W ow, that title sounds like a mouthful – maybe a little too much to take

on in a short article. However the seriousness of the problems I see is what motivates me to try. So here goes.

The public tendering process is inclined to attract inexpensive higher speed pumps. These higher speed pumps tend to have higher suction specifi c speeds (NSS). Higher NSS pumps have a narrow window of allowed fl ow rates from your pump before cavitation damage occurs.

It all started eight years ago when I replaced a high lift bowl assembly on a vertical turbine pump (VTP) in the City of Guelph. The City had asked the original equipment manufacturer (OEM) to identify the problem. They said it was corrosion. This was a bronze impeller in a potable water application (where mankind has been using bronze for decades) that all of a sudden is corroding. Furthermore only part of the impeller was corroding (Figure 1).

There were several other failures I received in our shop that sparked my curiosity and indicated that something serious was going on. I asked myself, is this corrosion or cavitation? What has changed?

I would like to bring one variable to the table that is more likely the root cause of the failure, that is, recirculation cavita-tion caused by running the pump off its best effi ciency point (BEP). But fi rst it is important to lay some groundwork.

There are three distinct types of cavita-tion. Classic cavitation occurs by running the pump without suffi cient Net Positive Suction Head (NPSH) and is related to suction conditions. The second type is known as recirculation cavitation. The third is pre-swirl or air induced cavita-tion. It is recirculation cavitation that is wreaking havoc at water plants across the Province.

Figures 2 and 3 show an impel-ler that has suffered both suction and discharge recirculation cavitation. Figure 2 shows that the suction vanes have cavitated away and the eye of the impel-ler is severely cavitated. Figure 3 shows severely cavitated discharge vanes.

Simply put, this type of cavitation is caused by high velocity fl uid recirculat-ing inside the pump causing a localized drop in pressure enough to cause the water to vaporize. When this vapor goes to higher pressure areas the resulting shock or condensing causes the typical cavitation explosion.

The damage is essentially the same as classic cavitation except that it occurs in distinctly different areas of the impeller.

Notice how some of the impeller shrouds and vanes are not damaged and other areas such as the discharge and suction vanes have literally disap-peared. The root cause here is cavitation, not corrosion as some would have you believe.

WHAT CAUSES THIS CAVITATION?As mentioned earlier in this article, there is a parameter known as NSS that pump manufacturers use to describe the suction characteristics of a particular impeller.

Pumps with higher suction specifi c speeds are less tolerant to fl uctuations in best effi ciency point (BEP). This is shown in Figure 4.

CAVITATION, CORROSION, & PUMP SUCTION SPECIFIC SPEEDBy Jeff Johnson, C.E.T., President, ASL Roteq Ltd.

Figure 1-Vertical turbine pump impeller. Figure 2- Suction vanes cavitation. Figure 3-Discharge vanes cavitation.

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As we lower the suction specifi c speed, the pump is more tolerant to fl uctuations in fl ow. The pump in Figures 2 and 3 has an NSS above 10,000. This suggests this pump is most stable operating at between 70% and 110% of the fl ow at BEP.

Figure 5 shows an impeller removed from a high lift pump in a water treatment plant. The duty point of this pump was selected at 79.5% BEP with a respectable 73.8% effi ciency. This pump has an NSS over 12,000. This impeller has suffered severe suction and discharge recirculation cavitation.

The chart in Figure 4 suggests that when this pump operates around 75% BEP it will leave the stable zone and start the cavitation process. This equates to a pressure change 5 psi (34.5 kPa) or 10 feet.

When you fi rst look at the 70% to 110% range you may think that is good for your plant. However, that assumes the pump you selected is at 100% BEP and it assumes your projections are 100% accurate. If one of these is under and the other over the 100% BEP it does not take much to put you in the danger zone.

PRESSURES VERSUS FLOWMany communities have experienced a reduction in water demand due to conser-vation. Many communities have grown more than projected and the demand has increased. Others have increased their level in the water tower or installed a UV system that adds head to the system.

All of these changes will cause change to the pressures your pumps see and could cause them to run off BEP and outside of a stable range.

CORROSIONSome may argue that this is some form of corrosion or in particular dezincifi cation (selective leaching of zinc from brass alloys). I am not qualifi ed to argue these points, other than to say the following:

• I would have expected corrosion toattack the entire impeller not just theareas where cavitation attacks.

• I would have expected corrosion toattack the bronze sleeves and bronzewear rings, which it did not.

• Dezincifi cation tends to occur instagnant or low velocity areas which thisis not.

• There does exist the potential for thecavitation and corrosion workingtogether to create or add to this problem.

WHAT CAN BE DONE?• Install pumps with lower NSS.

This may involve different pipingand/or electrics and can beexpensive.

• If you cannot change piping orsystem pressures consider usingstainless steel impellers whichprovide better cavitation andcorrosion resistance.

• Investigate why the pressuresare changing so much and makechanges.

• Consider a smaller pump for lowfl ows and a larger pump for higherfl ows.

• In existing installations wherepumps are operating to the right ofBEP consider the installation of anorifi ce plate to choke a pump backtowards BEP.

• Run your pumps at or near BEP.Modify valves or operatingparameters to ensure this.

• Consider a fl ow control valve afterthe pumps to maintain the correctback pressure on the pumps

• Cavitation damages are distinctive.An experienced pump engineer cantell the difference and guide you inthe right direction.

Figure 5-Severe cavitation.Figure 4- Stable operating window vs. suction specifi c speed.

“MANY COMMUNITIES HAVE EXPERIENCED A REDUCTION IN WATER DEMAND...”

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