The question and answer
A pump sits 2 m above the liquid surface of an open water tank. Suction losses are 1.5 m at the duty flow. With an illustrative manufacturer NPSH required of 3.0 m, how much margin is available?
NPSH available is 6.612 m, leaving 3.612 m above the assumed NPSH required. A lower tank level and higher suction loss can remove that margin.
1. Enter absolute pressures and the correct elevation sign
| Calculator input | Value |
|---|---|
| Liquid density | 998.2 kg/m³ |
| Pressure at liquid surface | 101.325 kPa absolute, an assumed atmospheric pressure |
| Liquid vapor pressure | 2.34 kPa absolute, rounded water value near 20°C |
| Liquid-surface elevation relative to pump | −2 m: the surface is below the pump |
| Suction-line friction loss | 1.5 m of liquid head |
| NPSH required | 3.0 m, illustrative pump-curve input |
Enter these values in the NPSH available calculator and press Calculate NPSH available. Use the water properties reference when checking another temperature; density and vapor pressure must describe the same liquid condition.
The suction-loss input includes entrance, pipe, fittings, valves and strainers up to the pump suction reference point. It excludes discharge-line losses. All four cases below assume the same operating flow and speed, so the assumed NPSH required remains 3.0 m.
2. Work through the equation
= (101,325 − 2,340)/(998.2 × 9.80665) − 2 − 1.5
= 10.11186 − 2 − 1.5 = 6.61186 m
Margin = 6.61186 − 3.0 = 3.61186 m
The reservoir free-surface velocity is assumed negligible. Do not subtract pump suction velocity head again from this reservoir-to-pump equation. Do not enter 0 gauge pressure into an input that requires absolute pressure.
3. Check low level and added suction loss
| Case | z (m) | Suction loss (m) | NPSH available (m) | Margin above 3 m (m) |
|---|---|---|---|---|
| Normal level | -2 | 1.5 | 6.612 | 3.612 |
| Level falls by 3 m | -5 | 1.5 | 3.612 | 0.612 |
| Higher suction loss | -2 | 3 | 5.112 | 2.112 |
| Low level and higher loss | -5 | 3 | 2.112 | -0.888 |
A 3 m fall in level removes exactly 3 m of NPSH available. An extra 1.5 m of suction loss removes another 1.5 m. In the combined case, available NPSH is 0.888 m below the assumed required value.
The low-level case alone has a positive 0.612 m arithmetic margin, but that does not establish an acceptable design. Use the pump manufacturer's margin guidance for the service and assess minimum level, maximum temperature, local atmospheric pressure and fouled strainers.
4. Decide what to change
Possible system changes include lowering the pump, raising the minimum tank level, reducing suction losses or selecting a pump with a different NPSH requirement at the duty point. Higher liquid temperature increases vapor pressure and can reduce available NPSH; recompute with matching properties.
NPSH available describes the system. NPSH required comes from the pump curve at the operating flow and speed. A curve based on NPSH3 uses a 3% head-drop criterion; it is not a promise of zero cavitation.
Reference: Hydraulic Institute Pump FAQs, NPSH and pump-system guidance. The operating conditions and required NPSH in this example are illustrative, not vendor data.