Pump Cavitation Indicator?
Pump Cavitation Indicator?
(OP)
We are currently in the process of starting up an open system with large Armstrong Inline Pumps (~100hp). The pumps are currently controlled by the BAS through VFDs. The balancer has now come in and claims that he is hearing cavitation as he ramps up the pumps and that we are not meeting the NPSHR (27'H2O). Through our calculations we have calculated a NPSHA of ~40'H2O. With the pumps off, he is claiming that the pressure on the suction side of the pump is only ~18'H2O. Something here doesn't seem right and I had personally ramped up the pumps to full capacity and only heard a little noise from the volute. The majority of the noise at that time was coming from the motor. Is there a rule of thumb to determine at what decibel level we can assume cavitation?





RE: Pump Cavitation Indicator?
RE: Pump Cavitation Indicator?
HVAC68
RE: Pump Cavitation Indicator?
Unless you have high temperature water and are on the moon, I doubt you have cavitation.
I think you're fine.... maybe bad bearings in the motor???
Good luck!
RE: Pump Cavitation Indicator?
After looking into it more, I believe that KLH is correct in that the discrepancy between our calculated and the 18' is in the atmospheric pressure. I think that the gauge on the suction side of the pump is only reading gauge pressure and not absolute, and so we have to add additional pressure to get the NPSHA. We had shown our NPSHA calcs to the balancer and he was convinced that NPSH does not include atmosphere, but I am sure he is incorrect.
HVAC68,
Yes, we did go back to the curves to make sure we were getting the proper performance that everything was matching up. It all seemed to align, except that a few individuals thought the pumps were running too noisy.
KLH,
Thanks, I forgot to take into account that the gauge measures GAGE pressure and so we have to add for atmosphere. I think we are at the point where most have sided with me and have thrown out cavitation. However, the lead engineer is saying that he has never heard pumps so loud and that he thinks there may be a problem. You may be right that there could be noise from the motor. That I believe is the loudest portion during operation. Currently these are 100hp split coupled inline pumps that are supported purely from the piping. So I believe that it may be an issue with the motor vibrating and it transferring the vibrations into the piping.
RE: Pump Cavitation Indicator?
RE: Pump Cavitation Indicator?
It's just a little frustrating at this point that no one seems to be listening to me because they feel that I don't have enough experience to know what I'm doing although I have been on working on site now for longer than anyone else involved.
Thanks all for the suggestions, I'll let you know how it goes...
RE: Pump Cavitation Indicator?
I was looking for an old post and came across this one in the process.
How did you ever make out with your pumps?
RE: Pump Cavitation Indicator?
Really the most frustrating thing of this entire debacle was that everyone jumped to conclusions that the system wasn't properly designed and that we had screwed something up when in reality, they just needed to properly balance and run the system.
RE: Pump Cavitation Indicator?
RE: Pump Cavitation Indicator?
It is unbelievable how misunderstood the concept of cavitation is. Everybody is an expert on cavitation, like your TAB firm, but none of them bother to even read up on the concept enough to calculate NPSHA/R correctly.
About the only time you will ever find cavitation in the HVAC realm is on condensate receivers or boiler feedwater, where the pumped liquid has a low NPSHA because it is very close to its boiling point. In the case where it is exactly at the boiling point the NPSHA is gravity head on the suction less the suction pipe losses.
You can definitely get flashing across a valve though.. If you are pumping hot water across a valve that is severely pinched closed, and there is alot of pressure (dead-headed), then the velocity at the throat of the valve can lower the normal pressure enough to cause flashing (Bernoulli's theorem) which is very loud.
RE: Pump Cavitation Indicator?