What is cavitation in pumps and propellers?

Cavitation is the formation of vapour bubbles in a liquid where the local pressure falls below the vapour pressure at the prevailing temperature, followed by their violent collapse where the pressure recovers. The liquid does not need to be hot; it boils because the pressure is low. Each collapsing bubble produces a micro-jet and shock wave that hammers the adjacent metal at enormous local pressure, and millions of such collapses erode, pit and fatigue the surface.

Aboard ship it appears in two main places. In pumps it occurs at the impeller eye, where velocity is highest and pressure lowest: the symptoms are a crackling noise like pumping gravel, vibration, fluctuating discharge pressure and falling capacity, and in severe cases complete loss of suction. Left running, the impeller comes out pitted and sponge-like around the vane inlets. On propellers, cavitation forms on the blade surfaces where the flow accelerates: it causes blade erosion, hull vibration, noise and loss of thrust, with characteristic pitting along the leading edges and tips of a worn or overloaded propeller.

The governing concept for pumps is net positive suction head, NPSH: the pressure margin above vapour pressure available at the impeller. The available NPSH must exceed what the pump requires, so anything that reduces it invites cavitation:

• High suction lift or a low tank level.
• Hot liquid, since vapour pressure rises steeply with temperature; condensate and boiler feed systems are the classic sufferers.
• Choked strainers, long or undersized suction lines, and partly closed suction valves.
• Running the pump far beyond its design flow.

Prevention is operational as much as design: keep suction lines short and clean, keep lifts small and tanks topped up, never throttle a centrifugal pump on the suction side, and slow the pump or cool the liquid when the margin is thin. Feed pumps get their NPSH from the head of the cascade tank or deaerator above them, which is why those tanks sit high in the ship. For propellers, designers limit blade loading and choose section shapes that delay cavitation inception; the crew's contribution is a clean, undamaged propeller and avoiding prolonged running heavily fouled or overloaded. Persistent cavitation is never just a noise: it is metal being removed, and the cause should be found and corrected, not lived with.