Pump NPSH available

NPSH available and margin for open tanks and closed vessels — with water vapour pressure from steam tables and suction-line friction.

Inputs

Units:

Liquid

Pumping temperatureuse the highest you expect

Vapour pressure at that temperature

Specific gravity

Viscosity

Suction source

Vessel

Site altitude

Pressure on the liquid surface (absolute)

Liquid at its boiling pointdeaerator outlet, condensate receiver, column bottom

Liquid level above pump centrelineuse the lowest level; negative = suction lift

Suction line

Friction loss

Suction line friction loss

Flow

Maximum flow (optional)check NPSH at max flow too; leave 0 to skip

Pipe inside diameter

Equivalent length incl. fittingspipe length + fittings, valves, strainer

Pipe roughnessnew steel 0.045 mm

Pump

NPSH required (from the pump curve)at this flow; 0 = skip

Results

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The Pump NPSH and Cavitation Excel tool does everything here and more: nPSH available for four cases side by side — tank or suction-gauge route, altitude, hot liquid, low level — against the vendor NPSH3 curve with margins, suction specific speed and vortex submergence. Print-ready report, any units, visible formulas, no macros.

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How this calculator works

NPSH available = pressure head on the liquid surface − vapour pressure head + liquid level above the pump − suction line friction. For water, density, viscosity and vapour pressure come from steam tables (vapour pressure by the IAPWS-IF97 equation). For an open tank, the air pressure is worked out from the site altitude. Friction is calculated by Darcy-Weisbach with the Swamee-Jain friction factor, or you can enter it directly.

The example is 80 °C water pumped at 120 m³/h from an open tank at 300 m altitude, with the level 3 m above the pump and 25 m equivalent length of 150 mm pipe. NPSH available is 7.8 m; with 4.5 m required, the margin is 3.3 m and the ratio 1.73.

NPSH required is not a safe limit. The maker's NPSHr is the point where the pump has already lost 3 % of its head to cavitation. Allow a margin — a ratio of at least 1.3 and 1 m (3 ft) is a common screening target, and hot water, high suction energy and long running hours need more. Hydraulic Institute guidance (ANSI/HI 9.6.1) and the pump maker set the margin for your service.

Limits: single liquid, no dissolved gas; for hydrocarbon mixtures use the bubble-point pressure as the vapour pressure. References: Perry's Chemical Engineers' Handbook §6 and §10; Hydraulic Institute ANSI/HI 9.6.1; IAPWS-IF97; Swamee and Jain (1976).

Learn the method step by step in Pumps Made Simple, Chapter 7.

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