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Pumps

Types of pumps used in the plant

Centrifugal PumpsCentrifugal pumps are used in industry for applications where largevolumes of fluids must be moved.Centrifugal pumps have been widely accepted in the petroleumindustry because of their versatility, simple construction, and lowinitial cost. Operating costs are lower for centrifugal pumps thanother types because of minimal maintenance and ease of repair.

Unlike positive displacement pumps, centrifugal pumps will not

continue to produce a head when operating against a closeddischarge. Centrifugal pumps perform best when pumping low-viscosity fluids.Centrifugal pumps can be classified in several ways. They can bedivided according to: The kind of impeller they contain.

The number of stages they have.

Their ais of rotation.

The method used to drive them.

Their configuration or appearance.

!ecause centrifugal pumps are available in a great "arity of stylesand have many different uses, they do not always resemble eachother. They all have the same operating characteristics.

Centrifugal Pumps Components

The ma#or parts of a centrifugal pump are the casing, impeller,shaft, coupling, bearings, and seals or packing $%igure &.&' (&.&)*

1

Figure 1.15 Schematic Drawing of Centrifugal Pump Figure 1.16 Typical Centrifugal Pump

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A. The casingis the largest and most visible part of the pump. +t

can be constructed of cast iron, steel, brone, or other special

material depending on type of service. The primary purpose of the

casing is to house and protect internal pump parts.

. An impeller is the part of a centrifugal pump which impartsenergy to the fluid being pumped. +t is firmly attached to the shaftand rotates at the same speed as the shaft. ost impellers aremade of cast iron, but stainless steel, plastic, or other materialscan be used for corrosive fluids.

C. The couplingwhich connects the driver to the pump transmitspower from the driver shaft to the pump shaft. Couplings must beable to withstand sudden changes in pump load or stoppage of thedriver. They must be fleible enough to handle misalignmentbetween the shafts as well as changes in the speed of the driver.

D. The shaftis connected to a driver $e.g., electric motor, engine,or steam turbine* and turns the impeller. The shaft is usually madeof steel and rotates at the speed of the driver.

!. earingssupport the shaft and reduce the friction as the shaftrotates in the casing. They also control the forward and backwardmovement $thrust* of the shaft, and control the side to side $radial*movement of the shaft so that rotating parts will not rub against thepump casing. !earings may be contained in the pump casing onsmall process pumps or in special housings on larger pumps.

F. Seals or pac"ingare used to prevent or reduce fluid leakagearound the shaft. ost centrifugal pumps in the petroleum industry

use mechanical seals. mechanical seal has a stationary ringsecured in a seal gland which is bolted to the casing and a rotatingring attached to the shaft. /acking is composed of a series ofpliable rings tightly pressed around the shaft in the stuffing bo.

P#S$T$%! D$SP&AC!'!(T P)'PS

$ntroduction/ositive displacement pumps are used in the petroleum industry

for low volume applications or where high pressure is re0uired tomove fluids

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1eciprocating pumps have a piston, plunger or diaphragm movingback and forth $reciprocating* in a cylinder

#perating Principle

/ositive displacement pumps work according to an old law ofnature2 that is, no two things can occupy the same space at thesame time. n eample: %ill a bucket to the top with water asshown in %igure 3.45 and then carefully drop a rock into thebucket. 6ome of the water is forced out of the bucket because therock and water cannot occupy the same space at the same time.The rock displaces a volume of water e0ual to the volume of therock.

%luid is drawn into the cylinder, then displacement by a piston,plunger, etc, and the fluid is forced out the cylinder. %luid pressureis increased in the pump by the piston, plunger, etc. pressingagainst the fluid in the cylinder.

!ecause of all the moving parts in positive displacement pumps,lubrication is very important.

+n addition to lubrication, positive displacement pumps need to becooled. /umps may be either air-cooled or water-cooled. ir-cooling relies on ambient air to cool the pump while water-cooling

re0uires the circulation of water or another coolant through thepump casing. +n larger pumps or in pumps moving high

3

Figure 1.*+ Positi,e Displacement Pump

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temperature fluids, water #ackets are commonly used. 7ater#ackets are channels located in the casing around the hot sectionsin the pump, such as bearings and packing locations.

+f the discharge line becomes blocked, the piston, plunger, etc.cannot displace the fluid in the cylinder and pressure will build up.8cessive pressure can cause the pump driver to stall or the pumpcasing and9or discharge piping to rupture. %or safety, positivedisplacement pumps should always have a safety relief valve andbypass line located a short distance beyond the discharge port.

reciprocating pump works with a back-and-forth, straight-linemotion.Piston:

%luid-end pistons convert mechanical energy into fluid movement.+n the steam end of the direct-acting pump, the piston converts thesteam into mechanical energy.Piston ring:

piston ring acts as a seal between a piston and the cylinder inwhich it is operating. The ring moves with the piston.Plunger:6ome pumps, like the one shown +n %igure 3.3&, use a plungerrather than a piston in the fluid end. The plunger slides back andforth in a stationary packing rather than carrying its own seal.Cylinder-The cylinder is a tubular chamber that contains the piston orplunger.Cylinder head:The cylinder head is a cap that seals the cylinder to allow pressurebuild-up.Stuffing o/:

stuffing bo is filled with packing to prevent fluid leakage from thecylinder. +t surrounds the plunger, piston rod, and connecting rod.

%al,es: slide valve controls the flow of steam into the steam end of thedirect-acting pump. On the fluid end of the pump, suction anddischarge valves control the flow of fluid into and out of the fluidcylinder. The valve on the steam end is mechanically actuated.The valves on the fluid end are material actuated.Cran"shaft:+n the power pump2 the crankshaft transmits motion from the primemover to the driving components in the power end of the pump.

Crosshead:

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The crosshead of the power pump converts the rotary motion ofthe crankshaft connecting rod into reciprocating motion.

0eciprocating Pump Types

A. Piston Pumps

One of the most familiar reciprocating pumps is the piston pump$%igure &.3*. The main components of this pump are the cylinder,piston, piston rings, suction valve, discharge valve, packing, andpump casing. The piston moves back and forth in the cylinder.8ach complete movement of this piston along the cylinder length iscalled a stroke. ovement of the piston toward the driving section

of the pump is called the backstroke $%igure &.35*. forwardstroke is movement of the piston away from the driving section ofthe pump. full stroke is the movement of the piston from one endof the cylinder to the other end and back to its original position

ear Pump

There are many types of positive displacement rotary pumps, andthey are normally grouped into three basic categories that includegear pumps, screw pumps, and moving vane pumps.

There are several variations of gear pumps. The simple gear pumpshown in %igure &; consists of two spur gears meshing togetherand revolving in opposite directions within a casing. Only a fewthousandths of an inch clearance eists between the case and thegear faces and teeth etremities. ny li0uid that fills the spacebounded by two successive gear teeth and the case must followalong with the teeth as they revolve. 7hen the gear teeth meshwith the teeth of the other gear, the space between the teeth is

reduced, and the entrapped li0uid is forced out the pumpdischarge pipe. s the gears revolve and the teeth disengage, thespace again opens on the suction side of the pump, trapping new0uantities of li0uid and carrying it around the pump case to thedischarge. s li0uid is carried away from the suction side, a lowerpressure is created, which draws li0uid in through the suction line

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ear pump

SC0!2 P)'PS.

