How To Work Out Npsh

Npsh margin is how much the npsha value exceeds the npshr.

How to work out npsh. Nphsa calculations should consider the temperature of the fluid and the distance between the pump and the suction source. The value of npsh available npsha shall be greater than npsh required npshr which is a function of pump and to be specified by pump manufacturer. To calculate npsh available take the source pressure data supplypressure add the atmospheric pressure data atmosphericpressure subtract the losses from friction within the pipeline result hazenwilliams. Nsph of pump is calculated as follow.

The formula to calculate npsh is. For example if nphsr is 10 feet nphsa must be at least 15 feet. Such an adjustment would result in npsh 3 curves being lifted for all pumps with discharge specific speeds less than about 1 500 and a lowering of npsh 3 curves for pumps with discharge specific speeds above that value. The npsh available normally shortened as npsha is a value calculated by the plant engineer.

In which pt is inlet total pressure p v is vapor pressure of the fluid r is density and g is acceleration of gravity. For the sake of example let s say that this pump is going to operate a 1000 gpm most of the time and 1350 gpm some of the time. As a general rule make sure that nphsr is less than nphsa by the larger of 5 feet or 10 of nphsa. For those that deal in si units or work professionally in the vacuum arena note that a torr is defined as 1 760 of an atmosphere and may be expressed as 1 mm hg where 760 mm hg also equals 29 92 inches hg.

See ansi hi 9 6 1 2012 for more information on this subject. There are guidelines for recommended or proper margins and i say the higher the margin the better. Npsh available ft m absolute pressure ft m vapor pressure ft m line loss ft m elevation difference ft m. For an example of how this might work in practice take a look at the npshr curve below.