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7/27/2019 pump-1.doc


Centrifugal Pump classification:

Centrifugal pumps can be classified depending on the design and installation

features.

• Casing design :

1) Volute casing – diffuser casing2) Vertical split casing – horizontal split casing

• Impeller design :

1) Semi open – open2) Single suction – double suction

3) Radial flow – mixed flow – axial flow

• Shaft suppor t

1) Overhung impeller also called Cantilever pump2) Simply supported shaft

• Suction condition :

-- Negative suction (vacuum or sub-atmospheric)-- Positive suction (flooded suction)

. No. of Impellers

Single stage –– two stage --- multi stage

• Installation :

Horizontal - vertical – submersible

Centrifugal pump can be further classified according to different casing designs,bearing arrangements, impeller arrangements etc.

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Different parts of a centrifugal pump and their functions:

Shaft:

It holds all the rotating elements of a pump. It drives the impeller.

Shaft sleeve:

It covers the shaft in the region where it is prone to corrosion, erosion or abrasion. It can be replaced when it wears out without replacing the entire shaft.

Impeller :

The impeller driven by the shaft imparts kinetic energy to the fluid by its dynamicaction.

Casing:

It converts the KE of the fluid into pressure energy. The available crosssection/volume for the liquid generally increases in the casing towards thedischarge nozzle.

Pump out vanes / Expeller vanes:

These vanes provided at the rear side / discharge side of the impeller drives thefluid away thereby reducing the pressure at the back of the Impeller. Thesevanes help to restrict the leakage of pumped liquid into the stuffing box thusreduces the load on the seal.

Auxiliary impeller :

It is a separate impeller positioned at the back of the impeller. It serves the samefunction of pumpout vanes.

Stuffing box:

The leakage of the pumped liquid into the atmosphere through the gap betweenshaft and casing is arrested in the stuffing box. It may house gland packing or amechanical seal.

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Impeller wear rings:

These are fitted at the suction side and discharge side of an impeller to protect itfrom wear and to reduce recirculation of discharge fluid back to suction.

Casing wear rings:

These are fitted in the casing at the front side and rear side of the impeller toprotect the casing from wear. These wear rings have little diametrical clearanceswith the Impeller wear rings restricting the leakage resulting in improvedvolumetric efficiency of the pump.

Vortex breaker / stop piece:

It is provided in the suction pipe near the impeller eye to avoid pre-rotation or

whirl of the fluid entering the impeller. This avoids turbulence in the fluid andreduces entry losses.

Casing throat bush:

This bush provided in the casing rear, before the stuffing box with littlediametrical clearances with the shaft restricts the leakage of fluid into the stuffingbox. This is generally made of a soft material like aluminum.

Lantern ring:

This is provided in stuffing boxes housing gland packing. This ring providescooling medium to the packing and also prevents the entry of air into the pump.

Deflector:

It is a ring fastened to the shaft at bearing end covers, which prevents the entryof water and dust into the bearing housing.

Bearings:

They take the axial loads and radial loads on the shaft. They also provide supportfor the static loads on the shaft.

Gland packing:

Soft and self-lubricating packing rings are provided in the stuffing box around theshaft sleeve to reduce the leakage of pumped fluid into the atmosphere.

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NPSH available:

It is the net positive suction head available at the eye of the impeller consideringall the head losses.

NPSHA = Pa /d + hs + Ps /d - Pv /d – hf – hi

Pa /d = atmospheric head acting on the liquid free surface

hs = static head from the free surface of the liquid to the centre of theimpeller.

d = density of the liquid at the operating temperature

Ps = suction pressure.

Pv = Vapour pressure of the pumped liquid at the operating temperature

hf = Head losses due to friction in the suction pipeline.

hi = Entry losses at the pump entry due to turbulence

hs and Ps terms are deducted it the suction source is below the centreline of thepump and when the suction pressure is negative respectively.

NPSH Required:

It is the pressure required at the impeller eye for satisfactory pumping. It is givenby the pump manufacturer. NPSHA > NPSHR for satisfactory operation of thepump.

a) NPSHR will increase with the fluid temperatureb) NPSHR will increase with the speed of the impeller c) NPSHR will increase with the capacityd) NPSHR will increase with decrease in viscosity

Also NPSHA decreases with increased capacity.

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Characteristic Curves of a centrifugal Pump:

Efficiency, power consumed and the head are plotted against the capacity or flow. We observe that Head falls parabolically, Power increases almost linearly

and efficiency varies as the flow across the pump increases. Efficiency rises to apeak point and then falls in a parabolic fashion. The pump should be made to runclose to the peak of efficiency curve. The head at zero flow or shutoff condition of discharge valve is called the shutoff head.

