Table of Contents
What Is NPSH?
Net Positive Suction Head (NPSH) is a measure of the pressure available at a pump's suction port relative to the vapor pressure of the liquid being pumped. It determines whether the liquid will remain in liquid phase or begin to vaporize (cavitate) as it enters the pump. Cavitation causes severe damage to pump impellers and reduces efficiency.
There are two types: NPSHa (available) is determined by the system design, and NPSHr (required) is specified by the pump manufacturer. For safe operation, NPSHa must always exceed NPSHr, typically with a safety margin of at least 0.5-1.0 meters. This calculator computes NPSHa from your system parameters.
NPSH Formula
Where Patm is atmospheric pressure, Pvapor is liquid vapor pressure, ρ is fluid density, g is gravity, hs is static suction head, and hf is friction losses in suction piping.
Cavitation Prevention
- Increase suction head by raising the liquid level or lowering the pump.
- Reduce friction losses with larger pipes and fewer fittings.
- Lower liquid temperature to reduce vapor pressure.
- Select a pump with lower NPSHr (slower speed, larger eye).
Frequently Asked Questions
What is a safe NPSH margin?
A common rule is NPSHa should exceed NPSHr by at least 1 meter or 1.5 times NPSHr, whichever is greater. For critical applications, higher margins are recommended. The Hydraulic Institute suggests a minimum 2:1 ratio for high-energy pumps.
How does altitude affect NPSH?
Higher altitude means lower atmospheric pressure, which directly reduces NPSHa. At 1500m elevation, atmospheric pressure drops to about 84 kPa (from 101.3 kPa at sea level), reducing the pressure head by about 1.7 meters of water.