NPSH Explained

If a pump is going to fail in its first year, cavitation is usually why. And cavitation is really just one inequality:

NPSH available  ≥  NPSH required  +  safety margin

Get that wrong and the pump eats itself from the inside.

What NPSH actually is

Net Positive Suction Head is the pressure at the pump inlet expressed in feet of the fluid being pumped, referenced above the fluid's vapor pressure. If NPSHa drops below vapor pressure anywhere inside the impeller, bubbles form, collapse, and blast pits into the metal.

• NPSHa — what the system provides. You calculate this.
• NPSHr — what the pump demands. Read this off the pump curve.

The NPSHa formula

NPSHa = 2.31 × (P_atm − P_vap) / SG   +   h_static   −   h_friction

All four terms in feet of fluid:

• P_atm — pressure on the fluid surface (atmospheric for open tanks; absolute tank pressure otherwise).
• P_vap — vapor pressure of the fluid at the pumping temperature.
• h_static — vertical distance from fluid level to pump centerline. Positive if flooded suction, negative if lifting.
• h_friction — line, valve, strainer and fitting losses on the suction side only.

Why hot water bites

Vapor pressure isn't linear. Water at 70°F has a vapor pressure of 0.36 psia. At 200°F it's 11.5 psia — that's 32× worse. On a boiler feed pump running from a deaerator at 220°F, atmospheric pressure contributes almost nothing to NPSHa, and every foot of friction matters.

How much margin?

The Hydraulic Institute says NPSHa should exceed NPSHr by the greater of 3 ft or 35%. Practically:

• Water at ambient: 20% margin is usually fine.
• Hot condensate, boiler feed: 50–100% margin.
• Hydrocarbons at the bubble point: use API 610 — much stricter.

Fixing a cavitation problem

In order of cost:

1. Cool the fluid (if the process allows) — lowers vapor pressure fast.
2. Shorten and straighten the suction line — cuts h_friction.
3. Raise the source tank or lower the pump — buys static head.
4. Pressurize the tank or add a booster pump.
5. Change to a pump with lower NPSHr (inducer, lower speed, larger eye).

Do the math

Use the NPSH Available Calculator to check your system. If you're sizing a boiler feed pump, the BFP Calculator accounts for feedwater temperature automatically. For a full sizing walk-through, see Pump Sizing 101.