6 replies to this topic

[ahmedensem]

Hello members,
on most docmentations, we speak of NPSH in meters (m) for a pump , but other documentation is in bar. I just would like know what is the physical meaning of NPSH in bar?

[S.AHMAD]

Dear Ahmed
1. The right/correct unit for NPSH is in "m of fluid".
2. The value of pressure depends on the density of fluid at the operating conditions. If the density change, so does the pressure, as correlated by:

Pressure = rho x g x H

3. The value of H in m does not change with density.
4. Hope the above gives you some food of thought.

[ahmedensem]

Dear S.AHMAD,
It's completly rignt what you said,but we can also give "bar" or "Pa" as unit for the NPSH.
To be clear,I'm asking for the fact that if the NPSH in bar or Pa can represent the minimal pressure in the pump's entry.?!

[kkala]

Coulson and Richardson, Chemical Engineering, vol 1, 3rd edition (Pergamon, 1977), pumping of fluids (p. 138): The NPSH is the amount by which the pressure at the suction point (expressed as head of liquid to be pumped) must exceed the vapor pressure of the liquid. It allows for drop in pressure and acceleration as the liquid flows into the impeller, as well as for irregularities in this flow pattern. Not obtained NPSH means that partial vaporization is likely to occur, causing the pump to be starved of liquid.
So NPSH physically means required absolute supply pressure at pump suction, minus liquid vapor pressure, to avoid cavitation. It is convenient to express it in head of liquid, since it is considered independent of liquid (*), depending only on impeller characteristics.
(*) Hydrocarbons tend to present lower NPSH required compared to water; but shop tests are conducted with water and supplier curves are based on water. I have not locally observed any NPSH relaxation for liquid hydrocarbon pumps; no relaxation is conservative.

Edited by kkala, 26 April 2012 - 09:11 AM.

[S.AHMAD]

Dear Ahmed
1. If you are asking whether NPSH can be expressed in unit of pressure, the answer is absolute YES as long as you understand that value (e.g 15 kPa, density 800 kg/kl) is only applicable/valid for that particular density. When the density changes (e.g 700 kg/kl), the NPSH value in kPa will also changed (13 kPa).
2. In other words, when we specify the NPSH in unit of kPa, we must also stated the density otherwise that value is useless. For example, if a pump vendor specify the pump NPSH required is 20 kPa. So how can you determine that particular pump is suitable for our use? unless the vendor also said that NPSHr = 20 kPa at density 800 kg/kl.
3. In the other hand, if the vendor specify the NPSH in m, it is applicable regardless of density.
4. Hope the above examples give a better insight.

Edited by S.AHMAD, 26 April 2012 - 09:01 PM.

[simadri]

Dear Ahmed,
I am completely agreed with Mr. S.AHMAD.

If you will follow pump curves provided by vendor, you will get a NPSH curve where you have to read the NPSH in head unit. When pump vendor does the experiments and prepare the curve, they do not use the liquid to be pumped in future as experiment media, even that is not possible as there are 'n' numbers of liquids which require to be pumped in the world. Since head is a density independent term, the pump curves will be independent of the liquid properties. Hence, if you want to change the type of liquid to be pumped in future, this pump curve will help you as head will not change.

[ahmedensem]

thanks friends