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There are many excellent pumptextbooks, but little isavailable to date on how to

determine the optimum time toinvest in a pump overhaul to restore lost performance. Whendeterioration causes a drop in plantproduction, overhaul is readilyjustified, as its cost is usually small inproportion. When the effect ofdeterioration is only to increasepower consumption, the best time tooverhaul for minimum cost can be calculated from condition-monitoring test results. Overhaulingpumps on a fixed time or breakdownbasis is rarely cost effective. Thecondition monitoring approachensures that pump overhauls, torestore performance, are carried outwhen necessary and economicallyjustified.

More than one method of conditionmonitoring may be appropriate.Methods should be chosen that will

economically detect each of thedegradation modes that are expected.Here we will consider onlyperformance monitoring and analysis.

The head-flowmethod

Head-flow measurement can be usedfor all pumps where flow, or arepeatable indicator of it, can be measured. It detects pumpdeterioration and any changes insystem resistance. Some test pointsnear the normal operating duty pointare sufficient to reveal the effects ofinternal wear. Figure 1 shows thehead-flow curve moving towards thezero flow axis by an amount equal tothe internal leakage flow.

A series of test readings at steadyconditions at 15 s intervals issufficient, taking the average valuesto plot. Speed must be measured forvariable speed pumps, and the head-flow data corrected to a standardspeed using the affinity laws.

Field tests occasionally give resultsslightly different to themanufacturer’s works tests as siteconditions for flow and pressuremeasurement rarely match thevarious standards as used in the works.However, for monitoring it is relativechanges we are seeking rather thanabsolute accuracy.

Non-intrusive flowmeters are

applicable in most cases. If apermanent flowmeter is installed aspart of a pump’s minimum flowprotection or in the process stream,and its condition is considered toremain constant, it can be used formonitoring. Such performanceinformation shows the extent towhich a pump has deteriorated, andpumps can be prioritized for overhaulon the basis of their relative wear. Abetter method will be described later.

The shut-off headmethod

The simple shut-off head test is onlypossible where zero flow can betolerated. This is not so with high-energy pumps, nor for those of highspecific speed. Unlike the head-flowtest, this test does not reveal thecondition of the system.

The test involves closing thedischarge valve for long enough to getsteady readings, and reading thesuction and discharge pressures. If itvaries from time to time, the liquidtemperature is needed to calculate thedensity, which is used to convert thepressure readings into head values.

Wear of vane outer diameters willshow readily. To show sealing ringwear, the pump head-flow curve needsto be relatively steep. (Note thatinternal leakage in pumps with arising curve will initially give anincrease in shut-off head.)

Optimize time for overhaul ofyour pumps using conditionmonitoringIn this article from Australia, Ray Beebe of Monash University considers how to determinethe optimum time for pump overhaul based on energy consumption. First, he explains howto use two simple condition-monitoring tests for pumps in general, before moving on todiscuss the specific case of multi-stage pumps fitted with a balance device. He describes thebalance device leakoff flow method and, from experience, illustrates its uses and pitfalls.

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Figure 1. Pump head-power-flow in newcondition, with test

points showinginternal wear.

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How to calculate theoptimum time foroverhaul

A number of possibilities are examinedbelow.

Plant production reduced

Where pump deterioration results in areduction in plant production promptoverhaul is usually simply justifiedwhere the cost of overhaul isinsignificant in proportion to the cost oflost production.

Intermittent operation

In a pumping installation such astopping up a water supply tank orpumping out where a pump runsintermittently to meet a demand,deterioration will result in the pumptaking more time to do its duty. Anyextra service time therefore results inincreased power consumption.

Production unaffected,output throttle valvecontrolled

Let us consider the case where pumpdeterioration does not initially affectplant production, and where the pumpruns at constant speed, with outputcontrolled by the throttle valve.Initially, internal wear may not causeany loss in production from the plant,as the control valve opens more fullyto ensure that pump output ismaintained. Eventually, as wearprogresses, pump output may beinsufficient to avoid loss ofproduction, or the power taken willexceed the motor rating.

Figure 1 shows the head-power-flowsite test characteristics of such a pump.The duty point at flow of 825 m³/h inthe new condition is A. The powerabsorbed by the pump is measuredwith test instruments, or read off the power-flow curve as 2150 kW (B). Ideally the power-flow

curve should be found on site, but if unavailable the works testinformation will suffice.

After some service, the worn pumptest points plotted indicate thatinternal wear has occurred. Thisresults in the operating point movingto C, as the system resistance curvelowers when the throttle valve isopened further. The required serviceflow is still maintained.

The increased power required whenworn can be estimated by extendingfrom the head-flow curve at constanthead from the operating point to D,and then dropping to intersect thepower-flow curve for new condition atconstant flow E. The basis for thisestimate is the assumption that, as thepump wears, the total flow throughthe impellers remains constant, butless of it leaves the pump to the systemas flow. If the pump was motor-driven,the power could be measured on test atextra expense.

