NPSH: what it is, how to calculate it and what margin to use
NPSH (Net Positive Suction Head) is the pressure headroom that keeps the liquid from vaporizing inside the pump. The installation offers a headroom — the available NPSH — and the pump demands a minimum — the required NPSH. When that equation fails, the result is cavitation. This guide explains both sides of the equation, how to calculate your NPSHa and what margin to keep.
Updated on July 8, 2026 · Reviewed by Hydro Pumps engineering
The idea in one sentence
Every liquid vaporizes when pressure drops below its vapor pressure — and a pumping system's lowest-pressure point sits at the impeller inlet. NPSH measures how much pressure 'surplus' exists at that point above vapor pressure: while the surplus is positive and sufficient, the liquid stays liquid and the pump works; when the surplus runs out, vapor bubbles form and cavitation begins.
The concept's elegance lies in separating responsibilities: available NPSH (NPSHa) is 100% the installation's responsibility — reservoir, piping, temperature. Required NPSH (NPSHr) is 100% a pump characteristic — it is on the manufacturer's curve, measured on a test bench. The installation offers; the pump demands.
Available NPSH: what the installation offers
NPSHa adds and subtracts four terms — all from the installation, none from the pump:
Pressure above the liquid (+). Open reservoir: the local atmospheric pressure — which decreases with altitude. Closed vessel: the actual vessel pressure.
Static height (+/−). Liquid above the pump adds (flooded suction); liquid below subtracts (suction lift). Watch the reservoir's actual minimum level, not the nominal one.
Friction losses (−). Every meter of pipe, bend, valve and strainer on the suction consumes pressure — the term that grows with the square of flow and that clogging multiplies.
Vapor pressure (−). The liquid's term: the hotter, the higher the vapor pressure and the lower the NPSHa. Water at 80 °C has nearly ten times the vapor pressure of water at 30 °C.
Required NPSH: what the pump demands
NPSHr is measured by the manufacturer on a test bench and published on the pump curve: it is the minimum suction energy for the pump to operate without noticeable performance loss from vaporization. Two behaviors matter in practice.
First: NPSHr grows with flow — typically steeply to the right of the design point. A pump running wide open, beyond nominal flow, demands far more suction than the design anticipated — and cavitates even in a correct installation.
Second: the published NPSHr is not the 'zero cavitation' point — by industry convention, it is the point where cavitation already degrades the head by 3%. Operating exactly at NPSHr means operating with some cavitation. That is where the need for margin comes from.
The margin: how much headroom to keep
The practice recommended by the Hydraulic Institute (ANSI/HI 9.6.1) for typical applications is to keep NPSHa at least 10% or 0.6 meter above NPSHr — whichever is greater. Severe services, high-energy pumps and fluids near saturation call for larger margins.
The margin covers what the calculation doesn't capture: reservoir level variation, piping aging and fouling, hotter days, transient off-point operation and the 3% convention embedded in the published NPSHr itself.
A practical design and diagnosis rule: if the NPSHa calculation only closes in the best-case scenario — maximum level, cold water, clean strainer — it doesn't close. Size for the worst realistic operating condition.
Diagnosis: when the equation fails
Suspected insufficient NPSH shows up as cavitation: gravel noise, broadband vibration, falling flow, impeller erosion. Confirmation combines three checks: measuring the actual pressure at the pump suction, surveying the installation's conditions (levels, temperature, losses) and comparing with the curve's NPSHr at the actual operating flow.
Corrections follow the cost hierarchy: restore level and clean strainers first; reduce suction losses and control temperature next; adjust flow and speed after that; and, ultimately, pump re-selection or an inducer-equipped impeller. The complete cavitation guide details the seven practical corrections.
Pump cavitating and the NPSH equation doesn't close?
We measure the actual suction conditions, confirm the diagnosis by vibration and propose the correction — from strainer cleaning to impeller re-selection.
NPSH FAQ
NPSHa (available) is the pressure headroom your installation offers at the suction — it depends on reservoir, piping and temperature. NPSHr (required) is the minimum the pump demands, measured by the manufacturer and published on the curve. The installation must offer more than the pump demands, with margin.
Yes, and it is frequently the most effective structural correction: friction losses drop drastically as diameter increases. Shortening the run, replacing elbows with long-radius bends and keeping strainers clean attack the same term of the equation.
Temperature: hotter liquid has higher vapor pressure, which directly reduces NPSHa. An installation with a tight margin closes the equation in winter and blows it in summer — correct sizing uses the actual maximum operating temperature, not the average.
Yes — it is the same phenomenon from another angle: the maximum height a pump can 'pull' is limited by atmospheric pressure minus losses, vapor pressure and NPSHr. In practice, at sea level with cold water, viable suction lift stays well below the theoretical 10 meters — and decreases with altitude, temperature and losses.
NPSHr is typically established by the manufacturer with cold water. For other liquids and temperatures the behavior can vary — hydrocarbons, for example, tend to cavitate less severely than water under the same conditions. In critical applications, selection considers the actual fluid with the manufacturer's criteria.
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Technical question about your installation's suction?
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