Net Positive Suction Head Available (NPSHA)

Look here first line

This is one of the better sites on pump.

And there is the explanations for next few pages.

All you guys bookmark McNally's site. He has most of the answers to commercial size centrifugal pump user questions.

Ed Weldon

Depends where the pressure tap is and the configuration of the inlet plumbing.

But, if the pressure tap is at the inlet of the pump it will be close.

By definition- NPSHA reads in absolute values. I know of no gauge that has a readout in absolute pressure.

You would have to add 14.7 PSI (or 1 BAR or 760 mm Hg), assuming you are at sea level, to the actual gauge reading to get a "reading" in absolute pressure. Or, have I missed something?

NPSHA is NPSH available. NPSHR is NPSH required.

Hello Energygod

Yes, you have . Also need to subtract the vapour pressure of the liquid (as sv13 mentioned) and add the velocity head (using velocity at the pressure tapping point). If you ignore velocity head it gives you a slightly safe (i.e. low) figure for NPSHA, but strictly speaking it should be included.

Cheers.........Codey

Absolute pressure gauges exist (many Web references to them), but it is surprising they aren't more commonly used. Compound pressure gauges (measuring pressure above atmospheric and vacuum below) are common, notably in refrigeration, but for some calculations you need to convert vacuum (inches Hg) into equivalent psi units (or the corresponding metric units in either case).

Pressure transducers are also available in absolute units.

Not quite. For calculating NPSH, you'll have to sustract the vapor pressure at the operating temperature. If the suction is from a vessel which is at saturated vapour pressure, for example deaerators, then a pressure gauge installed on the pump suction will give a good indiaction of the NPSH avaialble after adding the atmospheric pressure.

It does not matter whether the system is closed or not.

Dear Naif,

NPSHA or net positive suction head available is defined this way:

NPSHA = P + Z -hf - hvp

where p is the suction drum pressure, Z is the elevation (differential), hf is the friction loss and hvp is the vapor pressure. All are in unit of length e.g. meter of fluid

When the system is closed, the pressure reading at the suction is the gauge-pressure due to elevation plus the pressure of the suction drum (or P + Z). So in order to find the NPSHA, you need to firstly convert this pressure into absolute term and deduct friction loss and vapor pressure at the operating temperature. Express all terms in unit height of liquid.

In some cases, such as distillation column, the suction drum pressure is equal to the vapor pressure. so the formula for this case is

NPSHA = Z - hf

I some design, the suction friction loss is comparatively small, so in the case the NPSHA can be approximated by

NPSHA = Z = [suction pressure] - [drum pressure]

Hope u get the idea.

If the pressure gage is near the suction nozzle, the drum pressure and liquid lift will be included in the gage reading. Only Hf and hvp are to be accounted. Right?

Dear b.v. rao

Yes! you got it right.

If we measure the suction pressure with flow available then the gauge pressure measures the drum pressure plus elevation (or what u called lift) minus the friction loss. So in this case we can calculate the NPSHA by this formula

NPSHA = [suction pressure] - [vapor pressure]