NPSHr with different fluids
NPSHr with different fluids
(OP)
Hello,
I just need to clarify my mind on NPSHr.
Can someone tell me if the following sentences are true or not ?
1) Given two fluids with two different densities but with the same vapour pressure : then a pump will have the same NPSHr curve for both fluids. Only NPSHa will be different.
2) Given two fluids with two different densities and two different vapour pressures : then a pump will have different NPSHr curves and also a different NPSHa.
Thanks.
I just need to clarify my mind on NPSHr.
Can someone tell me if the following sentences are true or not ?
1) Given two fluids with two different densities but with the same vapour pressure : then a pump will have the same NPSHr curve for both fluids. Only NPSHa will be different.
2) Given two fluids with two different densities and two different vapour pressures : then a pump will have different NPSHr curves and also a different NPSHa.
Thanks.





RE: NPSHr with different fluids
If you do a search on this Forum, you could find information like this: thread407-235895: Do I need to correct NPSHR in pump curve for different S.G.?.
El que no puede andar, se sienta.
RE: NPSHr with different fluids
RE: NPSHr with different fluids
thanks for your inputs but they still do not tell me if my two sentences are correct or wrong.
Distinction has to be made between "two fluids with two different S.G. but the same vapour pressure" (CASE 1) and "two fluids with two different S.G and two different vapour pressure" (CASE 2).
CASE 1 will have the same NPSHr curve while CASE 2 will not.
Can someone confirm ?
Thanks
RE: NPSHr with different fluids
From Lobanoff and Ross, Centrifugal Pumps - Design and Application:
"The boiling of the liquid in the process of cavitation is a thermal process and is dependent on the liquid properties, pressure, temperature, latent heat of vaporization, and specific heat."
and also note
"From experience we know that up to 2,000 SSU we are safe in using water NPSHR values. Above 2,000 SSU, it is necessary to use the pump designer's judgment or experimentally determine the cavitation characteristics."
So the answer is, different liquids will have different NPSHR curves.
RE: NPSHr with different fluids
I notive that you gave the opposite answer to the same question some years ago : http://www.eng-tips.com/viewthread.cfm?qid=235895
RE: NPSHr with different fluids
Only if the substance is a long way from water will you get any appreciable difference.
Always remember NPSHr is NOT cavitation. It is measured as the point at which the differential head drops by 3%. Cavitation can start at much higher heads / pressures than NPSHr.
Question 2 is the same as question 1 (only with two variables instead of one) so has the same answer. - No. The curve is the same measured in feet or m of liquid.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: NPSHr with different fluids
Stement 2 is correct.
Also note that from this figure, even for the same fluid, the NPSHr is different when the vapor pressure at the operating temp has changed.
RE: NPSHr with different fluids
Can you paste that figure in?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: NPSHr with different fluids