6everal different types of screw pumps eist. The differencesbetween the various types are the number of intermeshing screwsand the pitch of the screws. %igure ' shows a double-screw, low-pitch pump2 and figure ) shows a triple-screw, high-pitch pump.6crew pumps are used aboard ship to pump fuel and lube oil andto supply pressure to the hydraulic system. +n the double-screwpump, one rotor is driven by the drive shaft and the other by a setof timing gears. +n the triple-screw pump, a central rotor meshes

with two idler rotors.+n the screw pump, li0uid is trapped and forced through the pumpby the action of rotating screws. s the rotor turns, the li0uid flowsin between the threads at the outer end of each pair of screws. Thethreads carry the li0uid along within the housing to the center ofthe pump where it is discharged.ost screw pumps are now e0uipped with mechanical seals. +f themechanical seal fails, the stuffing bo has the capability ofaccepting two rings of conventional packing for emergency use.

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Figure 4-Double-screw, low-pitch pump.

%igure ' Triple-screw, high-pitch pump

Selection criteria of plant pumps

Parameter CentrifugalPumps

0eciprocatingPumps

0otary Pumps

#ptimumFlow andCapacity

'edium34igh &ow Capacity &ow3'ediumCapacity

Pressure

Application

&ow3'edium

Pressure

4igh Pressure &ow3'edium

&ow Flow0ateCapaility

(o es es

0euires0elief %al,e

(o es es

Smooth orPulsatingFlow

Smooth Pulsating Smooth

%ariale orConstant

%ariale Constant Constant

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Flow

Self7priming (o es es

Performancewith

,iscosity

(ot suitalefor high

,iscosity

Suitale forhigh ,iscosity

#ptimum withhigh ,iscosity

The functions of the pumps in the plant

Should e prepared y 'opco

Start up and shut down procedures

The operator can prolong the life of his centrifugal pumps andlower the maintenance fre0uency and cost by using sound orderlyprocedures for starting and stopping centrifugal pumps.

A. Starting up Procedure1.The operator should check where applicable:a. Cooling water supply to pedestal is commissioned.. /ump #acket cooling water supply is commissioned.c. otor cooling is commissioned.d. have the right type oflubrication and that are lube oil levels.g. Check that the electric supply is available i.e., the switch gear isrest and okay.

(ote-it depends upon production practice, but in most cases, thespare pump in any service should be left with the suction valveopen and the pump under suction pressure. ?is should not stop theoperator carrying out all the routine checks before starring the

pump. There is no ecuse not to check oil levels or cooling water,etc. before starting a pump.8. 7ith the discharge valve cracked open, start the pump > ifelectric, by pressing the button.9.7atch discharge pressure gauge reading come up to normal. +fthe pump motor has an amp meter check the load. +f the loadremains higher than normal or if there is no discharge pressure,check for the reason and shut down if this condition remains. 6hutdown the pump and inform your supervisor immediately.

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5.+f the pressure is normal and the motor load is normal, open thedischarge valve slowly.6. 188!81 TO C?8C@ /U/ 18

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Aou can monitor:

6tuffing bo temperature. 6tuffing bo pressure.

=i0uid leakage out of the stuffing bo, or air leakage in. 6tuffing bo #acket inlet and outlet flow 6tuffing bo #acket inlet and outlet temperature. 6eal gland flush pressure, flow and temperature. The temperature, pressure and flow of the fluid between dual

seals. Convection tank temperature, pressure and level. Duench temperate and flow. "ibration.

T4! !A0$( CAS!

Aou can monitor:

Oil temperature to let you know if the oil is about to formvarnish or coke.

Oil level. Case pressure. 6haft movement or thrust

The amount of water present in the oil. 6haft speed. "ibration Cooling coil inlet and outlet temperature, pressure and flow.

Capacity control of the pumps in the plant

17 Centrifugal pumps

D$SC4A0! T40#TT&$(

2 6ince the pump eists to serve the re0uirements of theprocess, and one of the primary purposes of instrumentationis to adapt the e0uipment to the process, let us consider thepump from the point of view of the process. +t can be viewedas a constant pressure device with an internal restriction.

3 +t is the restriction that gives it the EcurveE. +t seems natural to

put a valve on the discharge to further restrict the pump. This

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has the effect of rotating the curve of the pump9valve systemclockwise around /pm, as can be seen in %igure &-3.

4t this point + must warn the reader that we are about to

encounter a paradigm shift.5 The combination of pump and valve will be presented as a

Eblack boE with a single characteristic curve which + shallterm the EmodifiedE pump curve.

6 The more traditional way of looking at the situation is fromthe point of view of the pump.

7 +t sees the process system curve as having rotated counterclockwise around /lm. %igure &-3 shows that the flow, D&, isthe same for both cases. The difference between the twopressures is the Belta / across the valve. 6ince the purposeof the pump is to serve the process re0uirements, and thepurpose of the valve is to adapt the pump to the process, itmakes sense to consider the valve to be part of the pumpsystem and to use the modified pump curve rather than themodified system curve in our discussion. +n any case it can

be seen that a discharge valve can be used to achieve any

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operating point on the system curve so long as the point isbelow the pump curve.

8 S)CT$#( T40#TT&$(

9 The second possibility for control using valves is to place thevalve in the pump suction line. This would have an identicaleffect on the characteristic curve, but the method has a fatalflaw > cavitation. Cavitation is a phenomenon that occurswhen the pressure of a li0uid is reduced below its vapourpressure and brought back up above the vapour pressureagain. !ubbles of vapour form in the li0uid and then collapseupon arriving at the higher pressure region. The collapse

occurs at sonic speed e#ecting minute #ets of etremely highvelocity li0uid. 7herever these #ets impinge on a solidsurface etreme erosion occurs. Over time even the hardestmaterials will be destroyed. Therefore it is of utmostimportance that this pressure reduction never occurs. +t isprevented by having sufficient pressure available at thepump suction so that the pressure drops that occur as theli0uid is drawn into the eye of the impeller are at all timesabove the vapour pressure of the li0uid at its current

temperature.

10 n eplanation of the term et /ositive 6uction ?ead$/6?* is in order. This is the pressure of the li0uid at thepump suction in terms of feet or meters of li0uid head abovethe vapour pressure of the li0uid. The actual /6? underoperating conditions is called /6? and the minimumre0uired by the pump to prevent cavitation is called /6?1.Clearly /6? must be greater than /6?1 to avoidcavitation. +t is safe to leave a margin of about one meter.

11 These peculiar definitions are very reasonable in termsof the pumps actual characteristic but they cause someproblems to the controls engineer. +t means that the gaugepressure e0uivalent of a given /6? is proportional to thedensity of the li0uid and is also affected by its temperature.The vapour pressure can rise dramatically as thetemperature rises. This means that the /6? can fallwithout a noticeable change in pressure.

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12 nything that would reduce the net positive pressure atthe pump inlet below the /6?1 must be absolutelyavoided. Thus suction throttling is never used to controlpump flow.

0!CC&! C#(T0#&

14 The third remaining possibility for pump control withvalves is to bleed some of the discharge flow back to thepump suction or to some other point on the supply side.Once again we can view the result as a modified systemcurve or as a modified pump characteristic. %igure &-; showsboth. 8ach curve is a rotation of the original: The modified

system curve as a clockwise rotation around /lm. ote thelittle EtailE at the left of the modified system curve. Thisrepresents the flow through the recycle valve before thedischarge check valve opens to the process. The modifiedpump curve has a counter clockwise rotation around thehypothetical intersection of the pump curve with the flowais.

