Learn the difference between suction lift and flooded suction.
by Jim Elsey
August 20, 2018

Last month, the NPSHa result for the 10-foot flooded suction Example 1 was 40 feet. The only change made this month for Example 2 was that the flooded suction was changed to a 10-foot suction lift with the NPSHa result of 20 feet.

Recognize that simply changing the conditions from a 10-foot flooded suction to a 10-foot suction lift changed the NPSHa by a difference of 20 feet.

Velocity Head (hvelocity): To Be or Not To Be?

I wish to explain a point of confusion for many of the readers. (I am guilty for not being consistent with this practice across many of my articles.) The component of velocity head (hvelocity) in the NPSHa formula is not required when you are calculating NPSHa using the formula method, but it is required when you are measuring for NPSHa.

There are two methods for the determination of NPSHa—either the calculation method or the empirical measurement for NPSHa. The fifth component in the NPSHa formula is velocity head and must be included if you are (measuring) determining the NPSH from a gauge reading on the pump suction. Velocity head is already included if the NPSH is established from a difference in elevation. The velocity head component is a positive value when used in the equation.

Measuring NPSHa = head of inlet + head of velocity – head vapor pressure.

Head of the inlet is equal to the absolute head + or – the static head – the friction head.

Equation 3

It is important to know that when measuring for NPSHa using a suction pressure gauge, the difference in elevation between the gauge and the pump impeller centerline (my proposed datum point for this example) must be corrected for the NPSHa result. If the gauge is above the datum point, add the result (how many feet or fraction thereof) and if the gauge is below the datum, subtract the elevation amount.

Most gauges used in the field are not accurate enough or in the wrong range/scale to measure for the specific purpose of NPSHa. Using a calibrated gauge in the proper range, with the proper scale, can yield good results.

References
Cameron Hydraulic Data Book, 16th edition

To read other articles in the 'Common Pumping Mistakes' column, go here.

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