Actual NPSH Test - Reciprocating Pump and What is a Safe NPSH Margin for a Reciprocating Pump?


Written by:
Terry Henshaw, P.E.

The collapse of the vapor during the discharge stroke is much "softer" with the light hydrocarbon, causing less damage to metal surfaces, and less shock transmitted to power end bearings. The power end would also be lightly loaded, and the components better able to absorb shock loads. The result would be a quiet, smooth running pump with a long life.

On the other hand, if this unit were pumping cool water at 1,000 psi discharge, the effect of cavitation would be much more pronounced. With 0.85 psi of NPSHA, capacity would be less, and the more severe collapse of the vapor bubbles would damage metal surfaces more and create larger shocks. Power end bearings (and other components), more heavily loaded (due to the higher discharge pressure), would be less able to tolerate the shock without damage.

How much additional NPSHA is required to eliminate all cavitation? Figure 1 might suggest that, since full capacity is achieved with 1.5 psi of NPSHA, that amount would eliminate all cavitation. It is true that the pump would be at rated capacity, and that no vapor‑collapse shock would occur on the discharge stroke. However, as Collier noted (1), cavitation can occur in a reciprocating pump without reducing capacity.

Incipient cavitation occurs during the initial portion of the suction stroke, because the plunger or piston is accelerating, then disappears during the latter portion because the plunger is decelerating. The pumping chamber is therefore completely filled with liquid at the end of the suction stroke, and pump capacity is not affected.

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.

References
1. Collier, S. E., "Know Your Mud Pump ‑ Part 5: Knocking", World Oil, Gulf Publishing Co., 1958/1959.

Pumps & Systems, January 2010

 

 

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