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MechanicalNPSHa Calculator
Compute the Net Positive Suction Head available to confirm a pump will not cavitate.
When to use this calculator
Use when checking suction conditions against the pump's required NPSH.
Required inputs
- Atmospheric pressure
- Vapour pressure
- Static suction head
- Suction friction loss
Expected outputs
- NPSHa
- Margin vs NPSHr
Formula overview
Pressure in Pa, head in m, ρ in kg/m³, g = 9.81 m/s².
NPSHa = (P_atm − P_vapour)/(ρg) + h_static − h_friction.Worked example
Example: P_atm = 101325 Pa, P_vap = 2339 Pa (water at 20°C), ρ = 998, h_static = 2 m, h_friction = 1 m
NPSHa = (101325 − 2339)/(998 × 9.81) + 2 − 1 = 10.1 + 1 = 11.1 m.Common mistakes
- Designing NPSHa to exactly match NPSHr without safety margins (should aim for ≥ 0.5–1.0 m margin).
- Using the vapour pressure at the wrong operating temperature.
FAQ
NPSH
cavitation
pump
suction
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