LOGOS Learning · 4
NPSH available formula: why the pump cavitates
NPSH available is the head left at the pump inlet above the vapor pressure: NPSHa = p_atm/(rho*g) - p_v/(rho*g) + dz - hf_suc. The pump runs free of cavitation only while NPSHa stays above the NPSHr from the pump curve, with a margin.
Lower the suction tank relative to the pump. The inlet pressure falls until the water boils inside the impeller.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. Installing the pump well above the suction level…
2. What reduces NPSH available?
3. Why is NPSHa slightly above NPSHr not enough?
Why this happens
Arrows compare with the suction level at the pump centerline (Δz = 0).
NPSH available
The energy left at the pump inlet above vapor pressure. The atmosphere pushes water in; the elevation helps or hurts; suction loss always hurts.
Cavitation criterion
Catalog NPSHr is usually NPSH3: at that point the pump has already lost 3 % of head to cavitation. So a ratio above 1 is not enough; the LOGOS calculator requires a ratio of at least 1.1 and a 0.5 m margin as the floor for general service, in line with HI 9.6.1.
Suction pressure (PIT-02)
With the tank below the pump, suction pressure falls below atmospheric and PIT-02 shows vacuum. When it nears vapor pressure, water boils at the impeller eye and the bubbles collapse on the vanes.
More flow, less margin
On the chart NPSHa falls with flow and NPSHr rises. Lowering the tank shifts the whole NPSHa curve down until it crosses NPSHr near the operating point.
Illustrative NPSHr curve of the teaching pump.
Formulas in plain text
- NPSH available
NPSHa = p_atm / (rho * g) - p_v / (rho * g) + dz - hf_suc- NPSHa = net positive suction head available (m) · p_atm = absolute pressure on the suction tank surface (Pa) · p_v = vapor pressure at pumping temperature (Pa) · rho = fluid density (kg/m3) · g = 9.81 m/s2 · dz = suction level minus pump centerline (m, negative for suction lift) · hf_suc = suction line head loss (m)
- Cavitation criterion
NPSHa >= NPSHr ratio = NPSHa / NPSHr- NPSHr = NPSH required by the pump, usually NPSH3 (m) · ratio = NPSH margin ratio (-)
- Absolute suction pressure
p_s,abs = p_atm + rho * g * (dz - hf_suc - v^2 / (2 * g))- p_s,abs = absolute pressure at the pump suction (Pa) · v = velocity in the suction pipe (m/s)
- Effect of flow
hf_suc ~ Q^2 and NPSHr rises with Q- Q = pump flow (m3/h)
Frequently asked questions
Why must NPSH available exceed NPSH required?
Below NPSHr the pressure at the impeller eye reaches the vapor pressure and the water boils. The bubbles collapse on the vanes, causing loss of head, gravel-like noise and erosion.
How much NPSH margin is enough?
Catalog NPSHr is usually NPSH3, where the pump has already lost 3 % of head, so a ratio of 1 is not safe. ANSI/HI 9.6.1 recommends margin ratios from about 1.1 up to 2.5 depending on the service; the LOGOS calculator uses at least 1.1 and 0.5 m as the floor for general service.
How does a suction lift affect NPSHa?
Every metre the liquid level sits below the pump centerline subtracts one metre from NPSHa, and the suction loss subtracts more. A flooded suction, with the tank above the pump, adds the elevation in your favour.
How does liquid temperature change NPSH available?
Vapor pressure rises quickly with temperature, so the p_v/(rho*g) term grows and NPSHa falls. Water at 25 C has about 3.2 kPa of vapor pressure, while at 100 C it equals sea-level atmospheric pressure.