NPSHA vs NPSHR

[arbitrader]

I am calculating NPSHA for specifying a 4,000 gpm gasoline pump.

I know manufacturer's pump curve prepare NPSHR curve based upon water. Therefore, I assume NPSHA conponents should all be referenced using water. Is this correct?

For example I am proposing that NPSHA would be 33' water (atmospheric head from an vented storage tank, less 17' water (vp of 7.5 psia gasoline converted to water, plus 3' water static suction head less 4' water friction head (suction losses based upon gasoline then converted to water.

 

[roaling]

In calculating NPSHA you should refer to gasoline applying correction factor, if any, on NPSHR with water.

 

[MortenA]

There is no "correction factor" as such - just remember that NPSHa includes that you subtract the vapour pressure of the medium. This can often be ignored for water unless the temperature is high.

You dont need to refer to "water". Calculated the NPSH a as your static head (incl ambient pressure) (inch, feet, meter whatever) - pressure loss in piping - vapour pressure and stick to "absolute pressure" even though this may stress some mechanical engineers.

Best regards

Morten

 

[roaling]

Dear Morten,
May be I was not clear. The correction factor to apply is on the NPSHR not NPSHA. In case of gasoline, I think that no correction is needed.

 

[arbitrader]

Maybe I was not clear in my question. Should atmospheric pressure be 33 feet of water or 33ft divided by SG of gasoline?

 

[roaling]

You have two ways: to make calculation referring to feet or to psi. If you chose psi then you have to convert to feet. Anyway, your assumption to divide atmospheric pressure in feet by S.G. is correct.

 

[MortenA]

OK if you move into specifying the pressure in psi/bar whatever then of course you need to calculate the pressure using the actual media (and thus include teh actual gravity of gasoline). If you however specify your liquid and the NPSH in liquid collumn - then you are beyound that.

Best regards

Morten