15

This family of curves shows several problems withrecycle control. %irstly, the pump is not rated to discharge

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more than the flow rate at the end of the curve. +t is possible,of course, to run the pump with a wide open discharge,minimum B /, but it is unhealthy for this particular pump torun at such a high rate. 8cessive flow may cause cavitation

damage. $8cess flow cavitation is not caused by /6?problems but by high velocity within the internal passages ofthe pump.* This restriction means that the minimumdischarge pressure may not be lower than the onecorresponding to the maimum flow. +n other words, themodified pump curve cannot reach all points on the systemcurve.

16 6econdly, although many pumps are capable of

operating near ero discharge pressure, the very flatpressure vs. flow curve for much of the lower range for mostpumps means a change of flow has very little effect on thedischarge pressure.

17 Thus it would take a very large amount of flow toproduce a small drop in pressure. +n control terms thismeans that control would be very FsloppyF. Bischargethrottling on the other hand, allows the pump to develop the

head that FsuitsF it. The unwanted pressure is dropped acrossthe valve. $ote that the curve for this particular pump risesrather steeply. +t will be more easily controlled than most.*

18 Thirdly, this method is often inefficient. %igure &-'shows a system curve, a pump characteristic, a dischargemodified characteristic, and a recycle modified characteristic.

bove these is a pump power re0uirement curve. +n the caseof discharge control, the pump is adapted to the process bydropping its discharge pressure. +f one follows the flow linevertically to the actual pump curve and then beyond to thepower re0uirement curve one arrives at its powerre0uirement. +n the case of recycle control, the pump isadapted by reducing the discharge flow. %ollowing thepressure line to the right to the actual pump curve and thenupwards to the power re0uirement curve one arrives at thepower re0uirement for recycle control. ote that the powerre0uirement curve tends to slope upward as flow increases.Therefore recycle control consumes more pump horsepower

than discharge throttling when both achieve the same

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operating point. This is not always so. +f the powerre0uirement curve were flat, there would be no difference.otice on the curve that there is a slight drop in horsepowernear the right hand end. +f circumstances were such that the

operating point corresponded to a downward sloping powercurve, recycle control would be more efficient. This is rare.

9 SP!!D C#(T0#&

20 There is, of course, one other means of adapting apump to the changing demands of the process: 6peedcontrol. The virtue of this method is that it reduces theenergy input to the system instead of dumping the ecess.

%igure &-) shows a system curve superimposed on a familyof curves for a variable speed pump. The curves reach allparts of the system curve below the full speed curve.Therefore this is an effective means of control. ote,however, that these curves have one feature in common withrecycle control: t the far left end of the system curve thepump curve and the system curve are almost parallel. $Theparticular pump chosen for this eample has a rather steeplyrising curve near shutoff. ost are considerably flatter.* +n

mathematical terms this means that the intersection is poorlydefined. +n practical terms this means that it is difficult tomaintain a precise operating point and that control is FlooseFat high turndown.

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21 +n practice, variable speed drives for centrifugal pumpsare still relatively uncommon. %or small pumps the powersavings are not significant and for large pumps theassociated electronics become very epensive. lso, they donot have the high reliability of valves. "ariable speed steamturbine drives are 0uite common in the larger horsepowerranges. 8lectric variable speed drives are used in certainspecialied applications such as pumps that are embeddedinside a high pressure vessel. +n such cases there are noalternatives.

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&ow flow damage prolem for centrifugal pumps

Centrifugal pumps have a minimum operating flow rate, belowwhich the pump should not be run for long periods without

sacrificing reliability. aturally it is best, from the standpoint oflong-term reliability and operating efficiency, to operate pumpsclose to their best efficiency point $!8/*, but there may be periodswhere reduced flow demand or system changes cause the pumpto run at reduced flow rates. inimum flow is usually epressed asa percentage of the flow at the !8/ of the pump, for a givenimpeller diameter. 8tended operation below recommendedminimum flow can lead to ecessive vibration, impeller damageand premature bearing and seal failures. 7ith most sealless

$magnetic drive and canned motor* pumps, the allowable run timebelow the minimum flow rate may be only a matter of minutesbefore significant damage to the pump can occur.

The recommended minimum flow rate varies considerablyfrom one sie and type of pump to another, ranging from &G to)G percent of the !8/ flow. The ma#or characteristics of thepump that influence the determination of minimum flowinclude the energy level $horsepower per stage* of the pump,the specific hydraulic design of the impeller inlet, themechanical design of the pump shaft and bearing system andthe cost and criticality of the pump. %or sealless pumps, theamount of heat generated in the canned motor or across themagnets also is a consideration, as is the specific heat of thepumped fluid. There is no accepted industry standard forminimum flow that applies to all pump types, and evendifferent manufacturers of the same pump type may have arange of acceptable minimum flows for a given application.That being said, however, the pump manufacturer is still the

best place to start for the recommended minimum flow for aparticular pump installation. 6ome manufacturers show thisinformation on the pump performance curve.

7hat to do, if anything, to protect the pump from theconse0uences of low-flow damage is an economic decisionmade by the user. This analysis considers the cost of the pump,minimum flow protection system and downtime9lost production,in addition to energy and maintenance costs. Other potential

factors may include health, safety and environmental risks.

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significant ma#ority $estimated at over 5G percent* ofcentrifugal pumps have no minimum flow protection

whatsoever. The vast ma#ority of the pumps installed annuallyare fairly low horsepower pumps used for transfer or cooling,which are not normally epected to operate over a wide rangeof flow. These pumps are unlikely to operate below theminimum flow point, ecept for the inadvertent closure of themain discharge valve or other inadvertent blockage of thesystem. %urthermore, many of these relatively low-cost pumpsare not deemed worth the capital ependiture for minimumflow protection. This is especially true for non-critical pumps

used in residential, commercial and light duty industrialservices.

%or the 4G percent or so of pump applications that do re0uireminimum flow protection, there are number of choices that theuser or system designer has. The determination of whichchoice to use considers accuracy, reliability, cost, andcriticality and is very specific to the application. One otherfactor in the selection process is whether the pump needs tobe protected for minimum flow in a modulating fashion $i.e.,keeping the pump operating but with a certain amount of flowbypassed*, or whether it(rs0uo2s sufficient to simply alarm ortrip off the pump in the event that the flow rate drops belowthe recommended minimum flow. %inally, additional protectionobtained from the same instrument should be considered$e.g., a power monitor can protect against both high and lowflow damage to a pump, while a relief valve will only protectagainst low flow.* %or the 4G percent or so of pumpapplications that do re0uire minimum flow protection, here are

&G different methods that may be considered for protection ofthe pump. ll of these are used in the industry, and somesystems use a combination of these methods for protectionagainst low-flow ecursions.

1. Continuous ypass

This may be the lowest capital cost method of protecting a

pump, whereby a bypass line with an orifice allows a fiedamount of flow to be pumped continuously back to the suction

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source. This always ensures that the pump delivers itsrecommended minimum flow, even if the main line is shut offcompletely. The most significant negative aspect of this systemis that the pump must be oversied in the first place to allow for

the continuously bypassed flow. 6econdly, and sometimes moreimportantly, is the fact that energy is wasted due to the etrahorsepower re0uired to accommodate the bypassed flow. Theremay also be a potential for product damage when being forcedthrough an orifice. 6till, this alternative is chosen by manyindustrial users for pumps in the range of 'G horsepower andbelow. The operating cost of a continuous bypass low-flowprotection system can be significant compared to other methods.%or eample, a 'G-hp 6+ process pump costs H3,34G more

per year to operate with a continuous bypass, compared with abypass system that only opens when the flow drops below thespecified minimum.

