what is NPSH for the water pump. Can you anyone explain in details? How can it calculate?

Answer:

Net Positive Suction Head Equation

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NPSH stands for Net Positive Suction Head. It usually refers to the amount of pressure needed at the inlet of the pump for the pump to work properly. If there is insufficient pressure pushing the fluid into the pump, and the pressure at the inlet is lower than the NPSH requirement, then the pressure will be so low in the sucking center of the pump that the fluid will boil (this is called cavitation). It usually makes a racket, reduces pump performance, and erodes away the pump. To make sure this doesn't happen you have to have enough head (as in some level in the tank feeding the pump) and low pressure drop on the way to the pump (meaning big enough lines, no obstructions, and not too many bends). Some pumps are designed for very low npsh requirements. These pumps can suck water from some depth without cavitating. No pump can suck room temperature water from fore than about 25 feet below the pump, because the vacuum needed to lift the water 25 feet or so is close to the vacuum needed to boil water at room temperature. You cannot easily calculate the npsh a pump needs. This is a function of the design and geometry of the pump. You can calculate the delivered npsh by figuring the pressure of the liquid in the feed tank (don't forget atmospheric pressure) and subtracting the pressure loss due to friction in the piping between feed tank and pump. This loss will be a function of flow rate. Faster flow, more frictional pressure loss.

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