NPSH Calculator: Net Positive Suction Head and Cavitation Margin

The net positive suction head available at a pump, and whether it is enough for the pump you chose.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Cavitation happens when the pressure at the pump inlet falls below the vapour pressure of the liquid, so vapour bubbles form and then collapse inside the pump. It makes noise, damages the impeller and cuts the flow. To avoid it, the NPSH available at the pump must exceed the NPSH required by the pump maker.

NPSH available is the head from the surface pressure above the vapour pressure, plus the height of the liquid surface above the pump, minus the friction and fitting loss in the suction line. Raise the tank, lower the pump, shorten or enlarge the suction pipe, or cool the liquid to increase it.

Formulas used

  • NPSH_a = \frac{p_a - p_v}{\rho g} + z - h_f
  • NPSH_a > NPSH_r + \text{margin}

Worked example

With the default numbers in the calculator above:

  1. Head from the pressure above the vapour pressure: 10.11 m
  2. NPSH available: 11.31 m
  3. Margin over the NPSH required: 8.308 m

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What margin should I leave?

Often at least 0.5 m to 1 m, or 10 to 30 percent over the NPSH required, more for hot liquids or fast changes of flow.

Why does hot water cavitate more easily?

Its vapour pressure is higher, so less pressure margin is left. Water vapour pressure is 2.3 kPa at 20 degrees Celsius but 47 kPa at 80.

Where do I find the NPSH required?

On the pump curve from the manufacturer. It rises with flow.

Related calculators

Further reading: White, Fluid Mechanics; Munson, Young and Okiishi, Fundamentals of Fluid Mechanics; Crane Technical Paper 410.

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