*. 'ulti7Component Control %al,e System

This type of system relies on a continuous flow measurement inthe system. 7hen the flow drops below the recommended

minimum flow, a signal is sent to a valve in the bypass line thateither opens it completely or modulates the valve so that itgradually opens. This valve may be a solenoid type if it is strictlyon9off, which is generally the least costly method, or may be apneumatically actuated control valve. This method of bypasseliminates the energy waste of continuous bypass, but relies onconsiderably more compleity than a continuous bypass system.The system includes multiple components, each of which couldfail. +t re0uires a power supply and, if pneumatically actuated, an

air supply. aintenance costs are typically higher than otheralternatives. s such, it is one of the more costly methods ofminimum flow protection. ?owever, it is deemed by many usersto be the best approach, especially if the system alreadyincludes a reliable method of flow measurement.

8. %ariale Freuency Dri,e

"ariable fre0uency drives $"%Bs* change the fre0uency of the

electric motor on the pump to slow the pump down when thedemand for lower flow is called for by the process. %or most

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systems, this keeps the pump operating near its !8/ at alltimes, and prevents the moving of the pump to a lowerpercentage of !8/ that causes the damage to pumps. "%Bsare being used more and more in process applications and

have eliminated the need for other minimum flow protectionwhen they are being used. $ote that with canned motor andmagnetic drive pumps, there will still be a minimum flowre0uired to carry away the heat caused by the motor ormagnetic flu, and to lubricate the bearings.* "%Bs arerelatively epensive, although the cost has reduceddramatically in recent years with rapid improvements intechnology. Other benefits of "%Bs include lighter loading ofpump seals and bearings, and the ability to (ld0uo2soft

start(rd0uo2 e0uipment at slower speeds, reducing the strainand high current caused by on-line starts.

%al,e9. Automatic 0ecirculation

This type of valve combines the features of a check valve anda bypass valve, and has a number of advantages compared toother approaches. Compared to the multi-component flowcontrol valve system, it has fewer components, re0uires lower

installation and operating costs, has less environmental effect$no dynamic seals* and does not re0uire air or electricity.Compared to systems that #ust shut down the pump, it keepsthe pump and the system operating $does not shut down theprocess*. Bisadvantages include its relatively high cost, andthe fact that these valves are not normally available in alloyshigher than stainless steel, thus eliminating many chemicalservices. lso, 1C valves are generally unsuitable for fluidscontaining solids.

5. 0elief %al,e

This simply relies on a pressure relief valve in the pumpdischarge piping being set to relieve back to suction when thepressure put out by the pump reaches a certain setpointpressure. The characteristic performance curve of allcentrifugal pumps is such that as the pump delivers a lowercapacity $flow*, the pressure $head* that the pump producesgets higher. 6ome pumps ehibit a steeper capacity versus

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pressure curve than others. pressure relief valve isparticularly appropriate for pumps with fairly steep capacityversus pressure curves. Thus, this is the normally chosenminimum flow protection system for regenerative turbine style

pumps. +t is also the accepted method for protection of manyfire pump systems. $ote: 7ith li0uid returning to the suction,the internal temperature of the pump may rise, so this maylimit the use of the relief valve for an etended period of time.

lso, relief valves are sub#ect to maintenance andtesting9calibration regimens.*

6. Pressure Sensor

This device relies on the fact that as the flow decreases with acentrifugal pump, the amount of pressure produced by thepump increases. This high-pressure signal is then used toeither open a bypass valve at a high-pressure $low-flow*indication, or to simply trip the pump. %or pumps with relativelysteep head versus capacity curves, this method can beeconomical and reliable. %or pumps with flatter head capacitycurves in the low-flow range, it is considered to be lessreliable than other approaches.

. mp eter

The amp draw of the electric motor varies across the range of flowproduced by a pump. %or many pumps, the amp draw of the pumpis lower at lower flow rates, and increases with increasing flow.Thus, it is possible with many pump types to monitor amp draw,and to alarm or trip the pump when the amps drop below a certainsetpoint level. 7hile this is a relatively inepensive way to protect

the pump against low flow damage, it has some potentialdrawbacks. +t may be sub#ect to unacceptable inaccuracy due tocurrent fluctuations in the system and the fact that the amp drawcurve can be fairly flat at lower flow rates. +n general, the lower thenominal speed of the driver, the less practical amp monitoringbecomes, due to the flatter curve and resulting smaller amp range.The device may also need to be disabled during start-up of thepump due to high current draw. On the plus side, an amp metercan also be used to protect a pump from damage due to ecess

flow.

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5. /ower onitor

/ower monitors measure motor horsepower. 6ince most

pump curves have a horsepower curve that rises withincreasing flow, it is possible to set the motor to shut off if thepower drops below a minimum setpoint, so this is a reliableprotection against low flow problems. /ower monitors aretypically more reliable than amp meters, since they are notsub#ect to fluctuating results with variations in line current. %orpumps with relatively flat head capacity curves, wherepressure measurements aren rs0uot reliable, the powermonitor may be the best choice for low-flow protection. /ower

monitors can be programmed to protect against ecessiveflow $high power*, as well as minimum flow $low power*. Theycan also be programmed to ignore momentary power spikeswhere an amp meter might trip the motor. They are ad#ustableto allow altering setpoints should the process re0uirementschange. They aren(rs0uo2t appropriate for many mied flowpumps, which may have a nearly flat horsepower curve as afunction of pump flow. +f the power monitor measures motorinput power rather than motor output power, it may not be asaccurate, since the efficiencies of small motors at low powercan be 0uite low.

+. %iration Sensor

6ome pump systems have vibration monitors to alarm or tripthe pump if the pump begins to vibrate ecessively. One of thethings that occur at lower flow rates is that the pump mayindeed vibrate significantly higher than normal. $ote that highvibration levels may also be an indication of other problemswith the pump, such as misalignment, imbalance of theimpeller or cavitation.* This device, while relatively epensive,is part of the low-flow protection system on many criticalprocess pumps. +f vibration is associated with pump wear orother factors, such as bearing degradation, it is also possibleto pro#ect the time of failure and plan preventive maintenance.

1;. Temperature Sensor

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t very low flow rates, the temperature of the pumped li0uidincreases because of the recirculation of the li0uid within thepump that goes on at lower flow rates. Thus, if the pumpdischarge is shut off by a closed main valve, the temperature

of the li0uid inside the pump will begin to rise. One method ofprotecting the pump against this occurrence is by monitoringthe temperature in the pump casing $or containment shell inthe case of a magnetic drive pump*, and tripping the pump offwhen the temperature rises above a certain setpoint value.This may be relatively inepensive but not necessarily tooreliable, because by the time it shuts the pump off, damagemay have already occurred in the pump.

Pumps operation prolems

#peration practices that cause freuent seal and earingmaintenance

6eals and bearings account for over eighty five percent $5'I* ofpremature centrifugal pump failure. +n the following paragraphs wewill be looking at only those operation practices that can, and willcause premature seal and bearing failure. Besign and

maintenance practices will be discussed in other papers in thisseries.

7hen pumps were supplied with #am packing, the soft packingstabilied the shaft to prevent too much deflection. +n an effort tosave flushing water and to conserve power, many of these samepumps have since been converted to a mechanical seal and theradial stabiliation the packing provided has been lost.

