avoiding cavitation damage extends pump life - waterworld oct_2007

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by Allan Budris

One way that the Best-of-Class users reduce the Life Cycle Costs of their pumpinstallations is by selecting pumps and designing pump systems that will avoidcavitation damage within their pumps. Cavitation damage can drasticallyshorten the life of the pump impeller, mechanical seals, bearings and possiblyother pump components. In other words it will reduce the mean-time-between-failure (MTBF), which will increase maintenance costs and pump down time.

There has been a lot written on the dangers of pump cavitation damage whenadequate suction pressure (Net Positive Suction Head Available - NPSHA) isnot provided; when pumps are operated at low flow rates (below the start ofsuction recirculation); and/or when sufficient straight runs of piping are notprovided upstream of the pump suction. It can require NPSHA values of fourtimes the published Net Positive Suction Head Required (NPSHR), or more, toavoid all cavitation in a pump. The start of Suction Recirculation, which occursin all centrifugal pumps at some reduced flow rate, can be as high as 85% ormore of the pump best efficiency flow rate (bep). The required minimum straightlength of pipe, up stream of the pump suction, may be as high as 15 times thepipe diameter or higher.

However, experienced pump users also know that most smaller pumps do not

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experience cavitation damage, even when operated under these unfavorableconditions. So the question is, specifically when must pump users take specialprecautions to avoid cavitation damage, and the associated increase inmaintenance costs?

Well, based my extensive experience with several major pump manufacturers, Iwas able to developed a method that predicts when pumps are susceptible tocavitation noise, vibration, and/or damage; and when they are free of thesedamaging affects. The method is called Suction Energy, with specific gatingvalues of Suction Energy identified for different pump types. The amount ofenergy in a pumped fluid which flashes into vapor and then collapses back intoa liquid in the high pressure areas of the impeller determines the amount ofnoise and/or damage from cavitation.

Suction energy is another term for the liquid momentum in the suction eye of apump impeller, which means that it is a function of the mass and velocity of theliquid in the inlet. Suction energy is defined as:

Click here to enlarge image

Based on the experience of hundreds of centrifugal pumps, the writer was ableto establish the following specific gating values for the start of High SuctionEnergy and Very High Suction Energy, for various pump types.Low Suction EnergyPumps with levels of suction energy below these values are considered to havelow suction energy. Generally speaking, low suction energy pumps are notprone to noise, vibration or damage from cavitation or recirculation. However,there could be detrimental effects on mechanical seals from the air or vaporswhich may be liberated from the liquid during the formation of the cavitationbubbles, under low NPSH margin conditions (below 1.1 - 1.3 NPSH marginratio).High Suction EnergyPumps with high suction energy and low NPSH margins, especially whenoperated in the suction recirculation flow range, may experience noise, vibrationand/or minor cavitation erosion damage with impeller materials that have lowcavitation resistance, such as cast iron. High suction energy starts at about3,560 rpm in end suction pumps with 6" and larger suction nozzles sizes, andsplit case pumps with 8"and larger suction nozzles. At ~1,780 rpm, high suctionenergy starts with 10 suction nozzle size end suction pumps, and 12" suctionsplit case pumps.

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Very High Suction EnergyPumps with very high suction energy and low NPSH margins, especially whenoperated in the suction recirculation flow range, may experience erosiondamage, even with cavitation resistant materials, such as stainless steel.NPSH MarginThe NPSHR of a pump does not represent the start of cavitation. It is actuallythe NPSHA that will cause the total head to be reduced by 3%, due to flowblockage from cavitation vapor in the entry of the impeller vanes. It can takefrom 1.05 to 2.5 times the NPSHR just to achieve the 100 percent head point,and typically 4 to 5 times the 3% NPSHR of the pump to totally eliminatecavitation. The question then becomes, how much NPSH Margin is reallyrequired to minimize cavitation damage, and achieve extended MTBF rates.


NPSH Margin Reliability FactorI developed the following graph (figure 1), based on field experience, to providea reasonable approximation of the reliability that can be expected with aqueousliquids, when various NPSH margin ratios are applied to High Suction Energypumps of increasing energy levels (Actual Pump Suction Energy divided by theHigh Suction Energy gating value for the pump type). These NPSH marginreliability factors are based on the fact that, above the gating suction energyvalues, cavitation becomes more severe. In other words, the greater the suctionenergy, the more important it is to suppress the residual cavitation that existsabove the NPSHR, to prevent damage. This reliability factor is only applicablewithin the allowable operating flow region, above the start of suctionrecirculation. Much higher NPSH margin values are required in the region of


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suction recirculation for high and very high suction energy pump applications.


Figure 2 can be used to approximate the start of suction recirculation for highsuction energy pumps, in the absence of actual test results. The three curvesshow the effect of specific speed on the start of suction recirculation, whichincreases with increasing specific speed and suction specific speed.

This new method for predicting cavitation damage in centrifugal pumps, whichhas been adopted by the Hydraulic Institute and major pump companies, cansave thousands of dollars in unnecessary maintenance costs.About the Author:Allan R. Budris, P.E., is an independent consulting engineer who specializes intraining, failure analysis, troubleshooting, reliability, efficiency audits andlitigation support on pumps and pumping systems. With offices in Washington,NJ, he can be contacted via e-mail at [email protected].

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avoiding cavitation damage extends pump life - waterworld oct_2007

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