Free · Fluid mechanics
Reservoir NPSH Available and Required Head Comparison
Calculate a reservoir-to-pump suction head budget using absolute surface pressure, vapor pressure, elevation and friction loss, then compare the entered manufacturer requirement.
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Calculator inputs
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How to use this calculator
- Enter the known values in the units shown. Results update as you type.
- Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
- Use the worked example to check the method. Reset restores the starting fields.
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Formula and method
For an open reservoir with 98.1 kPa absolute atmosphere, 9.81 kPa vapor pressure, density 1000 kg/m³ and g=9.81 m/s², a 2 m suction lift and 1 m loss give 6 m available head. Comparing 3 m required head does not by itself certify cavitation-free operation.
Head budget = (Psurface,absolute − Pvapor)/(ρg) + zsurface−datum − friction head loss
Worked example
Enter these known values and leave the other values blank.
- Reservoir is...
- open to atmosphere
- Reservoir position
- Below pump
- Atmospheric absolute pressure
- 98.1 kPa
- Surface-to-NPSH-datum elevation magnitude
- 2 m
- Friction head loss (H==l==)
- 1 m
- Liquid vapor absolute pressure
- 9.81 kPa
- Density (ρ)
- 1000 kg/m³
- Manufacturer required head at operating point
- 3 m
- NPSH available
- 6 m
- Liquid surface head
- 10 m
- Vapor pressure head
- 1 m
Assumptions and limitations
- Steady incompressible reservoir-to-pump head balance with negligible velocity at the reservoir free surface. Enter total suction-line losses, including entry, valves and fittings. Velocity head at the pump is included through the energy balance, not subtracted again.
- Use the same manufacturer NPSH reference datum for elevation and the requirement curve. The diagram uses the horizontal-pump centerline as a common example; other pump designs require their specified datum. Above adds the elevation magnitude; below subtracts it.
- Atmosphere and liquid vapor pressure are absolute pressures. Closed-reservoir pressure is gauge pressure relative to atmosphere; its contribution is ignored in open mode even if a previous closed-mode entry is retained.
- Supply density and vapor pressure for the actual fluid and operating temperature. No fluid properties, friction loss, duty-point pump curve or required application margin are inferred.
- Required NPSH may use a 3% head-drop criterion (NPSH3); exceeding it does not prove that cavitation is absent. Obtain the applicable criterion and margin from the pump manufacturer. A zero or negative computed budget flags conditions outside a positive available-pressure margin, not a certified single-phase operating state.
Common questions
Is closed reservoir pressure absolute or gauge?
It is gauge pressure. The model adds it to absolute atmospheric pressure in closed mode. Open mode uses atmosphere alone.
Does available head above required head guarantee no cavitation?
No. Requirements commonly reflect an NPSH3 test criterion and depend on operating point. Manufacturer guidance may require an additional margin and a different cavitation criterion.
References
- KSB: NPSH reference levels, pressure balance and cavitation criteria
- Hydraulic Institute summary hosted by DOE: NPSH3 definition and margin context
- Calculation definition and unit reference
For a reservoir open to atmosphere, surface pressure is atmospheric pressure alone. Closed-reservoir gauge pressure contributes only in closed mode; opening a previously pressurized reservoir must not retain that pressure in the head budget. All associated pressure/density/gravity inverses use the same mode-dependent relationship. Use positive density, gravity and absolute surface pressure, nonnegative vapor pressure, elevation magnitude and friction loss. A head budget may be zero or negative as a diagnostic; it does not establish a physically sustainable single-phase operating condition. Compare it with the entered manufacturer requirement and applicable operating-point margin rather than treating the comparison as a cavitation guarantee. The absolute reservoir surface pressure must not be below the specified liquid vapor pressure; otherwise the steady liquid-reservoir assumption fails even when elevation would make the numerical head budget positive.