NPSHa — Net Positive Suction Head Available
NPSHa = (patm − pvp)/(ρ·g) + zs − hf (+ v²/2g). All results in metres and feet of the pumped liquid.
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metres of NPSHa
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Term-by-term breakdown
| Term | Value | Metres |
|---|---|---|
| Pressure head (patm − pvp)/(ρ·g) | — | — |
| Static suction head zs | — | — |
| Friction loss −hf | — | — |
| Velocity head v²/2g | — | — |
| NPSHa total | — |
Negative NPSHa. The suction conditions cannot support this fluid at this temperature — the liquid would flash to vapor before reaching the pump. Increase zs, reduce hf, or cool the fluid.
NPSH margin & cavitation check
Margin = NPSHa − NPSHr. Enter the pump's NPSHr from the manufacturer's curve at your operating flow rate.
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NPSH margin
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NPSHa / NPSHr ratio
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Margin in feet
Guidance: margin ≥ 0.5 m (1.6 ft) is a common rule of thumb; the Hydraulic Institute recommends NPSHa/NPSHr ≥ 1.1. Margin below 0.5 m = thin, at or below zero = cavitation.
Reference tables & converters
Water vapor pressure vs temperature (Buck equation, ±0.1% vs steam tables), head unit conversion, and pressure-to-head conversion.
Water vapor pressure vs temperature
| Temp (°C) | pvp (kPa) | pvp (psia) |
|---|
Head converter (metres ↔ feet)
Pressure → head of fluid
h = p/(ρ·g). Useful for turning a gauge reading into head.
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Head in metres
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Head in feet
Worked example
Example. A pump at sea level draws water at 20 °C from a tank whose surface is 2 m above the pump centreline, with 0.5 m of suction-line friction loss and negligible suction velocity:
patm = 101.325 kPa, pvp = 2.339 kPa (water at 20 °C), ρ = 998 kg/m³, zs = +2 m, hf = 0.5 m.
NPSHa = (101.325 − 2.339)·1000 / (998·9.80665) + 2 − 0.5 ≈ 10.11 + 2 − 0.5 ≈ 11.61 m ≈ 38.10 ft
Type these values into Mode 1 to verify — you should get the same result.
Related tools
- Air Pressure at Altitude Calculator — get the correct local patm for your pump's elevation.
- Hydrostatic Pressure Calculator — pressure ↔ head with P = ρgh, full unit support.
- Bar to PSI Converter — pressure unit conversions for gauge readings.
- Flow Rate Calculator — find suction velocity v from flow rate and pipe diameter.
NPSH FAQs
What is NPSH?
NPSH stands for Net Positive Suction Head. It is the total head (energy per unit weight of fluid) available at the pump suction flange, above the fluid's vapor pressure, expressed in metres or feet of the pumped liquid. NPSH is the key quantity that determines whether a pump will cavitate.
What is the difference between NPSHa and NPSHr?
NPSHa (available) describes your system: it is the suction head your piping, tank elevation and fluid properties actually provide, computed as (p_atm - p_vp)/(rho*g) + z_s - h_f (+ velocity head). NPSHr (required) describes the pump: it is the minimum suction head the pump needs to avoid cavitation, read from the manufacturer's pump curve at your operating flow rate. The pump runs safely only when NPSHa exceeds NPSHr.
What is the NPSH formula?
NPSHa = (p_atm - p_vp)/(rho * g) + z_s - h_f + v^2/(2g). Here p_atm is atmospheric pressure, p_vp is the fluid's vapor pressure at pumping temperature, rho is density, g is 9.80665 m/s^2, z_s is the static suction head (positive for flooded suction, negative for suction lift), h_f is suction-line friction loss, and v^2/(2g) is the optional velocity head. Worked example: p_atm = 101.325 kPa, p_vp = 2.339 kPa (water at 20 C), rho = 998 kg/m^3, z_s = 2 m, h_f = 0.5 m gives NPSHa of about 11.61 m (about 38.10 ft).
What is cavitation in a pump?
Cavitation happens when the local pressure inside the pump drops below the fluid's vapor pressure, so vapour bubbles form in the liquid. When those bubbles travel into a higher-pressure zone they collapse violently, producing noise like gravel in the pump, vibration, pitting damage to the impeller, and loss of head and efficiency. A pump operates in cavitation when NPSHa falls below NPSHr (zero or negative margin).
What is a good NPSH margin?
Margin = NPSHa - NPSHr. A margin of at least 0.5 m (about 1.6 ft) is a common engineering rule of thumb for a comfortable cushion; the Hydraulic Institute (HI 9.6.7) recommends a margin ratio NPSHa/NPSHr of 1.1 or more, and larger margins for high-energy or critical pumps. A margin below 0.5 m deserves attention, and a margin at or below zero means the pump is cavitating.
How does temperature affect NPSH?
Higher temperature raises the fluid's vapor pressure, which subtracts directly from the (p_atm - p_vp) term and lowers NPSHa. For water, vapor pressure grows from 2.34 kPa at 20 C to 47.4 kPa at 80 C - that alone removes about 4.6 m of suction head. Hot-fluid pumping (boiler feed, condensate, hot oil) is therefore the classic low-NPSHa application and usually needs a flooded suction.
How does altitude affect NPSH?
Atmospheric pressure falls with altitude, shrinking the (p_atm - p_vp) term. At 1,000 m above sea level p_atm is about 89.9 kPa instead of 101.3 kPa, which costs roughly 1.2 m of NPSHa for water. Always use the local atmospheric pressure at the pump's elevation, not sea-level standard pressure, in the NPSHa formula.
What is the difference between flooded suction and suction lift?
In flooded suction the liquid surface is above the pump centreline, so the static suction head z_s is positive and adds to NPSHa. In suction lift the pump sits above the liquid surface, z_s is negative, and every metre of lift subtracts a metre of NPSHa. Enter z_s with its sign: positive above, negative below the pump.
Should I include the velocity head v^2/(2g) in NPSHa?
The velocity head at the pump suction flange is part of the strict definition of NPSHa, but it is usually small (for 2 m/s in water it is only 0.20 m) and many field calculations omit it as a conservative simplification. Include it when you want the full value or when suction velocities are high; otherwise 0 is a safe default.
My NPSHa is too low. How can I increase it?
Raise the supply tank or lower the pump (increases z_s), enlarge the suction pipe or shorten it (reduces friction loss h_f), remove unnecessary valves, strainers and fittings on the suction side, cool the fluid (lowers vapor pressure p_vp), or choose a pump with a lower NPSHr at your duty point. Increasing flow rate usually raises NPSHr and friction losses, so re-check the margin at the actual operating flow.