the distillate pump suction barrel is connected to the evaporator which is under vacuum, the minor air leak which occur through the gland packing of the standby pump creates a positive pressure inside the casing which pushes down the water level in this casing to a level below that of the cutless bearing which become dry. Action
A balancing line 15 mm diameter have been installed connecting pump casing to pump barrel to equalize the pressure in both and in order to keep water level in pump casing to avoid drying of the cutless bearing. Problem of dry running has occurred twice in a 5 mgd desalination plant. Cooling waterpump for 65 mw unitsfailure of muflcoupling nuts and studs
During the major overhaul of the cooling water pump in 1984, the muff coupling nuts were found seriously corroded and in brittle form which could result in a serious breakdown of the pump. The material of muff coupling nuts and studs is l-4460 DIN standard. Sample of the defective parts were sent to the manufacturer. The examination was carried out by him, it was found that damages were caused by a selective corrosion attack. The definite cause of this could not be clarified,
203
but there is no doubt that a considerable chlorination of the flow medium aggravates corrosion due to the fact that chlorination procedure was modified, and to be injected at the upstream of the band screen instead of in front of pump chamber. In view of operational safety the screw and nuts of muff coupling material are replaced to DIN l-4462, which is more resistant to corrosion attack. Seawater recirculating pump (4 mgd distiller)failure of journul bearing of the pump
The pump is double suction volute casing, the two radial bearings were white metal journal bearing with oil ring lubricated system, the thrust bearing is an angular contact ball bearing. In the field operation, the coupling side bearing metal have been damaged. On investigation by the manufacturer, the condition of damage were found as follows: a. Damage to the metal is concentrated at coupling side bearing. The anti coupling side is free from any damage. b. The place and area of damage metal at the upper metal side (upper shell of bearing). From the above condition, the radial load is applied into upward direction, which is unexpected. The oil ring is attached at the upper bearing metal and when upward direction radial load is applied, radial load receiving area of the metal is reduced and the oil film became insufficient, to the load thus the damage happened. Because of the reasons as above described upward direction load is applied to the pump it is difficult to protect this damage. So, the metal type bearing has been replaced by a spherical roller bearing which can withstand any directional load. Cavitation on brine recirculating pump impeller (4 mgd) distiller
The impeller of this pump is a mixed flow shrouded type with five impeller blades. By inspection of this impeller after = 20,000 hr it was found a cavitation effect appeared as erosion corrosion near the inlet of impeller blades. The following measurements have been taken: nominal thickness of impeller blade = 13 mm nominal length of impeller blade = 1075 mm
204
max. erosion 3 .I3thickness min. erosion 1.8 thickness
x x
200 mm length 130 mm
Partial load operation can cause such erosion corrosion by cavitation phenomenon also suction strainer blockage and in this condition continuously operating the pump at its nominal capacity can create cavitation effect on the impeller. A new impeller was installed and the old one was repaired by welding after receiving the manufacturer’s advice. High vibrationfor brine recirculating pump (5 mgd) distiller
When starting the brine recirculating pump it was noticed that the vibration value of BRP and its motor was extremely high. After investigation by the manufacturer and many trials to decrease the level of vibration by balancing of motor pump set at site and by improving the alignment. The vibration problem was solved by achieving a big enough difference between the fundamental natural frequency of the whole system and the rotating frequency of the pump. The technical solution is found in the connection of motor lantern and pump to be tight fit between motor stool and pump, which leads to the increase of fundamental natural frequency. This has been achieved by inserting thin plates between guide ring of motor lantern and the stationary ring at the thrust bearing lantern. After the above solution, the vibration data records confirmed the solution. Severe cracks in dl@erentparts of the seawater ing pumps
pumps
and brine recirculat-
Stress corrosion cracking has occurred in different parts of seawater
pumps and brine recirculating pumps at UAN causing a serious threat. The material of these parts is “Ni-Resist Ductile”. The stress cracking is probably internal stress resulting from lack of stress relief heat treatment or incorrectly performed stress relief. The cracks were found in the following pumps in the UAN area: a. Seawater pumps (4 mgd) distiller -
A big crack was developed in upper and lower column pipes, to put the pump back in service a new column pipes manufactured, locally by carbon steel coated. b. Seawater pumps (7 mgd) distiller - The set of guide vanes of the diffiser casing had cracks on every vane, the cracks appeared at the fillet between the van and outer casing, it appeared to be initiated at the top corner of this fillet and propogated down wards along the filter in the direction of the impeller.
205
These cracked vanes repaired by the manufacturer by welding and stress relieving. c. Brine recirculating pumps (5 mgd) - Two discharge elbows a big crack due to stress corrosion was noticed. The elbows have been replaced, recently big cracks were noticed in the diffuser vanes, some of these vanes already broken and separated from the diffuser. The crack of the diffiser vanes appeared on two pumps. d. Brine recirculating pumps (7 mgd) - On these pumps, cracks have developed on the diffuser vanes also, the diffuser vanes had been repaired by Belzona material. Finally, it is needed to mention that we have seawater pump and brine recirculating pumps some of them passed more than 85,000 h, its diffuser casing material is “stainless steel casting” and during inspection no sign of cracks or corrosion exist. Cooling water pumps 180 MW
Two modifications have been, done the first modification is by trimming the impeller to decrease pump head and flow, the second modification was in pump entry to decrease the high noise level. a. Trimming ofpump impeller
According to the specificationcooling water pump shall operate under the worst condition, low water level (-5.1 mt) and cleaning factor of condenser tubes 0.85. During the test period it was found that the results of the theoretical pressure loss calculation are higher as the actual pressure loss of cooling water pipes, also the low water level (-5.1 mt) was not reached, max. LWL was (-3.8 mt) during this period. For correction the delivery head a reduction in impeller of the cooling water pump by machining from 1442/904 to 1405/904 after that the impeller was balanced.
Q, m3/h H, mt
With modified impeller
Before
33,250 11
36,000 14
206
b. Modi@cation ofpump entry
When starting the cooling water pump it has been noticed that pump cannot be operated safely in continuous operation due to constant noise running and pressure pulsation. The manufacture studied this problem and stated that running noise and pressure pulsation can be reduced by removing the rib parts of the inlet rectifier (inlet cone). A complete study have been done by the manufacturer including model test to have complete information concerning the flow condition to the pump, an evaluation of the velocity profile in front of impeller has been done. Based on the model test, the modification has been done on the inlet cone (rectifier) and it gave positive response. 1. Modification of the inlet rectifier with 5 antivortex ribs to 3 shortened anti vortex ribs. 2. Arrangement of central longitudinal fin (splitter) in the pump chamber.
REFERENCES
1 Centrifugal Pumps for General Refinery Services (API Standard GlO) American Petrolium Institute. 2 The Hydraulic Design Pump Sumps and Intakes (M.J. Prosser Mami Mech. E.) Bhra Fluid Engineering (1977). 3 Allianz Hand Book of Loss Prevention (May 1987). 4 Kent’s Mechanical Engineers Hand Book. 5 Kubota Pump Hand Book. 6 Centrifugal Pump Lexicon (K.S.B.).