Net Positive Suction Head available, with cavitation risk check and full step-by-step working
kPa absolute (open tank ≈ 101.325)
kPa absolute, at operating temp
kg/m³
m — positive if liquid level is above pump, negative if below
m
m — optional, leave blank to skip the check
NPSH Available:
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What is NPSH?
Net Positive Suction Head (NPSH) measures how much pressure margin exists above the liquid's vapor pressure at the pump suction. If this margin runs out, the liquid flashes to vapor inside the pump — cavitation — which damages the impeller and collapses performance.
NPSHa = (Pa − Pv) / (ρ × g) + Hz − Hf
where Pa is source (usually atmospheric) pressure, Pv is the liquid's vapor pressure at operating temperature, Hz is static head (positive for flooded suction, negative for suction lift), and Hf is friction loss in the suction line.
The Cavitation Check
For safe operation: NPSHa > NPSHr + margin. NPSHr (required) comes from the pump manufacturer's datasheet — it's specific to each pump model and its operating point. A margin of at least 0.5-1 m above NPSHr is standard practice, since NPSHr itself is usually measured under ideal test conditions.
Why It Matters
Cavitation is one of the most common real-world causes of premature pump failure — it's rarely caused by a poorly selected pump itself, but by a suction-side design that doesn't provide enough NPSH margin (too much suction lift, undersized suction piping, or hot/volatile liquids with high vapor pressure).
Solved Examples (Practice Problems)
Click "Try This Example" to auto-fill the calculator above and see the full step-by-step working.
Example 1 — Cold water, flooded suction (safe)
Water at 20°C (Pv = 2.34 kPa, ρ = 998 kg/m³) is fed from a tank 3 m above the pump, with 0.8 m of suction line friction loss. Pump requires NPSHr = 3.0 m. Check if it's safe.
Example 2 — Hot water, suction lift (cavitation risk)
Water at 80°C (Pv = 47.39 kPa, ρ = 971.8 kg/m³) is drawn from a tank 2 m below the pump, with 1.2 m of suction line friction loss. Pump requires NPSHr = 4.0 m. Check if it's safe — this one isn't.
Example 3 — Light hydrocarbon, flooded suction
A light hydrocarbon (Pv = 25 kPa, ρ = 750 kg/m³) is fed from a tank 1.5 m above the pump, with 0.5 m of suction line friction loss. Pump requires NPSHr = 2.5 m.
Worked Solutions in Full
Each example is worked through in full, ending with the safety decision against the pump's required NPSH.
Example 1 — Cold water, flooded suction
Given: Water at 20 °C, tank 3 m above the pump. Pa = 101.325 kPa, Pv = 2.34 kPa, ρ = 998 kg/m³, Hz = 3 m, Hf = 0.8 m, NPSHr = 3 m
Step 4 — Compare with NPSHr: NPSHa is 8.87 m above NPSHr, so the pump is safe with a comfortable margin.
Answer: NPSHa ≈ 11.37 m against NPSHr = 2.5 m.
Fixing Example 2
Example 2 fails with NPSHa = 2.46 m. Moving the tank so the liquid sits 3 m above the pump (Hz = +3 m) gives NPSHa = 5.66 + 3 − 1.2 = 7.46 m, which clears the 4.0 m requirement by 3.46 m.
Practical Notes and Common Mistakes
NPSH is the suction-side check that goes with every centrifugal pump selection. It tells you whether the liquid can reach the impeller without boiling. Once the suction side passes, size the driver with the Pump Power Calculator.
Mistakes that give wrong answers
Gauge instead of absolute pressure. Both Pa and Pv must be absolute. An open tank at sea level is about 101.3 kPa absolute, not 0.
Wrong sign on static head. Positive when the liquid level is above the pump, negative for a suction lift.
Vapor pressure at the wrong temperature. Pv rises steeply with temperature, as Example 2 shows. Use the actual pumping temperature, including any heating in the line.
Forgetting that friction changes with flow. Suction-line loss rises with flow, and NPSHr also rises with flow, so check the highest flow the pump will see.
No safety margin. Datasheet NPSHr is measured under controlled test conditions. Keep at least 0.5 to 1 m of margin, and more for critical services.
Ways to raise NPSHa
Raise the liquid level or lower the pump.
Use larger suction piping and fewer fittings to cut friction loss.
Cool the liquid to lower its vapor pressure.
Pressurise the source vessel.
Choose a pump with a lower NPSHr, such as a lower-speed or larger-eye design.
Frequently Asked Questions
What's the difference between NPSHa and NPSHr?
NPSHa (available) is a property of your system design — the tank height, piping, and fluid. NPSHr (required) is a property of the specific pump — how much margin that pump's design needs to avoid cavitation, found on its performance curve or datasheet. Safe operation requires NPSHa to exceed NPSHr with margin.
Where do I find the vapor pressure of my liquid?
For water, standard steam tables give vapor pressure at any temperature (2.34 kPa at 20°C, 47.39 kPa at 80°C, 101.325 kPa at 100°C). For other liquids, check the fluid's physical property data sheet or a process simulator — vapor pressure rises sharply with temperature for most liquids.
Why is static head negative in my case?
If the pump sits above the liquid surface it's drawing from (suction lift), gravity works against the flow reaching the pump, so Hz is entered as negative. If the liquid source is elevated above the pump (flooded suction), gravity helps, so Hz is positive.
What happens if I'm right on the edge — NPSHa only slightly above NPSHr?
This is risky in practice even if technically "passing" — NPSHr from a datasheet is measured under controlled test conditions, and real installations have uncertainties (actual friction losses, fluid temperature swings, tank level variation). This is exactly why a 0.5-1 m margin is standard rather than treating NPSHa = NPSHr as acceptable.
Related Tool
Confirmed your suction side is safe? Head back to the Pump Power Calculator to size the motor.