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Calcimator

Pump NPSH Calculator

Calculate Net Positive Suction Head available (NPSHa) and pump shaft power. Verify cavitation margin against required NPSH from the pump curve.

Inputs

%

Results

NPSHa (m)

12.61

NPSH margin (m)

9.61

NPSH ratio (NPSHa/NPSHr)4.2
Pressure head (m)10.35
Vapor pressure head (m)0.24
Hydraulic power (kW)4.08
Shaft power (kW)5.44
Shaft power (HP)7.29
Nss1,591.62
How to Use This Calculator
  1. Enter Surface pressure (kPa abs), Static suction head (m), and Suction line friction loss (m).
  2. Set Vapor pressure (kPa abs), Fluid density (kg/m³), and NPSHr from pump curve (m).
  3. Adjust Flow rate (m³/hr), Total dynamic head (m) as needed.
  4. Review NPSHa (m) and NPSH margin (m).
  5. Use NPSH ratio (NPSHa/NPSHr) and Pressure head (m) to inform your decision.

How the result changes with Surface pressure (kPa abs)

Surface pressure (kPa abs)NPSHa (m)NPSH margin (m)
1,000104.4101.4
3,500359.75356.75
6,500666.18663.18
9,000921.53918.53

What each input means

Surface pressure (kPa abs)
Absolute pressure at the liquid surface (atmospheric = 101.325 kPa).
Static suction head (m)
Height of liquid surface above pump centerline. Negative if pump is above liquid.
Suction line friction loss (m)
Total friction head loss in the suction piping.
Vapor pressure (kPa abs)
Vapor pressure of the liquid at pumping temperature (water at 20°C ≈ 2.34 kPa).
Fluid density (kg/m³)
Density of the pumped fluid.
NPSHr from pump curve (m)
Net Positive Suction Head Required — from manufacturer's pump curve.
Flow rate (m³/hr)
Volumetric flow rate for power calculation.
Total dynamic head (m)
Total head the pump must deliver (suction + discharge + friction + elevation).
Pump efficiency (%)
Overall pump efficiency at the operating point.

What each result means

NPSHa (m)
Net Positive Suction Head Available. Must exceed NPSHr to avoid cavitation.
NPSH margin (m)
NPSHa − NPSHr. Positive values indicate safe operation; target > 1 m.
NPSH ratio (NPSHa/NPSHr)
Safety ratio; HI recommends > 1.2 for general service.
Pressure head (m)
Pressure at liquid surface converted to head of fluid.
Vapor pressure head (m)
Vapor pressure converted to head of fluid.
Hydraulic power (kW)
Power transferred to the fluid: ρgQH.
Shaft power (kW)
Required motor/shaft power accounting for pump efficiency.
Shaft power (HP)
Shaft power in horsepower.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Surface pressure (kPa abs) = 101.325, Static suction head (m) = 3, Suction line friction loss (m) = 0.5, Vapor pressure (kPa abs) = 2.34 = 9 input(s) provided
  2. Calculate NPSHa
    NPSHa = pressureHead + staticHead - frictionLoss - vaporHead
    12.61 = 12.61
  3. Calculate NPSH margin
    NPSH margin = npsha - npshr
    9.61 = 9.61
  4. Calculate NPSH ratio
    4.2 = 4.2
  5. Calculate Pressure head
    Pressure head = Ps_Pa / (density * g)
    10.35 = 10.35

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