Pump NPSH Margin Calculator — Available NPSH, Cavitation Risk Index & Suction Head Validator
Calculate available NPSH using suction pressure, vapor pressure, fluid density, static suction head, suction losses, and required NPSH.
Enter the known values and review the calculated result
Input parameters
Use consistent values and select the intended engineering units.
Suction conditions
Fluid properties
Geometry
Hydraulic losses
Pump data
Results
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Method, application and limitations
Review the calculation method, intended application and engineering assumptions before using the result in a design decision.
Formula and calculation method
Available NPSH formula:
Hp = (psuction − pvapor) / (ρ · g)
NPSHa = Hp + z − hloss
ΔNPSH = NPSHa − NPSHr
CRI = (ΔNPSH / NPSHr) · 100
where:
- NPSHa — Available NPSH (m)
- Hp — Pressure head from suction pressure and vapor pressure (m)
- psuction — Suction pressure (Pa)
- pvapor — Vapor pressure (Pa)
- ρ — Fluid density (kg/m³)
- g — Gravitational acceleration, 9.81 m/s²
- z — Static suction head (m)
- hloss — Suction line losses (m)
- NPSHr — Required NPSH (m)
- ΔNPSH — NPSH margin (m)
- CRI — Cavitation risk index (%)
When to use this calculator
When to use this calculator:
- Check whether available NPSH is higher than the pump required NPSH for a defined suction line.
- Calculate NPSH margin for a pump using suction pressure, vapor pressure, fluid density, static suction head, and suction losses.
- Evaluate cavitation risk when suction line losses reduce the available suction head.
- Identify whether suction pressure, fluid vapor pressure, or suction losses are limiting the pump inlet condition.
- Estimate the required head improvement when available NPSH is lower than required NPSH.
How to interpret the result
Available NPSH is defined as the suction-side head available above the fluid vapor pressure after static suction head and suction line losses are included.
NPSH margin is defined as available NPSH minus required NPSH. A positive margin means available NPSH is higher than required NPSH. A negative margin means required NPSH is higher than available NPSH.
Cavitation risk index is defined as the NPSH margin divided by required NPSH and expressed as a percentage.
- Safe — NPSH margin is greater than 1 m.
- Warning — NPSH margin is from 0 m to 1 m.
- Unsafe — NPSH margin is lower than 0 m.
- Invalid — suction pressure must be greater than zero, vapor pressure must be lower than suction pressure, fluid density must be greater than zero, suction line losses must be zero or higher, and required NPSH must be greater than zero.
Available NPSH increases with suction pressure and static suction head. Available NPSH decreases with vapor pressure, fluid density, and suction line losses.
The result is used to compare suction-side available NPSH against pump required NPSH and identify whether suction-side head improvement is required.
Calculation example
Example:
A user wants to check whether a pump suction line has enough available NPSH before selecting the pump.
- psuction — Suction pressure = 120000 Pa
- pvapor — Vapor pressure = 3000 Pa
- ρ — Fluid density = 1000 kg/m³
- z — Static suction head = 1.5 m
- hloss — Suction line losses = 0.8 m
- NPSHr — Required NPSH = 3.0 m
NPSHa = 12.63 m, ΔNPSH = 9.63 m, CRI = 321.0%.
Assumptions and limitations
- Pressure head is calculated from suction pressure minus vapor pressure using fluid density and gravitational acceleration equal to 9.81 m/s².
- Available NPSH is calculated from pressure head, static suction head, and suction line losses.
- NPSH margin is calculated as available NPSH minus required NPSH.
- Cavitation risk index is calculated as NPSH margin divided by required NPSH and multiplied by 100.
- Suction line losses must be zero or higher.
- Required NPSH must be greater than zero.