NPSHr with different fluids

[Hervesj]

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.

 

[21121956]

Hello everybody:

If you do a search on this Forum, you could find information like this: thread407-235895.



El que no puede andar, se sienta.

 

[georgeverghese]

See also figure 10-25 in Perry Chem Engg Handbook 7th edition for NPSHr correction for fluids to cold water NPSHr.

 

[Hervesj]

Hello,

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

 

[TenPenny]

Different liquids will have different NPSHr. However, it is extremely unlikely that you will ever find any NPSHr curves for a pump based on anything other than water.

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.

 

[Hervesj]

Thanks TenPenny.

I notive that you gave the opposite answer to the same question some years ago :

http://www.eng-tips.com/viewthread.cfm?qid=235895

 

[LittleInch]

IMHO, the pump curve NPSHr will be the same for all fluids ( so the answer is no), but this measured in feetor m, just like the differential head. Hence pressure will change depending on SG of the fluid. Convert everything to feet (or m) head in absolute pressure and it all works much better.

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.

 

[georgeverghese]

When you look at fig 10-25, statement 1 is only approximately correct - there is still a minor difference in NPSHr for 2 different fluids even when the vapor pressure of the 2 fluids is the same.

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.

 

[LittleInch]

George,

Can you paste that figure in?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

LI, Tried to earlier, but couldnt from my electronic copy of the 7th edn. The text makes a reference to the Hydraulics Institute Standards for this graph if you have a copy of this instead. This graph is new to the 7th edn, and isnt in the previous 6th edn.