The bad operating practices include:

1unning the pump dry will cause over-heating and ecessivevibration problems that will shorten seal life. ?ere are some of thecommon reasons why a pump is run dry:

%ailing to vent the pump prior to start-up. 1unning the tank dry at the end of the operation cycle. 8mptying the tank for steaming or introduction of the net

product.

1unning on the steam that is being used to flush the tank.

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6tarting the standby pump without venting it. "enting ahaardous product can cause a lot of problems with theli0uid disposal. any operators have stopped venting for thatreason.

Tank vents sometimes freee during cold weather. This willcause a vacuum in the suction tank, and in some casescould collapse the tank.

6ump fluids are often dirty, corrosive or both. The controlrods for the float switch will often Egum upE or corrode andgive a false reading to the operator. ?e may think that thereis an ade0uate level, when in fact, the tank is empty.

Bead heading the pump can cause severe shaft deflection as thepump moves off of its best efficiency point $!.8./.*. This translatesto ecessive heat that will affect both the seal and the bearings aswell as causing the seal faces to open, and the possibility of theimpeller contacting the volute when the shaft deflects.

6tarting the centrifugal pump with a shut discharge valve isstandard practice with many operation departments. Theconcern is to save power without realiing the damage that isbeing done to the mechanical seal, impeller, wear rings andbearings.

6ome pumps are e0uipped with a recirculation valve thatmust be opened to lessen the problem, but many times thevalve is not opened, or the bypass line is clogged or not ofthe correct diameter to prevent the ecessive head. notherpoint to remember is that if the bypass line is discharged tothe suction side of the pump the increased temperature cancause cavitation.

fter a system has been blocked out the pump is started withone or more valves not opened.

Bischarge valves are shut before the pump has beenstopped.

Operating off of the best efficiency point $!.8./.*. Changing theflow rate of the li0uid causes shaft deflection that can fail themechanical seal and over-load the bearings.

6tarting the pump with the discharge valve closed to savepower.

The level in the suction tank is changing. 1emember that thepump pumps the difference between the discharge and

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suction heads. +f the suction head varies, the pump moves toa different point on its curve.

ny upset in the system such as closing, throttling oropening a valve will cause the pump to move to a new point

on the curve as the tank fills. /umping to the bottom of a tank will cause the pump to move

to a different point on the curve as the tank fills. 6omesystems were designed for a low capacity positivedisplacement pump and have since been converted to acentrifugal design because of a need for higher capacity.Centrifugal pumps must discharge to the top of the tank toprevent this problem.

+f the discharge piping is restricted because of product build

up on the inside walls, the pump will run throttled. This is oneof the reasons that it is important to take periodic flow andamperage readings.

+ncreasing the flow will often cause cavitation problems.

6eal environmental controls are necessary to insure longmechanical seal life. +t is important that operations understandtheir function and need because many times we find the controlsinstalled, but not functioning.

Cooling-heating #ackets should show a differentialtemperature between the inlet and outlet lines. +f the #acketclogs up, this differential will be lost and seal failure willshortly follow.

!arrier fluid is circulated between two mechanical seals.There may or may not be a differential temperaturedepending upon the flow rate. +f a convection tank isinstalled, there should be a temperature differential betweenthe inlet and outlet lines. The line coming out of the top of the

seal to the side of the tank should be warmer than the linefrom the bottom of the tank to the bottom of the seals,otherwise the system is running backwards and may failcompletely. The level in the tank is also critical. +t should beabove the tank inlet line or no convection will occur. 6omeconvection tanks are pressuried with a gas of some type.any original e0uipment $O.8..* seal designs will fail if thisdifferential pressure is lost.

6ome seal glands $./.+. type* are e0uipped with a 0uench

connection that looks like the seal is leaking water or steam.+f there is too much steam pressure on this 0uench

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connection, the ecessive leakage will get into the bearingscausing premature failure. The steam is often used to keepthe product warm to prevent it from solidifying, crystalliing,getting too viscous, building a film on the faces etc.

Operating people fre0uently shut off the 0uench to stop thecondensate from leaking. %lushing fluids are used for a variety of purposes, but most of

the time they are used to get rid of unwanted solids. Theflush can be closely controlled with a flow meter or throttlingvalve. The amount of flush is determined by the seal design.

s an eample, those designs that have springs in theproduct re0uire more flush.

+t is important to check that the stuffing bo has been vented

in vertical pumps. The vent should be coming out of the sealgland and not the stuffing bo lantern ring connection.

There are some additional things that all operators should know toinsure longer rotating e0uipment life. s an eample :

echanical seals have an 5'I or more failure rate that isnormally correctable. This is causing unnecessary down timeand ecessive operating epense. 6eals should run until thesacrificial carbon face is worn away, but in more that 5'I of

the cases the seal fails before this happens. There are five different causes of cavitation. Aou should know where the best efficiency point $!.8./.* is

on a particular pump, and how far it is safe to operate off the!.8./. with a mechanical seal installed.

Aou should be aware that washing down the pump area witha water hose will cause premature bearing failure when thewater penetrates the bearing case.

=earn about the affect of shaft =39B; on pump operation.

@now how the pumped product affects the life of themechanical seal and why environmental controls arenecessary.

+f you are not using cartridge seals, ad#usting the openimpeller for efficiency will shorten the seal life. +n most casesthe seal will open as the impeller is being ad#usted to thevolute. Burco pumps are the best eample of the eceptionto this rule. The popular Burco pumps ad#ust to the backplate causing a compression of the seal faces that can

create mechanical seal Eover heatingE problems.

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Cycling pumps for test will often cause a mechanical sealfailure unless an environmental control has been installed toprevent the failure.

echanical seals should be positioned after the impeller has

been ad#usted for thermal growth. This is important on anypump that is operated above 4GGJ% $&GGJC* or you willeperience premature seal failure.

6ome elastomers will be affected by steaming the system. great deal of caution must be eercised if a flushing fluidsuch as caustic is going to be circulated through the lines orused to clean a tank. !oth the elastomer and some sealfaces $reaction bonded silicone carbide is a good eample*can be damaged. +f the elastomer is attacked, the failure

usually occurs within one week of the cleaning procedure. The stuffing bo must be vented on all vertical centrifugalpumps or otherwise air will be trapped at the seal faces thatcan cause premature failure of many seal designs.

ost original e0uipment seal designs cause shaft damage$fretting* necessitating the use of shaft sleeves that weakenthe shaft and restrict pump operation to a narrow range atthe !.8./..

?ere are a few common misconceptions that cause friction

between maintenance and operation departments

6hutting the pump discharge valve suddenly, will blow theseal open.

ll ceramics cold shock. ?igh head, low capacity consumes a lot of power. The pump must come into the shop to change a mechanical

seal. +f you use two hard faces or dual mechanical seals in slurry

applications, you will not need flushing water with itscorresponding product dilution.

+f you use metal bellows seals for hot oil applications, you willnot need the stuffing bo cooling #acket operating.

+t is O.@. to use an oversied impeller because throttlingback will save power.

few more thoughts on the sub#ect

Operators should receive proper schooling on the troubleshooting and maintenance of pumps. +n the military and

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many modern plants, the operator and the maintenancemechanic are often the same person. +f the operator knowshow the pump works he will have no trouble figuring out thesolution to his problem. Too often he is told to keep the flow

gage at a certain point, or between two values withoutunderstanding what is actually happening with thee0uipment. +f the operator recognies cavitation he can tellthe maintenance department and help them with their troubleshooting.

s you wander around the plant look out for painters thatpaint the springs of outside and double mechanical seals.There is a trend to putting two seals in a pump forenvironmental reasons and the painting of springs is

becoming a common problem. +f someone is ad#usting the impeller make sure he isresetting the seal spring tension at the same time.

