Worked problem

Pump Cavitation Check: NPSH Available vs. Required

Calculate NPSH available for 60 °C water from an open tank at 1,500 m elevation and compare it with NPSH required plus a stated margin.

Mechanical, HVAC & Fluids · Last reviewed October 17, 2026

Given

A pump draws water from an open tank at a site 1,500 m above sea level. The water surface is 1.2 m above the pump centerline, and the suction line losses at the design flow are 0.8 m. The water is at 60 °C, with a vapor pressure of 19.94 kPa and a density of 983.2 kg/m³. The manufacturer gives NPSH required = 4.5 m at the duty flow. For this exercise require NPSH available to exceed NPSH required by at least 1.0 m.

Find

NPSH available, and whether the pump is acceptable. Then repeat for 80 °C water.

Solution

NPSHa = (P(atm) − P(vapor)) ÷ (ρ × g) + z(static) − h(friction)

Atmospheric pressure at 1,500 m ≈ 101.325 × (1 − 2.25577 × 10⁻⁵ × 1,500)^5.256 = 84.6 kPa.
Pressure term = (84.6 − 19.94) × 1,000 ÷ (983.2 × 9.81) = 6.70 m.
NPSHa = 6.70 + 1.2 − 0.8 = 7.10 m.
Requirement = NPSHr + margin = 4.5 + 1.0 = 5.5 m. NPSHa is 7.10 m, so the pump passes with 2.6 m above NPSHr.

Answer

NPSHa = 7.1 m against NPSHr = 4.5 m (required with margin: 5.5 m)  |  acceptable

The same pump with 80 °C water

At 80 °C the vapor pressure is 47.39 kPa and the density 971.8 kg/m³. The pressure term becomes (84.6 − 47.39) × 1,000 ÷ (971.8 × 9.81) = 3.90 m, and NPSHa = 3.90 + 1.2 − 0.8 = 4.30 m. That is below even NPSHr (4.5 m), so the pump would cavitate.

Case	NPSHa	Needed	Result
60 °C, 1,500 m	7.10 m	5.5 m	Pass
80 °C, 1,500 m	4.30 m	5.5 m	Fail
60 °C at sea level	8.84 m	5.5 m	Pass

Common mistakes

Forgetting the vapor pressure, which rises quickly with temperature and takes away most of the margin for hot water.
Using sea-level atmospheric pressure at a high-elevation site.
Leaving out suction friction, or counting the static height with the wrong sign for a suction lift.
Comparing NPSHa at one flow with NPSHr at another. NPSHr rises with flow.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

Questions

What happens when NPSHa is too low?

Vapor bubbles form at the impeller inlet and collapse as the pressure recovers. That causes noise, vibration, loss of head and damage to the impeller over time.

Is a 1.0 m margin a standard?

No. It is an exercise value. Margins used in practice depend on the application, the pump type and the owner or standards requirements.

How can I raise NPSHa?

Raise the liquid level, shorten or enlarge the suction line, lower the liquid temperature, or lower the pump.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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