Net Positive Suction Head (NPSH) is one of the most critical parameters in pump selection and installation. It directly impacts whether a pump will operate smoothly or suffer from cavitation — a destructive condition that can rapidly damage impellers, casings, and bearings.
NPSH represents the absolute pressure available at the pump suction compared to the vapour pressure of the liquid. It determines whether the liquid will remain in a liquid state as it enters the pump or begin to vaporise (leading to cavitation).
NPSHa (Available)
The suction head actually available from the system. Determined by your system conditions and can be improved through design changes.
NPSHr (Required)
The minimum suction head needed by the pump, as provided by the manufacturer’s pump curve. This is a pump characteristic.
Critical Rule
The Physics Behind NPSH
When liquid pressure at the pump inlet falls below its vapour pressure, the liquid begins to form vapour bubbles. As these bubbles are carried into the higher-pressure region of the impeller, they collapse violently. This collapse is cavitation.
Effects of Cavitation
Excessive vibration and noise (a “gravel” or “crackling” sound)
Erosion and pitting of impeller surfaces
Reduced flow and head performance
Shortened pump life
Components of NPSHa
NPSHa Formula
Patm
Atmospheric pressure head (≈ 10.3 m at sea level)
Hs
Static suction head (positive if source above pump, negative if lift)
Hf
Friction losses in suction piping (m)
Pv
Vapour pressure of liquid (higher temp = higher vapour pressure)
ρg = Liquid density × gravity constant
Example Calculation
Application: Centrifugal pump drawing water from a river
- Elevation of pump: 3 m above water level → Hs = −3 m
- Suction pipe friction losses: Hf = 0.5 m
- Atmospheric pressure: Patm = 10.3 m (sea level)
- Water temperature: 30°C → Vapour pressure ≈ 4.3 kPa = 0.44 m
Calculate NPSHa:
If the pump curve shows NPSHr = 5.0 m, then:
Safety Margins in Practice
While the formula requires only NPSHa > NPSHr, best practice is to allow a margin of 0.5–1.0 m, or 10–20% above NPSHr (whichever is greater). This accounts for fluctuations in suction level, water temperature, and fouling in the suction line.
Temperature Fluctuations
Higher temperatures increase vapour pressure, reducing NPSHa
Water Level Changes
Seasonal or operational level variations affect static head
Suction Line Fouling
Debris and buildup increase friction losses over time
Reducing NPSH Problems
Improve Suction Design
Keep suction lifts as low as possible (< 5 m for most pumps)
Use larger suction pipe diameters to minimise friction
Keep suction lines short and straight; avoid sharp elbows
Use eccentric reducers (flat side up) at the pump to avoid air pockets
Ensure adequate submergence at suction intakes to prevent vortexing
Reduce Fluid Temperature
Cooler fluids have lower vapour pressure → higher NPSHa
Avoid recirculating hot fluid into suction lines without cooling
Pressurise the Suction Source
If possible, use a flooded suction (pump below liquid level)
For closed systems, increase inlet pressure with a pressurised tank or booster pump
How NPSHr is Determined
NPSHr values are obtained through testing. Manufacturers measure the NPSH at which pump performance drops by 3% head (the “3% drop criterion”). The test is performed under controlled conditions, but in the field, conditions may vary — which is why maintaining margin above NPSHr is essential.
The 3% Drop Criterion
NPSHr is defined as the suction head at which cavitation causes a 3% reduction in pump head. At this point, cavitation has already begun — operating at NPSHr means some cavitation is occurring.
Common Mistakes
Ignoring vapour pressure at elevated fluid temperatures
Assuming pump can handle maximum theoretical suction lift (10.3 m)
Undersizing suction pipework, increasing friction losses
Using vertical elbows at the pump suction (air trap risk)
Not checking NPSHa after system changes (longer suction lines, added strainers)
Summary
Net Positive Suction Head (NPSH) is the cornerstone of reliable pump operation. By ensuring NPSHa is greater than NPSHr — with adequate margin — pumps can avoid cavitation, maintain efficiency, and achieve long service life. Correct suction design, temperature control, and system pressure management are key strategies in achieving this balance.