+f the pump is getting hot or making ecessive noises, reportit immediately. fter the failure, it does no good to tellmaintenance that it was making noise for two weeks.

+f you are the floor operator it is common knowledge thattaking temperature and pressure readings is very boring,especially on those gages that are located in hot or awkwardlocations. void the temptation to EradioE these readings.%rom hot to failure is a very short trip.

aintenanceFs favorite epression is Ethere is never time todo it right, but there is always time to fi it.E Try to keep this inmind when the pressure is on to get the e0uipment runningagain.

Bo not let cleaning people direct their Ewash downE hosesdirectly at the pump. 7ater entering the bearings through thelip or grease seals is a ma#or cause of premature bearingfailure. ost water wash downs are used to dilute and wash

away seal leakage. 6top the leak and you have eliminatedthe reason for the hose.

great many motor and electrical problems are caused bythese same wash down hoses.

Cooling a bearing outside diameter will cause it to shrink andthe bearing will get hotter as the radial load increases. @eepthe water hose and all other forms of cooling off of thebearing casing.

Two asic 0euirements for Troule7Free #peration ofCentrifugal Pumps

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Centrifugal pumps are the ultimate in simplicity. +n general thereare two basic re0uirements that have to be met at all the times fora trouble free operation and longer service life of centrifugal

pumps.

The first re0uirement is that no cavitation of the pump occursthroughout the broad operating range and the second re0uirementis that a certain minimum continuous flow is always maintainedduring operation.

clear understanding of the concept of cavitation, its symptoms,its causes, and its conse0uences is very much essential in

effective analyses and troubleshooting of the cavitation problem.

Kust like there are many forms of cavitation, each demanding auni0ue solution, there are a number of unfavorable conditionswhich may occur separately or simultaneously when the pump isoperated at reduced flows. 6ome include:

Cases of heavy leakages from the casing, seal, and

stuffing bo Beflection and shearing of shafts

6eiure of pump internals

Close tolerances erosion

6eparation cavitation

/roduct 0uality degradation

8cessive hydraulic thrust

/remature bearing failures

8ach condition may dictate a different minimum flow low

re0uirement. The final decision on recommended minimum flow istaken after careful Ltechno-economicalM analysis by both the pumpuser and the manufacturer.

The conse0uences of prolonged conditions of cavitation and lowflow operation can be disastrous for both the pump and theprocess. 6uch failures in hydrocarbon services have often causeddamaging fires resulting in loss of machine, production, and worstof all, human life.

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Thus, such situations must be avoided at all cost whether involvingmodifications in the pump and its piping or altering the operatingconditions. /roper selection and siing of pump and its associatedpiping can not only eliminate the chances of cavitation and low

flow operation but also significantly decrease their harmful effects.

)nderstanding Ca,itation

+n the above, two basic re0uirements for trouble free operation andlonger service life of centrifugal pumps are mentioned in brief.

1. P0!%!(T CA%$TAT$#(

Cavitation of the pump should not occur throughout its operatingcapacity range.

4. '$($'$

Continuous operation of centrifugal pumps at low flows i.e.reduced capacities, leads to a number of unfavorable conditions.These include reduced motor efficiency, ecessive radial thrusts,

ecessive temperature rise in the pumping fluid, internal re-circulation, etc. certain minimum continuous flow $C%* shouldbe maintained during the pump operation.

Operating a pump under the condition of cavitation for even a shortperiod of time and have damaging conse0uences for both thee0uipment and the process. Operating a pump at low flowconditions for an etended duration may also have damagingconse0uences for the e0uipment.

The condition of cavitation is essentially an indication of anabnormality in the pump suction system, whereas the condition oflow flow indicates an abnormality in the entire pumping system orprocess. The two conditions are also interlinked such that a lowflow situation can also induce cavitation.

The concept of cavitation is eplored in detail under followingtopics:

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&. eaning of the term Ncavitation in the contet of centrifugalpumps.4. echanism of cavitation.3.

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+n any discussion on centrifugal pumps various terms like vaporpockets, gas pockets, holes, bubbles, etc. are used in place of theterm cavities. These are one and the same thing and need not be

confused. The term bubble shall be used hereafter in thediscussion.

In the context of centrifugal pumps, the term cavitation implies adynamic process of formation of bubbles inside the liquid, theirgrowth and subsequent collapse as the liquid flows through the

pump.

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aseous ca,itation occurs when any gas $most commonly air*enters a centrifugal pump along with li0uid. centrifugal pump canhandle air in the range of P I by volume. +f the amount of air isincreased to )I, the pump starts cavitating. The cavitation

condition is also referred to as ir binding. +t seldom causesdamage to the impeller or casing. The main effect of gaseouscavitation is loss of capacity.

The different types of cavitation, their specific symptoms andspecific corrective actions shall be eplored in the net part of thearticle. ?owever, in order to clearly identify the type of cavitation,let us first understand the mechanism of cavitation, i.e. howcavitation occurs.

Unless otherwise specified, the term cavitation shall refer tovaporous cavitation.

*. 'echanism of Ca,itation

The phenomenon of cavitation is a stepwise process as shown in%igure &&.

Step One, Formation of bubbles inside the liquid being pumped.

The bubbles form inside the li0uid when it vaporises i.e. phasechange from li0uid to vapor.But how does vaporization of the liquid occur during a pumpingoperation?

"aporiation of any li0uid inside a closed container can occur ifeither pressure on the li0uid surface decreases such that itbecomes e0ual to or less than the li0uid vapor pressure at the

operating temperature, or the temperature of the li0uid rises,raising the vapor pressure such that it becomes e0ual to or greaterthan the operating pressure at the li0uid surface. %or eample, ifwater at room temperature $about o %* is kept in a closedcontainer and the system pressure is reduced to its vapor pressure$about G.'4 psia*, the water 0uickly changes to a vapor. lso, if theoperating pressure is to remain constant at about G.'4 psia andthe temperature is allowed to rise above o %, then the water0uickly changes to a vapor.

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Kust like in a closed container, vaporiation of the li0uid can occurin centrifugal pumps when the local static pressure reduces belowthat of the vapor pressure of the li0uid at the pumping temperature.

OT8: The vaporisation accomplished by addition of heat or thereduction of static pressure without dynamic action of the li0uid isecluded from the definition of cavitation. %or the purposes of thisarticle, only pressure variations that cause cavitation shall beeplored.

Temperature changes must be considered only when dealing withsystems that introduce or remove heat from the fluid beingpumped.

o understand vaporization, two important points to remember are!&. 7e consider only the static pressure and not the totalpressure when determining if the system pressure is less thanor greater than the li0uid vapor pressure. The total pressure isthe sum of the static pressure and dynamic pressure $due tovelocity*4. The terms pressure and head have different meanings andthey should not be confused. s a convention in this article, theterm LpressureM shall be used to understand the concept ofcavitation whereas the term LheadM shall be used in e0uations.

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Figure 11- Phenomenon of Ca,itation

Thus, the key concept is - vapor bubbles form due to vaporiationof the li0uid being pumped when the local static pressure at any

point inside the pump becomes e0ual to or less than the vaporpressure of the li0uid at the pumping temperature.

4ow does pressure reduction occur in a pump system?

