How this calculation works
Available NPSH compares liquid pressure head at the suction condition with its vapor-pressure head. Reservoir elevation can add head, while suction losses consume it. This is a different question from how much hydraulic power a pump needs to raise discharge pressure.
NPSHa = (surface absolute pressure − vapor absolute pressure)/(density × g) + surface elevation above pump − suction lossA worked example
At 101.325 kPa surface pressure and 2.339 kPa vapor pressure, their difference is 98.986 kPa. With density 1000 kg/m³, that equals about 10.094 m of liquid head. A surface two metres above the pump and one metre of suction loss give about 11.094 m available. Subtracting entered required head and an extra allowance is a transparent comparison, not an automatic pump approval.
Good to know
Steady large-reservoir head balance only; no fluid-property lookup, transient cavitation analysis, manufacturer verification or safe-operation certification. Use actual liquid, temperature and operating-point data. NPSH availability and the manufacturer’s required value must use compatible definitions and datum conventions.
What the model answers
Available NPSH compares liquid pressure head at the suction condition with its vapor-pressure head. Reservoir elevation can add head, while suction losses consume it. This is a different question from how much hydraulic power a pump needs to raise discharge pressure.
The page starts from a large reservoir with negligible free-surface velocity. It uses a head balance to the pump datum and keeps required NPSH as an externally supplied equipment value. It does not derive that requirement from a pump name.
The Hydraulic pump power and energy calculator estimates power from flow and pressure rise. Available suction head is a separate inlet condition and cannot be inferred from output power alone.
Measure and enter consistently
Reservoir surface and vapor pressures are both absolute. Gauge pressure cannot be substituted without converting its reference. Vapor pressure must correspond to the actual liquid and operating temperature; no fluid table is selected automatically.
Positive surface elevation is above the pump datum, while a suction lift is negative. Enter suction loss at the intended operating flow. Required NPSH must likewise correspond to that flow and the relevant manufacturer definition.
Interpret the result
The result retains negative available head and negative margins. Clipping them to zero would hide inconsistent or unfavorable scenarios. The extra-margin field is supplied by the user rather than treated as a universal rule.
A positive comparison does not certify cavitation-free service. Transients, entrained gas, liquid composition, inlet geometry and the equipment maker’s requirements can matter beyond this steady reservoir balance.
Work through the numbers
At 101.325 kPa surface pressure and 2.339 kPa vapor pressure, their difference is 98.986 kPa. With density 1000 kg/m³, that equals about 10.094 m of liquid head.
A surface two metres above the pump and one metre of suction loss give about 11.094 m available. Subtracting entered required head and an extra allowance is a transparent comparison, not an automatic pump approval.
Where this model stops
Steady large-reservoir head balance only; no fluid-property lookup, transient cavitation analysis, manufacturer verification or safe-operation certification.
Use actual liquid, temperature and operating-point data. NPSH availability and the manufacturer’s required value must use compatible definitions and datum conventions.
A few questions, answered
Why must both pressures be absolute?
The available pressure above vaporization depends on their physical reference. Mixing gauge and absolute values creates an incorrect difference.
Does positive headroom guarantee safe operation?
No. It is a comparison under the entered steady assumptions and does not replace the equipment and system assessment.