If the fluid only cavitates when the stagnation pressure drops below the vapor pressure, then why is NPSHr anything other than it just needs to be greater than 0?

@@Cray25 The minimum amount of suction head required depend on the pump size, impeller design, rpm, restriction at inlet, etc. The hand in the pool demonstration will explain this. If your hand is close to the top when you quickly push down on and into the water the amount of static head above your hand was low at the start, thus you see more bubbles when you push down. The deeper your hand is at the start of the hand demonstration the height of the water above your hand at the start is higher (more suction head), the less bubbles are produced. So NPSH (min. static head) is dependent on pump design and also the fluid viscosity being pumped.

@@oppanheimer superb explanation...👍

x2

Minor losses are generated a each pipe fitting, valve, etc and are estimated using this equation: Hm = k(V^2)/(2g). The k value is something you look up. V, the velocity of the water passing through the fitting/valve.

Available and required

Do you mean: "What does it mean if NPSHa is 3m? how is this observed physically?" NPSHa of 3m means that there is a total of 3m of absolute pressure head acting on the pump suction flange. Absolute pressure head accounts for atmospheric pressure, and the vapor pressure as well as the water depth. NPSHr of 3m means that the pump mechanics itself require at least 3 m of absolute pressure head in the fluid in order to avoid creating a cavity in the fluid. Hopefully, those answers are helpful, but please feel free to re-ask your question if I misunderstood.

@@kennethwlamb thanks sir, atlast my doubt cleared

​​ Before l have also the same doubt why the Required should be less than the Available until I get this point that the NPSH Required is the minimum value should be satisfied to avoid cavitation hence the NPSH Available from the system should be higher than the required . Another point we need two simultaneous constraints to be satisfied for the suction process 1st the absolute pressure at the eye of the impeller should be less than the atmospheric pressure and 2nd that absolute pressure should be higher than the vapor pressure at the the same time and I hope Dr Lamb clarify my answer if it is correct or have any shortage 🌹🌹

I've never been asked that question before :). I think it would be impossible for a pump, that is operating, to have an NPSHr = 0. I don't think it is physically possible.

I think the Nsphr is derived by manufacturers by experiment and it should be positive value it represents the minimum required head at the suction impeller side to avoid cavitation so that it cannot be zero meaning it exists already