My liquid ring pump is knocking and losing vacuum — is it cavitation, and what fixes it?
Probably yes, and in a liquid ring pump cavitation has one specific cause: the suction pressure has fallen to the vapour pressure of the service liquid at its working temperature. At that point the liquid ring itself begins to boil. The vapour bubbles collapse again as they are carried into the compression side, and that collapse is the knocking you hear. It is not a mechanical fault — the pump is being asked to work below the floor its own service liquid sets.
That floor moves with temperature, which is why the same pump cavitates in August and runs quietly in January. Service water at 15 °C puts the floor near 17 mbar; at 30 °C it is already around 42 mbar; at 40 °C around 74 mbar. A recirculated seal water loop that has warmed up through the day walks the floor upward while the process keeps asking for the same vacuum. Nothing changed on the pump.
Two checks separate cavitation from ordinary wear or bearing noise. First, cavitation noise tracks the operating point: throttle the suction slightly or drop the service water temperature and it fades within seconds, while bearing noise does not care. Second, look at where the damage is. Cavitation erodes the impeller blade tips and the porting face in a pitted, sponge-like pattern; wear from normal running polishes those surfaces flat instead.
The fixes go in order of cost. Lower the service liquid temperature or raise its flow — often that alone restores the margin. Next, fit a cavitation protection line: a small controlled air bleed into the suction that stops the pump ever reaching its own floor, at the price of a little capacity. Then, respect the minimum suction pressure for your liquid temperature rather than the figure on the curve, which assumes 15 °C water. If the process genuinely needs deeper vacuum than the liquid allows, the answer is an ejector stage ahead of the pump, not pushing the pump further down.
If you want a straight answer rather than a guess, send the pump model and serial number, the service liquid and its temperature at the inlet, the suction pressure you are actually running at, whether the seal liquid is once-through or recirculated, and whether the noise changes when you throttle the suction. Those five facts decide whether this is cavitation, and if it is, which of the fixes above applies to your installation.