NPSHANet Positive Suction Head Available
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NPSHA = 2.31/s ([P.sub.a] - [P.sub.vp]) + ([V.sup.2.sub.a] - [V.sup.2.sub.s]/61.3) + ([Z.sub.a] - [Z.sub.s]) - [H.sub.a [right arrow] s]] (1)
Many engineers get concerned about maintaining NPSHA in open cooling tower systems and consequently insist on elevating the tower basin well above the pump suction.
NPSHA = [2.31/1] (14.7 - 0.5) + [([0.sup.2] - [12.sup.2])/61.3] (0) - 2 = 28.5 ft (2)
If the NPSHa is greater than the NPSHr, then cavitation should not occur.
For NPSHa, derating of engine power can be linked to coolant temperature limits.
By considering NPSHa relative to NPSHr up front, this software identifies the potential for cavitation and any related problems early in the engine design cycle.
If the noise level and/or vibration can be stopped with just a small decrease in the flow, the problem is likely cavitation because the NPSHR has been reduced and the NPSHA has been increased by the lower flow.
Fortunately, in the pool industry, the net positive suction head available (NPSHA) necessary to avoid cavitation can be put into a very simple formula:
As long as the NPSHA is greater than or equal to the Net Positive Suction Head Required (NPSHR) on the pump curve, cavitation will not occur.
You will have little trouble identifying a pump with inadequate NPSHA. It will be cavitating, and you will hear the hydraulic noise.
If cavitation-free pump operation cannot be achieved by this approach and the recirculator is equipped with a capacitance probe for liquid level sensing/ control, it may be possible to raise the vessel's liquid operating level to increase the available net positive suction head (NPSHa).