1. Understanding NPSH
- NPSH (Net Positive Suction Head) is the available pressure head at the pump suction above the liquid’s vapor pressure.
- Two types:
- NPSH Available (NPSHa): What the system provides.
- NPSH Required (NPSHr): What the pump needs (given by manufacturer).
The pump will operate safely only if:
NPSHa greater than or equal to NPSHr
2. General Formula for NPSHa
3. Case A: Flooded Suction (Open Atmosphere Tank)
Here the pump is below the liquid level, tank open to atmosphere.
4. Step-by-Step Example
Given:
- Water at 30°C
- Atmospheric pressure = 101.3 kPa
- Pump suction is 2 m below tank liquid level (flooded suction)
- Suction pipe friction loss = 0.5 m
- Vapor pressure of water at 30°C ≈ 4.24 kPa
5. How to Find Vapor Pressure
- Vapor pressure depends on temperature of the liquid.
- Sources:
- Steam tables (for water).
- Manufacturer’s fluid property charts.
- Formula (approximate for water):
6. Pipe Friction Loss (hₓf)
7. Quick Checklist for NPSH Calculation
1. Get atmospheric pressure (convert to head).
2. Measure static suction head (positive for flooded).
3. Find vapor pressure at liquid temperature.
4. Calculate pipe friction losses.
5. Apply formula → Compare with pump’s NPSHr.