Minimum bypass valve:

In multistage centrifugal pumps, the temperature of the fluid rises rapidly if thepump is run with the discharge valve closed. Due to the temperature rise of thefluid, the internal clearances will be affected owing to different thermal expansioncoefficients of static and rotary parts in the pump and may lead to rubbing andconsequent seizure of the pump. Pumps handling condensate are prone tocavitation also, as slight temperature rise may lead to evaporation of condensateoperating close to the saturation curve.

The minimum by pass valve provided in the discharge line recycles the dischargefluid back to suction and dissipates the heat generated in the pump.

The quantity of fluid to be bypassed is designed according to the pump designflow, head and temperatures. The d/s of minimum bypass valve should not beprovided with isolation valve to avoid any inadvertent closing of the valve.

System head curve:

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It is the sum of all heads (absolute). The operating point is the intersection of thiscurve and the characteristic curve. This operating point is to be kept close to thehighest efficiency point for good performance of the pump.Radial thrust:

Radial thrust occurs on the shaft due to unequal forces acting side ward on theimpellers in the volute casing. This becomes more prominent when the flow isless (i.e. head is high). Another cause of radial thrust on the shaft is itsSelf-weight acting vertically downward.

Radial thrust can be avoided / reduced by providing a double volute casingwhere the force acting on the impeller due to the volute is greatly reduced.

Another way of balancing radial thrust is providing a diffuser casing where thepressure is uniform through out the casing around the impeller.

Axial thrust and it’s balancing:

In a centrifugal pump impeller, the unequal force components acting on the faceand back of the impeller due to differential pressures lead to an axial thrust onthe shaft towards the suction. This becomes more predominant in high-pressuremultistage pumps as the number of impellers arranged in tandem increase.

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Thrust = Pd Ab – Pd A1 – Ps (A2+ A3)

Thrust = Pd Ab - Ps (A1+A2+ A3) – (Pd - Ps )A1

Thrust = Ab (Pd - P s) -- A1(Pd - Ps )

Thrust = (Pd - Ps) (Ab - A1 )

But Ab = pi/4 ( (impeller O.D)2 - (shaft O.D )2 )

and A1 = pi/4 ( (impeller O.D)2 - (wear ring O.D )2 )

Hence net Thrust = (Pd - Ps) pi/4 ( (wear ring O.D )2

- (shaft O.D )2

)

This shows that net thrust is positive and directed towards suction.

This thrust can be balanced in small single and two stage pumps by:

1) Maximizing A1 in the impeller 2) Using a double suction impeller 3) Using Opposed impeller design4) Providing balancing holes5) Provided both suction and discharge wear rings.

6) Reducing wear ring outer dia.

However, in multistage High-pressure centrifugal pumps, balancing devices suchas balancing disc or balancing piston are used.

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1) Increase of balancing line pressure indicates wear of balance piston.

2) Balancing disc adjusts clearance automatically so as to maintainequilibrium. This cannot be used in volatile and inflammable services asrubbing between Balancing disc and counter balancing disc may lead tofire. A lift off device is required in this type of an arrangement. The lift off device pulls the balancing disc away from the counter balancing disk for some time initially during startup of the pump. The balancing disk isreleased when sufficient pressure is developed in the final discharge. Therelease in automatic. Disc arrangement doesn’t require thrust bearings.

3) Balancing piston reduces thrust to a large extent. But during the start of apump, when the discharge pressure is not developed fully, thrust loadsexist and thrust bearings are to be installed to take care of such loads.

4) Both disk and piston arrangements are simultaneously used. The pistonacts when the disk fails.

Pump standards:

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built up by direct welding / spray welding and machined later to the standarddiameters. Proper care is to be taken during welding in selection of electrodes and welding procedures. Bushes of lesser thickness peel off during running.

4) Excessive runouts: High runouts (> 0.1mm.) can be reduced by plastic

bending if the shafts are long and of lesser diameter. Proper fixing of shaft isrequired in bending operation as it may lead to runout in other areas of theshaft. Shafts are to be kept in the bent position in the direction required for some time for proper retention of shape.

5) Runouts in multiple shafts: Runout of individual shafts are to be addedabsolutely to get the maximum possible runout of the shafts’ assembly.Individual shafts are to be checked for runout at different areas and max.runout is to be considered which is to be within limits (< 0.05 mm say).Bends in individual shafts can be minimised as described in the earlier part.

6) Impeller and brg. Locknut threading failures: If the thread damage isminor, the threads can be reworked. If the damage is serious, threads of higher/lower size can be machined depending on the requirement.

