NPSH available is the suction-side pressure head above the liquid's vapor pressure, referenced to the pump centerline. It must comfortably exceed the pump's required NPSH or the pump will cavitate.
Worked example
Open water tank at 25 °C, 2 m flooded suction, 0.5 m suction friction. ≈ 11.5 m NPSHa — plenty of margin over a typical 3–4 m required NPSH.
Inputs
Suction vessel pressure (kPa abs) — Open tank = atmospheric (101.325).
Vapor pressure at temp (kPa abs) — At pumping temperature — see the Vapor Pressure tool.
Specific gravity
Static head (+ flooded / − lift) (m)
Suction friction loss (m) — Suction-line head loss — see the Pipe Pressure Drop tool.
NPSH required (optional) (m) — From the pump curve. Leave 0 to skip the margin check.
Frequently asked questions
What sign does static head take?
Positive when the liquid surface is above the pump (flooded suction), negative when the pump is above the source (suction lift). Lift directly reduces NPSHa.
How much NPSH margin is enough?
A common floor is about 0.6 m (or a ratio above 1.1), but the proper margin depends on the service and energy level per ANSI/HI 9.6.1 — high-energy pumps need more.
Why does hot liquid hurt NPSHa?
Vapor pressure rises sharply with temperature, and it's subtracted directly. Pumping near the boiling point leaves little margin, which is why hot services often need flooded suction.
Assumptions
Head terms are in metres of the pumped liquid at pumping temperature.
Static head is positive for flooded suction, negative for suction lift.
Acceleration head (reciprocating pumps) and transient effects are not included.
Required NPSH margin should follow ANSI/HI 9.6.1 / API 610 for the specific service, not a single rule of thumb.
Pipe Pressure Drop — Frictional ΔP and head loss for single-phase flow in a circular pipe (Darcy–Weisbach, Swamee–Jain friction factor).
Fittings Pressure Drop (K-factors) — Minor losses through elbows, tees, and valves by the ΣK excess-head method — the companion to straight-pipe friction.
Pipe Velocity & Reynolds — Line velocity, Reynolds number, and flow regime from flow rate and pipe inside diameter.
Friction Factor (Moody) — Darcy (and Fanning) friction factor from Reynolds number and relative roughness — Colebrook–White solved exactly, with the Swamee–Jain explicit fit for comparison.