Compute the available Net Positive Suction Head (NPSH_a) of a pump and compare it against the required NPSH (NPSH_r) from the pump curve. Head-balance form: NPSH_a = (p_surface_abs - p_vapor) / (ρ·g) + H_static - h_friction. Surface and vapor pressures are absolute; H_static is positive for flooded suction, negative for suction lift. Returns margin (NPSH_a - NPSH_r), ratio (NPSH_a/NPSH_r) and a classification: safe, marginal (margin < 0.5 m), or cavitation likely. If NPSH_r = 0, only NPSH_a is reported. Pressure in Pa/kPa/bar/atm/psi, length in m/ft, density in kg/m³/g/cm³/lb/ft³.
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Key facts
Category
Math & Numbers
Input types
number, select
Output type
json
Sample coverage
4
API ready
Yes
Overview
Calculate a pump’s available Net Positive Suction Head (NPSH_a) from absolute surface pressure, vapor pressure, static head, friction loss, liquid density, and gravity, then compare it with the pump’s required NPSH (NPSH_r) to assess cavitation risk.
When to use
Check whether a pump has enough suction head for a selected operating condition.
Compare calculated NPSH_a with NPSH_r from a pump curve.
Evaluate how suction lift, vapor pressure, or friction loss affects cavitation margin.
How it works
1Enter absolute surface pressure and absolute vapor pressure, selecting Pa, kPa, bar, atm, or psi.
2Provide static head, where a positive value represents flooded suction and a negative value represents suction lift.
3Enter suction-line friction loss, liquid density, gravity, and optionally the pump’s required NPSH.
4The calculator returns NPSH_a, margin, ratio, and a classification of safe, marginal, or cavitation likely when NPSH_r is provided.
Use cases
Pump selection and preliminary hydraulic design for centrifugal pump systems.
Suction-piping checks for flooded tanks and suction-lift arrangements.
Troubleshooting cavitation risk by testing pressure, temperature-related vapor pressure, elevation, and friction-loss changes.
Examples
1. Open tank with flooded suction
Pump engineer
Background
A water pump draws from an open tank with the liquid surface 2 m above the pump centerline.
Problem
Verify that the available NPSH exceeds the pump’s required NPSH.
How to use
Enter surface pressure as 1 atm, vapor pressure as 2.34 kPa, static head as 2 m, friction loss as 0.5 m, density as 1000 kg/m³, gravity as 9.81 m/s², and NPSH_r as 3 m.
Pressure units: atm and kPa; head units: m; density unit: kg/m³.
Outcome
NPSH_a is approximately 11.5902 m, with an 8.5902 m margin and a safe classification.
2. Pump with suction lift
Fluid systems designer
Background
A pump draws water from a source located 3 m below the pump, and the suction line has 0.8 m of friction loss.
Problem
Check whether the suction-lift arrangement provides sufficient NPSH for the pump.
How to use
Enter surface pressure as 1 atm, vapor pressure as 2.34 kPa, static head as -3 m, friction loss as 0.8 m, density as 1000 kg/m³, gravity as 9.81 m/s², and NPSH_r as 2.5 m.