Capacity is reduced only when cavitation is so severe that complete bubble collapse does not occur until the plunger is on its discharge stroke. Even when the bubbles completely collapse on the suction stroke, some damage can occur. During collapse, the liquid impinges on the face of the plunger, chewing metal away, similar to the damage seen in the eye of a centrifugal pump impeller exposed to similar cavitating conditions.
The resulting shock is transmitted through the open suction valve and into the suction line, sometimes causing vibration and noise. The shock is also transmitted through the plunger and crosshead assembly, echoing in the power end. Such a knock is often construed as a power end problem, such as a loose rod or broken gear tooth.
No known attempt has been made to quantify the extra margin of NPSHA required to preclude all cavitation in reciprocating pumps. Until that occurs, the author suggests a 50 percent NPSHA margin for cool‑water type applications over the 0 percent capacity‑drop NPSHA.
1. Collier, S. E., "Know Your Mud Pump ‑ Part 5: Knocking", World Oil, Gulf Publishing Co., 1958/1959.
Pumps & Systems, January 2010