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In the example, the power requiredwhen worn is shown in Figure 1 by theprojection from 825 m3/h to the testcurve to find 640 m head, then acrossto the site test – new pump curve, thendown to the power curve, to find 2300kW at E.

The extra electricity consumption istherefore 2300 – 2150 = 150 kW ÷motor efficiency (here approx. 90%) =167 kW.

If the sealing clearances are known, byprevious experience of correlationwith measured performance, or if thepump is opened up already, the extrapower consumed, which is likely to besaved by overhaul, can thus beestimated.

Using this method, a number of pumpsof varying wear conditions could beprioritized for maintenance, based ontheir increased power consumptionand their relative costs of overhaul,that is, the cost/benefits.

Finding the optimum timefor overhaul For this example, the head-flow testpoints were obtained following 24months of service since the pump wasknown to be in new condition. Anoverhaul would cost $50 000;electricity costs $0.10/kWh; and thepump operates for 27% of the time onaverage.

Our test shows that the rate ofincreasing cost/month has reached 167× 0.10 × 0.27 × 720 = $3240/month(taking an average month as 720 h).

As the time now is 24 months, $3240 ÷24 gives the average cost rate ofdeterioration as $135/month/month.Where O is the cost of overhaul and Cthe cost rate of deterioration, theoptimum time for overhaul, T, can becalculated from:

T = √(2O/C)

This gives T as 27.2 months.

It is better to calculate and plot theaverage total cost/month values for arange of times. Seen clearly will be thecost impact of doing the repairs at someother time, such as at a scheduled plantshutdown.

Calculating costsIt is also possible to calculate the totalaverage cost per month, month bymonth. In this example, take the timeat 22 months.

The average cost of overhaul is:

$50 000 ÷ 22 = $2273/month

The average cost of extra energy is:

$135 × ½ × 22 = $1485/month

The total average cost/month is the sumof these two figures = $3578/month.

Repeat this calculation for severalmonths, perhaps using a spreadsheet,and look for the minimum total cost,which is at 27.2 months. If plotted ascost/month against time, the resultingcurves will show the cost per month ofoverhaul dropping with time, with thecost of lost energy increasing with time.Usually the total cost curve is fairly flatfor ±20% or so.

This method is correct only if the wearprogresses at a uniformly increasing ratewith time, but this is not unusual.Information may not be available tomake any other assumption, but

decision makers have to startsomewhere! Paresh Girdhar hascompiled a spreadsheet for the method(available from www.goldson.free-online.co.uk/programs.htm).

Note that some relatively small pumpsmay never justify overhaul on savingsin energy use alone, but repair may bejustified on reduced plant productionrate or reliability considerations.

Production unaffected,output speed controlled

Let us now consider the situation wherepump output is controlled by variablespeed, and pump deterioration does notaffect production, at least initially. Forvariable speed pumps, the effect of wearon power required is greater. This isbecause the power increases inproportion to the speed ratio cubed.

Unless the pump output is limited bythe pump reaching its maximum speed,or by its driver reaching its highestallowable power output, then noproduction will be lost.

Figure 2 shows the performance of avariable speed pump. When new,operation at 1490 rpm meets thedesired duty, at operating point A,requiring 325 kW power (B). Aftersome time in service, internal leakagehas increased such that the pump mustrun at 1660 rpm to meet the requiredduty – still point A.

To estimate the power required in theworn state, the head-flow curve mustbe drawn for the higher speed in thenew condition. Several head-flowpoints are selected and corrected to thehigher speed. Multiply each flow by thespeed ratio, and multiply eachmatching head by the speed ratiosquared. This will result in the head-flow curve at 1660 rpm in the newcondition.

The head at the duty flow (A) isprojected across to meet the head-flowcurve at 1660 rpm (the new condition– C). Projection downwards at

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Figure 2. Variable-speed pump head-power-flow in new condition atoriginal service speed, showing how to estimate power when wornand running at a higher speed.

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constant flow leads to the increasedpower required at 425 kW. Hence theextra power required is 31% more. Thepower can be measured for electricallydriven pumps, but it is expensive toobtain accurately. For steam-drivenpumps, as in this example, it isimpossible to measure the power.

The same calculations as before arefollowed to find the time for overhaulfor minimum total cost.

Optimization usingshut-off head results

The shut-off head test information canalso be used to estimate power used in the worn state, applying the optimization calculationsexplained previously. Head-power-flowcharacteristics in the new state areneeded as before, and the operatingpoint must be known. Note the powerrequired at operating point as before.

An overlay trace of the head-flowcurve in the new condition is placedover the new curve and moved to theleft horizontally until the curve cutsthe head axis at the value of shut-offhead obtained on the test. The trace isnow in the position of the worn head-flow curve that is being experienced.Exactly the same process can befollowed as explained above.

