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Pipelines, Piping and Fluid Mechanics engineering FAQ

Flow in Pipe

NPSHa - Net Positive Suction Head available by itsmoked
Posted: 12 Jan 10 (Edited 16 Jan 10)

This FAQ is a reduction of tbarkerjr's question and BigInch's response on: 12 Jan 2010.

I found it to be clear, concise, and informative.

Thanks you two!


The only two that are important are NPSHa and NPSHr.  You don't calculate NPSHr, you get it from the mfgr's pump data. You only have to calculate NPSHa which must be >= NPSHr.

Use absolute pressures. (If tank is open or closed, it makes no difference to the equation, as long as you calculate the tank's absolute pressure correctly).  Vapor pressure is always absolute.

NPSHa = p(on_fluid_surface)/y - p(v)/y + v(s)^2/2g + h(e) - h(l)

Note: The velocity term can be ignored normally as it doesn't contribute significantly.

p(on_fluid_surface)absolute pressure on_fluid's_surface
y = specific weight of fluid
v(s) = velocity of fluid
g = acceleration of gravity
p(v) = vapor pressure
h(e) = elevation from surface to pump
h(l) = head loss

The sign convention for h(e) is positive when the fluid surface is above the pump and negative when below the pump's Center Line(CL).

Dropping the velocity term is common as it's almost always relatively small, not worth calculating, and yields a conservative result.


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