7) Failures due to improper dismantling of bearing assemblies, coupling hubs: Generally shafts are made of hardened carbon steels and martensiticstainless steels. (SS-410, 430). Proper jacking is to be done while removingcoupling hubs. Bearing inner races should be removed by knocking on allsides using softer brass/ Cu / Al rods. In case heating is required, the raceshould be heated uniformly with neutral / oxidising flame cooling the adjacentarea of the shaft. Simultaneously knocking / jacking may be employed.

Usage of Anti- seize compounds in high interference and high temp. serviceprone to fretting, galling etc. avoids jamming of bearings and coupling hubs.

8) Excessive loading of shafts: The process parameters should be studiedthoroughly and the loads on the shaft are to be estimated. The suitability of the shaft should then be assessed.

Failures related to Impellers :

• Impeller with excessive clearance on the shaft.

• Impeller locknut / Helicoil failures

• Impeller wear ring failures

• Impeller seizure / rubbing in the casing

• Acoustic failures of impellers

• Pitting on impellers in erosive fluids.

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Repairs on impellers :

• Impellers with excessive clearance: Impeller should have a push fit or a

minimum clearance (<0.01 mm) with the shaft. If the clearance is high, itleads to impeller keyway failures and locknut failures. Excessive clearancecan be reduced by fitment of a bush (with interference) of sufficient thicknessafter machining the impeller. This requires keyway milling on the bush.

Alternatively metal build up and machining can be employed. This method isdifficult as the impeller is generally cast and weld deposition poses problems.

Alternatively a shaft of higher suitable diameter may be made.

• Impeller locknut / Helicoil failures: New locknuts and helicoils are to be

used wherever possible. Helicoils can be replaced by double locknut in caseof unavailability of spare helicoils.

• Impeller wear ring failures: Impeller wear rings are to be fit properly. Proper

clearances (design / protocol ) are to be maintained and material of the wear ring should be superior. CI / Hardened steels should be used.

• Impeller seizure / rubbing in the casing: Impellers seize / rub because of

lower clearances i.e. the front clearances, the back clearances and the wear ring clearances. Impeller locking and positioning on the shaft need to bechecked thoroughly during assembly to avoid problems during running.Corrective action can be taken depending on the type of failure.

• Acoustic failures of impellers: Acoustic failures occur because of the

matching of impeller vanepass frequencies and the impeller angular speedsor their harmonic frequencies. They also occur because of the impeller speed

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process conditions, cavitation , and higher pump speeds can be probablereasons of bearings failures. Remedy can be employed by studying theexact reason behind the failures.

• High bearing temperatures: Incorrect oil grade, improper lubrication, excess

lubrication, higher loads, improper alignment and improper CW flow to thebrg. housing water jacket may lead to high bearing temperatures. Theproblem can be solved by studying the reason.

Note :Radial play is to be at a minimum depending on the diameter and length of shaftfrom bearings. Axial float is to be adjusted such that the thermal expansion of thebearing races is to be taken care of.

Sealing failures :

• Leakage due to ovality of stuffing box.

• Lantern ring failures

• Packing failures

• Seal flushing failures

• Frequent seal failures

• Rubbing of seal faces

• Shaft sleeve failures (scoring)

• Mech. seal internals’ failures

Repairs that can be done for seal failures :

• Stuffing box ovality: The ovality of stuffing box has to be removed by

machining for correct sealing.

• Lantern ring failures : Correct Lantern and CW arrangements are required

for proper sealing and sleeve life.

• Packing failures: Proper packing material which has low friction coefficient,

which is self lubricating and which is compatible with the service liquid is to beused for long life of packing and to avoid scoring of sleeves.

• Seal flushing failures: Correct seal flush flow required for the seal is to be

ensured by providing proper orifice in the flushing line. Seal quench is to beprovided wherever prescribed by the seal manufacturer. Pumping rings are tobe installed wherever recommended.

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System head = total suction head + total disch. head + headdue to friction losses (all absolute)

Operating point is the intersection of system head curve and pump

characteristic curve.

• Pipe line stresses due to improper piping of suction and discharge lines.

Stresses are to be lesser than allowed by the pump manufacturer.

• Failures like seizure of wear rings, balancing piston etc. because of rise in

temperature of operating fluid due to closure of discharge valve for a longer time than that allowed by the pump manufacturer. This can be avoided by theinstallation of Auto recirculation valve.

• Improper functioning of ARV resulting in seizure of high pressure multistage

centrifugal pumps like BFW pumps.

• High velocities (more than allowed) in the suction and discharge lines

sometimes due to insufficient size of the pipelines leading to erosion in the

pipes.

Normal fluid velocities:

Suction line - 1 to 1.5 m/sec.Discharge line - 2.5 to 3.5 m/sec

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