How to Determine Net Positive Suction Head

Net Positive Suction Head (NPSH) is one of the most used terms for pumps (including eductors) and also one of the least understood. Simply stated, NPSH is a method of evaluating a set of suction conditions for a pump. This formula takes into account variables for the particular liquid being pumped and the conditions under which it is being pumped. It corrects them to a set of standard conditions. This allows the user to reliably predict the performance of a given pump when liquids with varying temperatures, specific gravities, and vapor pressures are being pumped. This formula also corrects for friction loss in the pump suction.

When being used with eductors, it is important that the NPSH be calculated for both the motive and the suction fluids. This is because the motive fluid is also exposed to the reduced pressure in the suction chamber of the eductor. Failure to consider this could result in the motive or suction fluid flashing to a gas as it enters the suction chamber. This would cause the eductor to pump reduced or no suction fluid, or possibly cause the motive fluid to flow out the suction port. When determining the NPSH of both, the one with the higher vapor pressure will be the limiting liquid.

NPSH should always be calculated at the centerline of the eductor. This calculation will result in an adjusted pressure in feet of liquid (water). It must be equal to or greater than the NPSH on the table (see page 1) to achieve the performance at these conditions.

NOTE: Many eductor manufacturers specify performance in feet of water lift at 60 to 70°F. As a rule, this can be changed to NPSH by subtracting the lift from 33 Ft, or in the case of a positive suction head by adding it to 33 Ft.

Calculate the NPSH of the suction fluid by using the following formula:

NPSH = 2.31 x (Ps-Pvp)/Sg + Hs - Hf Hf = Friction loss in the suction piping at full flow conditions. Specified in feet of liquid. This frequently can be found in a hand­book on flow through pipes if the piping arrangement is known. The Jacoby-Tarbox eductor sizing program will estimate friction loss if needed.

Hs = Vertical feet that the liquid is being moved above or below the centerline of the eductor. This number will be negative if the fluid is below, or positive if it is above, the eductor.

Ps = Pressure in the suction vessel in PSIA (Pounds per Square Inch Absolute). If the vessel is at atmospheric pressure, specify normal atmospheric pressure at the application site. (Normal pressure at sea level is 14.7 PSIA.)

Pvp = Vapor pressure of the liquid being pumped at the highest temperature that it will be pumped.

To calculate for the NPSH of the motive fluid, change the Pvp to the vapor pressure of the motive fluid and recalculate using all other parameters of the suction conditions. To determine vapor pressure for water, go to a set of steam tables. If the liquid is a water-based slurry or solution, these tables will generally provide a conservative estimate. For other chemicals the vapor pressure can generally be obtained from the supplier or a chemical handbook. Estimates can be used if it is understood that these may cause some problems if the estimated value is lower than the actual vapor pressure of the liquid.

Sg = Specific gravity of the liquid being pumped.