The reduction in local static pressure at any point inside the pumpcan occur under two conditions:

&. The actual pressure drop in the eternal suction system is

greater than that considered during design. s a result, thepressure available at pump suction is not sufficiently highenough to overcome the design pressure drop inside the pump.

4. The actual pressure drop inside the pump is greater than thatconsidered during thepump design.

Step Two@ rowth of ules

Unless there is no change in the operating conditions, newbubbles continue to form and old bubbles grow in sie. Thebubbles then get carried in the li0uid as it flows from the impellereye to the impeller eit tip along the vane trailing edge. Bue toimpeller rotating action, the bubbles attain very high velocity andeventually reach the regions of high pressure within the impellerwhere they start collapsing. The life cycle of a bubble has beenestimated to be in the order of G.GG3 seconds.

Step Three@ Collapse of ules

s the vapor bubbles move along the impeller vanes, the pressurelocalied hammering effect can pit the pump impeller. The pittingeffect is illustrated schematically in %igure &4.

fter the bubble collapses, a shock wave emanates outward fromthe point of collapse.

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+n nutshell, the mechanism of cavitation is all about formation,growth and collapse of bubbles inside the li0uid being pumped. !uthow can the knowledge of mechanism ofcavitation can really help in troubleshooting a cavitation problem.

The concept of mechanism can help in identifying the type ofbubbles and the cause of their formation and collapse. Thetroubleshooting method shall be eplored in detail in the net partof the article.

et let us eplore the general symptoms of cavitation and itsaffects on pump performance.

87 eneral Symptoms of Ca,itation and its Affects Pump

Performance and Pump Parts

/erceptible indications of the cavitation during pump operation aremore or less loud noises, vibrations and an unsteadily workingpump. %luctuations in flow and discharge pressure take place witha sudden and drastic reduction in head rise and pump capacity.

Figure 12: Collapse of a Vapor Bubble

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Bepending upon the sie and 0uantum of the bubbles formed andthe severity of their collapse, the pump faces problems rangingfrom a partial loss in capacity and head to total failure in pumpingalong with irreparable damages to the internal parts. +t re0uires a

lot of eperience and thorough investigation of effects of cavitationon pump parts to clearly identify the type and root causes ofcavitation.

detailed description of the general symptoms is given as under.

QReduction in capacity of the pump:

The formation of bubbles causes a volume increase decreasingthe space available for the li0uid and thus diminish pumping

capacity. %or eample, when water changes state from li0uid togas its volume increases by approimately &,GG times. +f thebubbles get big enough at the eye of the impeller, the pumpLchokesM i.e. loses all suction resulting in a total reduction in flow.The une0ual and uneven formation and collapse of bubblescauses fluctuations in the flow and the pumping of li0uid occurs inspurts. This symptom is common to all types of cavitations.

QDecrease in the head de,eloped-

!ubbles unlike li0uid are compressible. The head developeddiminishes drastically because energy has to be epended toincrease the velocity of the li0uid used to fill up the cavities, as thebubbles collapse. s mentioned earlier, The ?ydraulic 6tandards+nstitute defines cavitation as condition of 3 I drop in headdeveloped across the pump. =ike reduction in capacity, thissymptom is also common to all types of cavitations.

Thus, the hydraulic effect of a cavitating pump is that the pump

performance drops off of its epected performance curve, referredto as break away, producing a lower than epected head and flow.The %igure &; depicts the typical performance curves. The solidline curves represent a condition of ade0uate /6?a whereas thedotted lines depict the condition of inade0uate /6?a i.e. thecondition of cavitation.

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Figure 18- Pump Performance Cur,es

Anormal sound and ,irations:

+t is movement of bubbles with very high velocities from low-pressure area to a high-pressure area and subse0uent collapsethat creates shockwaves producing abnormal sounds andvibrations. +t has been estimated that during collapse of bubblesthe pressures of the order of &G atm develops.

The sound of cavitation can be described as similar to small hardparticles or gravel rapidly striking or bouncing off the interior partsof a pump or valve. "arious terms like rattling, knocking, cracklingare used to describe the abnormal sounds. The sound of pumpsoperating while cavitating can range from a low-pitched steadyknocking sound $like on a door* to a high-pitched and randomcrackling $similar to a metallic impact*.

/eople can easily mistake cavitation for a bad bearing in a pump

motor. To distinguish between the noise due to a bad bearing or

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cavitation, operate the pump with no flow. The disappearance ofnoise will be an indication of cavitation.

6imilarly, vibration is due to the uneven loading of the impeller as

the miture of vapor and li0uid passes through it, and to the localshock wave that occurs as each bubble collapses. "ery fewvibration reference manuals agree on the primary vibrationcharacteristic associated with pump cavitation. %ormation andcollapsing of bubbles will alternate periodically with the fre0uencyresulting out of the product of speed and number of blades. 6omesuggest that the vibrations associated with cavitation produce abroadband peak at high fre0uencies above 4,GGG ?ert. 6omesuggest that cavitation follows the vane pass fre0uency $number of

vanes times the running speed fre0uency* and yet another indicatethat it affects peak vibration amplitude at one times running speed.ll of these indications are correct in that pump cavitation canproduce various vibration fre0uencies depending on the cavitationtype, pump design, installation and use.

The ecessive vibration caused by cavitation often subse0uentlycauses a failure of the pumps seal and9or bearings. This is themost likely failure mode of a cavitating pump.

Damage to pump parts-

Ca,itation erosion or pitting

Buring cavitation, the collapse of the bubbles occurs at sonicspeed e#ecting destructive micro #ets of etremely high velocity $upto &GGG m9s* li0uid strong enough to cause etreme erosion of thepump parts, particularly impellers. The bubble is trying to collapsefrom all sides, but if the bubble is lying against a piece of metal

such as the impeller or volute it cannot collapse from that side. 6othe fluid comes in from the opposite side at this high velocity andbangs against the metal creating the impression that the metal washit with a Eball pin hammerE. The resulting long-term materialdamage begins to become visible by so called

/its $see %igure &;*, which are plastic deformations of very smalldimensions $order of magnitude of micrometers*. The damagecaused due to action of bubble collapse is commonly referred as

Cavitation erosion or pitting. The %igure &; depicts the cavitationpitting effect on impeller and diffuser surface.

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Cavitation erosion from bubble collapse occurs primarily by fatiguefracture due to repeated bubble implosions on the cavitatingsurface, if the implosions have sufficient impact force. The erosion

or pitting effect is 0uite similar to sand blasting. ?igh head pumpsare more likely to suffer from cavitation erosion, making cavitationa Lhigh-energyM pump phenomenon.

The most sensitive areas where cavitation erosion has beenobserved are the low-pressure sides of the impeller vanes near theinlet edge. The cavitation erosion damages at the impeller aremore or less spread out. The pitting has also been observed onimpeller vanes, diffuser vanes, and impeller tips etc. +n some

instances, cavitation has been severe enough to wear holes in theimpeller and damage the vanes to such a degree that the impellerbecomes completely ineffective. damaged impeller is shown in%igure &'.

Figure 19- Photographic !,idence of Ca,itation

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The damaged impeller shows that the shock waves occurred nearthe outside edge of the impeller, where damage is evident. Thispart of the impeller is where the pressure builds to its highest point.This pressure implodes the gas bubbles, changing the waters

state from gas into li0uid. 7hen cavitation is less severe, thedamage can occur further down towards the eye of the impeller. careful investigation and diagnosis of point of the impeller erosionon impeller, volute, diffuser etc. can help predict the type andcause of cavitation.

