Inputs

All pressure inputs are absolute. For an open tank, surface pressure is local atmospheric pressure, not 0 psig.

Liquid density

Density unit

Pressure at liquid surface, absolute

Surface pressure unit

Liquid vapor pressure, absolute

Vapor pressure unit

Liquid-surface elevation relative to pump centerline

Elevation unit

Positive if the liquid surface is above the pump; negative for suction lift.

Suction-line friction loss

Loss unit

NPSH required (optional)

NPSHr unit

Results

NPSH available

—

NPSH available

—

Margin above NPSHr

—

Equation and assumptions

NPSHₐ = (Psurface,abs − Pvapor)/(ρg) + z − h_f

This reservoir-to-pump form assumes negligible velocity at the supply free surface and that the entered suction friction loss includes the entrance, pipe, and fittings up to the pump suction flange. NPSH required is pump-specific and must come from the pump manufacturer at the operating flow and speed.