There is a distinct sound a cavitating pump makes — like gravel being dragged through the casing. It is not gravel. It is millions of vapour bubbles forming in the low-pressure region near the impeller eye and then collapsing violently as they reach higher pressure. Each collapse is a tiny implosion, and together they pit the impeller, erode the casing, shake the bearings, and quietly destroy a pump that is otherwise perfectly sized for its duty.
The cause is almost always the same: not enough pressure on the suction side. The pump has plenty of power, the discharge piping is fine, the motor is happy — but the water reaching the inlet is too close to its boiling point. The tool for diagnosing and preventing this is NPSH, net positive suction head, and this article walks through what it means and how to compute it.
Why this calculation matters
Cavitation is one of the few pump failure modes that a healthy machine can suffer through no fault of its own design. It is purely a suction-side problem, and it is entirely predictable before installation if you do the arithmetic.
The consequences are not subtle. Sustained cavitation erodes impeller vanes, drops head and flow, raises vibration, and shortens seal and bearing life. In severe cases the pump loses prime altogether. Yet the fix is rarely exotic — lower the pump, shorten the suction line, increase the pipe diameter, or cool the fluid. The hard part is knowing you have a problem before metal starts disappearing. That is what an NPSH check buys you: a clear margin number you can defend.
The core formula
Every fluid boils when its local pressure drops to its vapour pressure. Inside a pump, the lowest pressure is at the impeller eye. If the pressure there falls to the vapour pressure of the liquid, the liquid flashes to vapour — cavitation begins. NPSH is the bookkeeping that keeps that from happening.
There are two quantities, and you must compare them.
NPSH available (NPSHa) is set by the installation. It is the suction-side pressure head the system actually delivers to the pump inlet, above the vapour pressure:
NPSHa = (p_atm - p_v) / (rho*g) - z_lift - h_f
where p_atm is the pressure on the liquid surface, p_v is the fluid's vapour pressure at its temperature, rho is density, g is 9.81 m/s^2, z_lift is the height the pump sits above the liquid surface, and h_f is the friction head lost in the suction line. If the pump sits below the liquid surface, z_lift is negative and helps you.
NPSH required (NPSHr) is a property of the pump itself, measured by the manufacturer and published as a curve against flow rate. It is the minimum suction head the pump needs to avoid cavitation internally.
The rule that governs the whole subject is simple:
NPSHa > NPSHr
The gap between them is the cavitation margin. A common practice is to keep NPSHa at least 0.5 to 1.0 m above NPSHr, and more for critical or high-energy pumps. Notice what NPSHa depends on: it falls as the pump is raised, as the suction line gets longer or narrower, and — crucially — as the fluid gets hotter, because vapour pressure climbs steeply with temperature.
A worked example
A pump draws water at 25 C from an open tank. At that temperature, density rho = 997 kg/m^3 and vapour pressure p_v = 3.17 kPa. The tank is open to atmosphere, so p_atm = 101.3 kPa. The pump sits 2.0 m above the water surface, and the suction line contributes 0.5 m of friction head loss.
Step 1 — the pressure-head term. Convert the net pressure above vapour pressure into metres of head:
(p_atm - p_v) / (rho*g)
= (101300 - 3170) / (997 * 9.81)
= 98130 / 9780
= 10.03 m
Step 2 — subtract the suction losses. Take away the lift the pump must overcome and the friction in the suction pipe:
NPSHa = 10.03 - z_lift - h_f
NPSHa = 10.03 - 2.0 - 0.5
NPSHa = 7.5 m
Step 3 — compare with the pump's requirement. Suppose the pump's datasheet gives NPSHr = 4.0 m at this flow rate. The margin is:
margin = NPSHa - NPSHr = 7.5 - 4.0 = 3.5 m
NPSHa exceeds NPSHr by 3.5 m, so cavitation is avoided with a healthy margin. There is room here for the line to foul slightly or the fluid to warm a little before the pump is at risk.
It is worth seeing how thin this can get. If the same water were at 80 C, its vapour pressure rises to roughly 47 kPa, the pressure-head term drops to about 5.6 m, and NPSHa falls to around 3.1 m — now below the 4.0 m requirement. Same pump, same piping, hotter water, and the pump cavitates. Temperature is the variable that catches people out.
Common mistakes
Using vapour pressure at the wrong temperature. Vapour pressure is not a constant. Between 25 C and 80 C it rises by more than an order of magnitude. Always use p_v at the actual pumped temperature, not a textbook room-temperature figure.
Confusing NPSHa and NPSHr. NPSHa belongs to the installation; NPSHr belongs to the pump. You cannot change NPSHr by re-piping, and you cannot change NPSHa by buying a different pump. Keep the two roles straight.
Ignoring that NPSHr rises with flow. NPSHr is a curve, not a number. Pushing a pump to higher flow increases what it requires, so a pump that is safe at duty can cavitate when run out to the right of its curve.
Forgetting suction-line fittings. Elbows, a foot valve, a strainer, and a partly closed isolation valve all add to h_f. A friction estimate that counts only straight pipe is optimistic, and the error always works against you.
Designing with zero margin. Meeting NPSHa equal to NPSHr exactly leaves nothing for fouling, warm-up, or a fouled strainer. Build in a real margin and treat it as part of the specification.
Try the interactive NovaSolver calculator
The arithmetic above is easy to do once, but tedious to repeat as you test different layouts. The Pump Cavitation & NPSH Calculator on NovaSolver computes NPSH available from suction head, flow rate, suction pipe diameter and length, and fluid temperature, then compares it against the pump's NPSH required and reports the margin and cavitation status live. You can switch the fluid — water at several temperatures, seawater, diesel — and watch the margin shrink or grow as you raise the pump or lengthen the suction line.
Related calculators
- Centrifugal Pump Curves Simulator — find the operating point first, since NPSHr depends on the flow rate the system settles at.
- Centrifugal Pump Cavitation calculator — a focused look at cavitation behaviour in centrifugal pumps specifically.
- Pump Affinity Laws calculator — see how changing speed shifts both flow and the suction conditions the pump faces.
The full collection lives in the fluid mechanics tools hub.
Closing note
Cavitation is a suction-side disease with a discharge-side reputation — people blame the pump when the piping is at fault. The cure is the NPSH check: compute NPSHa from atmospheric pressure, vapour pressure, lift, and friction; compare it against the pump's NPSHr; and keep a deliberate margin between them. Pay particular attention to fluid temperature, because vapour pressure is the term that quietly erodes your margin. Do the calculation before the pump is bolted down, and you will never have to identify cavitation by the sound it makes.