The etent of cavitation erosion or pitting depends on a number offactors like presence of foreign materials in the li0uid, li0uidtemperature, age of e0uipment and velocity of the collapsing

bubble.

Trouleshooting of the pumps

The following is a guide for troubleshooting centrifugal pump andpump systems.

Failure to Deli,er &iuidA. /ump not primed.

. +nsufficient speed.C. Bischarge head too high.D. 6uction lift too high $over &' feet* insufficient /6?, check withvacuum gauge.!. +mpeller passages partially clogged $plugged*.F. 7rong direction of rotation.. ir leaks or pockets in suction line.

Ca,itations

Cavitations are caused by a lowering of li0uid pressure at theimpeller eye-giving rise to vapor formation. This is followed by thesudden collapse of the vapour bubbles as pressure increasescausing damage to pump by pitting and erosion.

earing 4ousing &urication

The correct level of oil in the bearing housing is kept by the

lubricating oil level bottle provided that the bottle contains an oillevel.

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Pump 0outine Chec"s

%or centrifugal pumps check the following:

Bischarge pressure. 6uction pressure.

/ressure differential at suction strainer.

!earing temperature.

oise $cavitation*

echanical seal leakage.

Cooling medium temperature.

=ube oil system $/,T and level*

/ower consumption in mps. $means pump loading*

$nsufficient Pressure&. 6ped too low.4. ir or gases in li0uid.3. echanical defects.

7earing rings worn.

+mpeller damaged.

+nternal leakage due to defective gasket.

;. 7rong direction of rotation.

$nsufficient Capacity&. ir leaks in suction or stuffing boes.4. 6peed too low.3. Total head higher than that for which pump is rated.;. 6uction lift too high $over &' feet* or insufficient /6?, check

with vacuum gauge.'. +mpeller passages or piping partially clogged $plugged*.

). +nsufficient suction head for hot li0uid.. echanical defects. 7earing rings worn.

+mpeller damaged.

+nternal leakage due t defective gasket.5. 7rong direction of rotation.

Pump %irates&. isalignment.4. %oundation not sufficiently rigid $grounding broken*.

3. +mpeller partially clogged, causing imbalance.;. echanical defects.

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(oisy Pump #peration

&. ?ydraulic noise: Cavitation.

+nsufficient /6?.

ir in li0uid.

4. echanical defects: 6haft bent.

!earing warn.

1otating parts binding.

0educing Capacity

Centrifugal pumps should not be operated at a greatly reducedcapacity or with closed discharge valve, because the energyre0uired driving the pump is converted into heat and thetemperature of the li0uid may reach the boiling point. +f this occurs,the rotating parts are eposed to vapour with no lubrication andthey will be damaged.

Trouleshooting the positi,e displacement rotary pump

(o liuid discharge

The pump is not primed. /rime it from the outlet side bykeeping the outlet air vent open until li0uid comes out thevent.

The rotating unit is turning in the wrong direction. "alves are closed or there is an obstruction in the inlet or

outlet line. Check that the flange gaskets have their centercut out.

The end of the inlet pipe is not submerged. Aou can eitherincrease the length of the inlet pipe into the li0uid level orraise the level in the tank.

The foot valve is stuck. strainer or filter is clogged. The net inlet pressure is too low. bypass valve is open.

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There is an air leak some where in the inlet line. ir cancome in through gaskets or valves above the fluid line.

The stuffing bo is under negative pressure. /acking isallowing air to get into the system. Aou should convert the

packing to a mechanical seal The pump is worn. The critical clearances have increased. 6omething is broken. Check the shaft, coupling, internal

parts, etc. There is no power to the pump.

The pump is putting out a low capacity

The pumpFs internal clearances have increased. +t is time tochange some parts.

The net inlet pressure is too low2 the pump is cavitating. strainer or filter is partially clogged. The speed is too low. Check the voltage. The tank vent is partially froen shut. bypass line is partially open. relief valve is stuck partially open. The inlet piping is damaged. 6omething ran over it corrosion resistant liner has collapsed in the inlet piping. ir is leaking through the packing. Aou should go to a

mechanical seal.

The pump looses its prime after it has een running for awhile

The li0uid supply is ehausted. Check the tank level2sometimes the float is stuck, giving an incorrect levelreading.

The li0uid velocity has increased dramatically.

The li0uid is vaporiing at the pump inlet. bypass line is heating the incoming fluid. n air leak has developed in the suction piping.

The pump is using too much power

The speed is too high. The li0uid viscosity is higher than epected. The discharge pressure is higher than calculated The packing has been over tightened. Aou should convert to

a mechanical seal.

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rotating element is binding. isalignment could be theproblem or something is stuck in a close clearance andbinding the rotating element.

!/cessi,e noise and ,iration

1elief valve chatter. %oundation or anchor bolts have come loose. The pump and driver are misaligned. The piping is not supported properly. The li0uid viscosity is too high. The pump is starving. Check

the temperature of the incoming li0uid. Check to see if thesupply tank heater has failed.

8cessive noise or a loss of capacity is fre0uently caused bycavitation. ?ere is how the /6? re0uired was determined initially:

7ith the pump initially operating with a G psig. inlet pressure andconstant differential pressure, temperature, speed and viscosity2 avalve in the inlet line is gradually closed until cavitation noise isclearly audible, there is a sudden drop off in capacity or there is a'I overall reduction in output flow. Cavitation occurs with:

loss of suction pressure. n increase in fluid velocity. n increase in inlet temperature.

4ere are some common causes of ca,itation prolems-

foot valve or any valve in the suction piping is sticking. 6omething is occasionally plugging up the suction piping. +f

the pump suction is coming from a river, pond or the ocean,grass is a strong possibility.

loose rag is another common cause. collapsed pipe liner. filter or strainer is gradually clogging up. The tank vent partially freees in cold weather. The sun is heating the suction piping, raising the product

temperature close to its vapor point. The level in the open suction tank decreases causing vorte

problems that allow air into the pump suction. 6everal pumps in the same sump are running, decreasing

the level too much.

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The suction tank float is stuck. +t will sometimes show ahigher level than you really have.

discharge recirculation line, piped to the pump suction,opens and heats the incoming li0uid.

6ometimes the suction lift is too high. The increase in pipefriction will reduce the suction head. The vapor pressure of the product is very close to

atmospheric pressure. The pump cavitates every time it rainsbecause of a drop in atmospheric pressure.

The tank is being heated to de-aerate the fluid. 6ometimes itis being heated too much.

The process fluid specific gravity is changing. This canhappen with a change in product operating temperature or if

a cleaner or solvent is being flushed through the lines. The source tank is changing from a positive pressure to avacuum due to the process.

packed valve in the suction piping is at a negative pressureand air is leaking in through the packing.

The tank is being pumped dry. The inlet piping has been moved or altered in some way. ?as

a foot valve, strainer, elbow, or some other type of hardwarebeen installed in the suction pipingR

?as a layer of hard water calcium or some other type of solidformed on the inside of the suction piping reducing its insidediameter over some period of timeR

Aou are eperiencing rapid pump wear.

There are abrasives in the li0uid you are pumping causingerosion problems. Aou may have to go to a larger pumprunning at a slower speed.

There is some corrosion in one or more of the pump

elements. There is a lack of lubrication. Aou have a severe pipe strain problem. +t could have been

caused by thermal growth of the hardware. Too much misalignment. The pump is running dry. 7hen all else fails the best way to reduce /6? re0uired is

to select a larger pump and run it at a slower speed.