Is NPSH Really That Tricky?

The sound you hear is me banging my head on the desk.

I personally think NPSH is a fairly simple calc and an easy to understand concept - but I first did a pump calc in my first month at work after graduating and have done them regularly over the last 20 some years. I also saw the moth eaten impellor taken out of a cooling water pump due to NPSH problems.

It is a critical property to be calculated when specifying a pump service. I am a process engineer so my job is to specify the function of the pump which I take to be flowrate, head and NPSH and to provide the fluid conditions temp, density viscosity to allow the rotating mech engineer to size the pump and driver.

If you do not pay attention to the NPSH and if say after the pump is specified some bright spark says "ohh we can save $10m if we reduce the elevation of that column" reducing the NPSH available it would be like putting diesel fuel in the tank of an F1 Ferrari and expecting to get the same performance.

NPSH is a function in part of the geometry of a system and the physical properties of the fluid but it is the job of the engineers to specify that geometry correctly so that the NPSH available exceeds the NPSH required by the pump.

If you do not pay attention to NPSH and NPSHa is too low then cavitation will occur causing the pump to fail - typically due to vibration and destruction of the impellor.

The normal route in my workplaces has been that process are given some typical values for NPSH etc from previous jobs and calculate the min height required for a vessel to meet the NPSH. This is given to layout who see how this fits with their limitations. meanwhile mechanical get some preliminary data for the pump from vendors. there is then a meting with Layout, Process and Mech in which all this information is brought together and elevations agreed for vessels. If something does not fit and/or would be prohibitively expensive process review there design and then mech go to other vendors perhaps more exotic / expensive designs to get pumps with a lower NPSH.

It is a fundamental that I was taught early on to have pump as close as possible to the suction source with as litle pipe and equipment between the vessel and the pump suction when pumping saturated (boiling) fluids. The one exception is when there is a feed / bottoms exchanger which sub cools the bottom product en route to the pump.