Monitoring multi-stage pumps usingbalance leakoff flow

Measurement of the balance deviceleakoff flow has long beenrecommended as a simple way ofinferring internal condition on multi-stage pumps which have this designfeature.

Multi-stage pumps of split casing,ring-section design usually have allthe impellers facing towards the

suction end. To overcome theresulting axial thrust, some dischargeflow is led through an annularclearance to act against a balancedrum or balance disk. The resultingforce is self-adjusting with pump flowor speed, and results in a smallerresidual thrust loading on thebearings. Figure 3 shows such anarrangement.

The balance leakoff flow will increaseas the annular clearance between thedevice and the pump casing increaseswith wear. It is therefore likely thatthe clearances of the wearing rings ateach impeller will also have worn atthe same rate. This is particularly sowith the older-generation pumpsrotating at nominal 3000 rpm, andhaving up to 11 stages.

The attraction of this method is thatthe balance leakoff line is quite smallrelative to the main flow line, and apermanent flow metering device is

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therefore not expensive. Devices suchas Annubar™ flow elements havebeen fitted, relaying the flow to apanel meter or the control room.

Case study

At a power plant with several suchvariable speed pumps, permanentbalance flow metering was readregularly and the trends determinedusing a computer program originallybought for examining vibration data.

Initially, the plotted flows were erratic,until tests were run which revealedthat the flow varied directly with thepump speed, the relationship beingvery close to linear (i.e. 20% morespeed, 20% more flow). This meansthat, for routine monitoring, the speedmust be measured and the flowcorrected to a standard speed valuebefore trending.

Figure 4 shows the trend over someyears of service. A nominal leakageflow of 17 l/s was selected as theindicator of the need for overhaul. Asthis flow is equivalent to 11% of theduty flow, when balance device leakoffflow reached this level, an extra 250kW of power was being consumed.Additional to this would have been thepower wasted due to internalrecirculation, assuming that thewearing ring clearances were also worn.

Unfortunately, at the subsequentoverhaul for the pumps in this examplethe rings were replaced withoutmeasurement of the clearances. It isimportant to find the actual conditionso that this can be correlated with thepredicted condition.

The method described earlier could beused here to calculate when theinvestment in overhaul would bebalanced by the savings in wastedenergy.

Some problems

Elsewhere, on a set of pumps of anothermulti-stage design, both head-flow andbalance flow were measured for someyears, but no correlation was foundbetween the indications of conditiongiven by the methods.

Condition monitoring by performanceanalysis using head-flow alone wasdeveloped for another six pumps of adifferent type. These pumps had 11stages, and operated at constant speedof nominal 3000 rpm. The head wasmeasured using standard test qualitypressure gauges, and the flow foundusing differential pressuremeasurements across the permanentorifice plate installed in the suctionline. This flow element was provided toinitiate the operation of the minimumflow device. Although it was notinstalled in the long straight pipe runsthat are required in flow standards, forcondition monitoring it is repeatabilitythat is required.

During routine tests, one pump showeda test point well below the datum

curve. The pump was duly dismantledand, with what I perceived as ill-concealed glee, the maintenanceengineer reported that the interstageclearances were not worn. Our faith inthe value of our tests suffered acredibility crisis!

A day later, the balance seat area wasreached. It was found to be severelyeroded, showing that water had flowedthrough the stationary annular gapbetween the balance device sleeve andcasing, and left the pump behind thescrews that retained the balance seat.Balance leakoff flow had obviouslybeen very high and, if it had beenmeasured, would have pointed to thisdamage. Extensive building up andmachining of the casing was required.The pumps were modified toincorporate an O-ring seal on thestationary gap.

In conclusion

It seems that it would be unwise to relyon measurement of balance flow alonefor condition monitoring. Both head-flow and balance flow should bemeasured for the best total picture ofcondition. Calculation of the optimumtime to overhaul could be based on ahigher than normal balance leakoffflow. Where possible, the balancevalve/sleeve/seat should be dismantledfor inspection and replacementwithout opening the pump. Retestingafterwards would reveal whether thepump had to be fully dismantled for itsinterstage clearances to be restored. ■

CONTACTRay BeebeSchool of Applied Sciences andEngineeringMonash University Gippsland CampusChurchill, Victoria 3842, Australia.Tel: +61-3-5122-6496Fax: +61-3-5122-6738E-mail: Ray.Beebe@eng.monash.edu.auwww.gippsland.monash.edu.au

f e a t u r e w i n d p u m p s

Figure 3. Cross-section of horizontally-split multi-stage pump,showing thrust balance device.

Figure 4. Data plot of balance device leakoff flow for a multi-stagepump.

Based on papers first presented atINT-PEC – 1st First InternationalConference on Power and Energy,Monash University, Australia,December 1999.

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