OpenSpecEngineering

Finds the net positive suction head available (NPSHA) at a pump’s suction and compares it with the NPSH the pump requires (NPSHR). A pump cavitates when the pressure at its inlet falls to the liquid’s vapor pressure. Cavitation causes noise, lost head and eroded impellers. Pump users and system designers use the page to check a suction layout before buying a pump.

Equation

NPSHA = (P_atm − P_vp)/(ρ·g) + h_s − h_f

P_atm is the absolute pressure on the liquid surface and P_vp the liquid’s vapor pressure, both in Pa. ρ is the liquid density in kg/m³ and g = 9.80665 m/s². h_s is the static height of the liquid surface above the pump centerline, negative for a suction lift. h_f is the friction and fitting loss in the suction line at design flow, in m [2]. The margin is NPSHA − NPSHR.

Inputs

• Site Elevation fills P_atm from the standard atmosphere [3] for an open tank or sump. For a closed vessel, choose Custom and enter the vessel’s absolute pressure.
• Fluid fills the vapor pressure and density. Water presets from 5 to 100 °C come from IAPWS-IF97 [4], and ethanol and methanol at 25 °C from CoolProp [5]. For any other liquid, choose Custom and enter both at the pumping temperature.
• NPSH Required comes from the pump maker’s curve at your flow rate.

Limits and common mistakes

• The status says OK when the margin exceeds 0.6 m, Caution when it is between 0 and 0.6 m, and Fail otherwise. NPSHR on a curve is usually the point of a 3 % head drop, where the pump already cavitates. HI 9.6.1 [1] recommends a margin ratio NPSHA/NPSHR of about 1.1 to 2.5 or more, depending on the pump and its service, so check that too.
• Using gauge pressure for P_atm. It must be absolute.
• Forgetting temperature. Hot water’s vapor pressure cuts NPSHA by meters.
• Taking h_f at a low flow. Use the highest flow the pump will see, where NPSHR is also highest.
• Measuring h_s from the tank bottom. Use the lowest operating liquid level.

Worked example

A plant at 1,500 m lifts 60 °C water from an open sump whose lowest level is 2 m below the pump centerline. The suction line loses 0.8 m at design flow, and the pump needs 4 m NPSHR there.

1. Choose Site Elevation 1500 m — 84.556 kPa.
2. Enter Static Suction Head −2 m and Suction Line Friction Loss 0.8 m.
3. Choose Fluid Water, 60°C. The vapor pressure fills in 19.946 kPa and the density 983.2 kg/m³.
4. Enter NPSH Required 4 m.
5. Press Solve. The Solution shows: NPSH Available (NPSHA): 3.90096 m
   Margin over NPSH Required: -0.0990374 m
   Status: Fail — cavitation risk

Check by hand: the atmosphere is worth 84,556/(983.2 × 9.80665) = 8.77 m of head and the vapor pressure 2.07 m. Then 8.77 − 2.07 − 2 − 0.8 = 3.90 m. Setting the pump 1.5 m below the sump level instead raises NPSHA by 3.5 m, to 7.40 m.