Cavitating pump
Cavitating pump
(OP)
Hi!
I have a pump that serves a cooling tower. It was designed for 340GPM and 68 feet of head. In the installed condition, it is moving the 335GPM but head pressure is on 43 feet and its VFD is running at 45 HZ (2832RPM, max speed is 3600RPM). At the pump inlet the pressure is barely 1PSI, and the pump has cavitated.
I was not there for the balancing so I am assuming the balancer ran the pump at 60Hz, got too much flow, and they slowed the pump down. Was that the incorrect thing to do? Is the pump size incorrect (68 feet design but only 43 feet drop in the system) How can I keep my flow but get my head pressure up so I don't cavitate?
Here is the design curve, and the operating curve.

I have a pump that serves a cooling tower. It was designed for 340GPM and 68 feet of head. In the installed condition, it is moving the 335GPM but head pressure is on 43 feet and its VFD is running at 45 HZ (2832RPM, max speed is 3600RPM). At the pump inlet the pressure is barely 1PSI, and the pump has cavitated.
I was not there for the balancing so I am assuming the balancer ran the pump at 60Hz, got too much flow, and they slowed the pump down. Was that the incorrect thing to do? Is the pump size incorrect (68 feet design but only 43 feet drop in the system) How can I keep my flow but get my head pressure up so I don't cavitate?
Here is the design curve, and the operating curve.






RE: Cavitating pump
Very interesting, but nothing to do with cavitation. Your delivery head pressure has nothing to do with the inlet pressure which is what is causing your cavitation.
The pump is running towards the RHS of the pump curve which can increase NPSHR and also increase the cavitation curve quite a lot more than NPSHR. Its also running at a flow> the BEP which is not a great idea.
You don't say what the temperature of the incoming fluid is, but with a hot water system it's not hard to run into cavitation issues, which, as said are NOT the same as NPSH.
~Ask the vendor for a cavitation curve for your pump. You might be quite surprised.
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Also: If you get a response it's polite to respond to it.
RE: Cavitating pump
Remember that NPSHR is based on ABSOLUTE pressures, not gauge pressure. So you have an additional 30 feet up your sleeve that you probably didn´t realise. Take away the losses, temperature effects, altitude of the installation, etc., and you should still be OK.
I think I know why you presented TWO DIFFERENT pump curves. The first you mentioned is the design. The second is the actual selection which is a different curve. Not sure what happened to the original series 0306-010.0 design model but that would have been much better suited to the 340 gpm. Now you are stuck with a pump that will always be operating to the right of BEP.
Aside from changing the pump to a model that is better suited to pump 340 gpm, I think the best option you have is to lower the design flow rate to about 280 gpm IF the cooling tower circuit is OK with that. Just drop the rpm further or trim the impeller. Don´t do any more than these options. I am assuming of course that cavitation is occurring.
RE: Cavitating pump
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Cavitating pump
Have you investigated imposing some additional head on the system by throttling a discharge valve?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Cavitating pump
You might only need an extra 2-3m head on the inlet.
Many questions - no answers yet.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Cavitating pump
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Cavitating pump
I don't think the cavitating has anything to do with the pump discharge. There is a 3-way bypass valve from the return back to the supply. It is more or less about 10 feet above the pump. The cooling tower basin is more or less 10 feet above the bypass valve. I read a few places (tower manufacturer install guides) that if the bypass doesn't empty into the tower sump, that it can actually go into negative pressure and draw air into the system, and this air gets into the pump.
See page 9: http://www.baltimoreaircoil.com/english/resource-l...
So we are going to experiment with modulating the bypass from 0% to 100% and see if that effects the inlet pressure at the pump.
RE: Cavitating pump
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Cavitating pump
Without it, it is just running in circles and applying a shotgun approach to the problem / cure.
Your call,there are a lot of members with pump trouble shooting experience in E-T but little experience in fortune telling.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Cavitating pump
See my strap line, but I really like the fortune telling bit....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Cavitating pump
The design flow is met & the VFD is properly set so pump is not wasting energy. Lower head at start is to be expected because the new pipe & chiller tubes are not yet fouled. Open water piping system will corrode more than closed system so de sign allows for more pressure drop.
Make sure air has been bleed from the piping system. The bypass valve is normally open with chiller off. Set controls to gradually close valve when pump start & when condenser water supply temperature is above 60 F. Valve should not be controlled to modulate because during winter lowering the flow to the cooling tower will promote ice built up in the cooling tower fill.
RE: Cavitating pump
Hence your opening statement is without basis.
Also NPSH is not the same as cavitation. Cavitation often/usually happens at a higher head than NPSH, sometime by several feet.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Cavitating pump
Gage reading in ft of water absolute = 1/.433 + 33.96 = 36.27
Correction for gage to pump centerline elevation = say + 4/12 = 0.33 ft
Deduct absolute vapor pressure of condenser water at 85 degrees = - (1.213 in Hg absolute x 1.13406 ft H2O/in Hg) = - 1.376 ft of water absolute
Add velocity head in ft of water = .00259 x (GPM)^2 / (inch dia)^4 = .00259 x 340 ^2 / 3.068^2 = 3.38 ft
NPSH = 36.27 + 0.33 - 1.376 + 3.38 = 38.60 ft vs 12.8 NPSH required so pump should not cavitate.
You should not hear gurgling sound at pump.
RE: Cavitating pump
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Cavitating pump
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.