Testing for Net Positive Suction Head Available (NPSHA) 16-04

Centrifugal pumps are designed to pump liquids, not air. We are interested in the amount of NPSH available at the pump's suction flange because we want to insure there is enough head (pressure) available to prevent the fluid from vaporizing and causing gas bubbles. There are times when the available NPSH can be questionable, so you would be wise to either measure or calculate the NPSH available in the following conditions:

You can calculate the available net positive suction head (NPSHA), at the suction of a pump, if you know:

To make the calculation, use the following equation:

hsv = hgs + ha + hvs - hv

Where:

As an example, we will begin by connecting a mercury manometer to the 2 inch pipe at the suction nozzle of your pump

To calculate NPSHA we will need all of our units in feet of head. We will start with the 14.7 inches of mercury:

Next we will convert barometric pressure to feet:

The velocity head = 1.4 feet at 100 gpm through a 2 inch pipe. (We got this number from a standard pipe friction loss table. Look under the column V2/2g)

Now we will convert the Vapor pressure to head:

Going back to our original formula: 

hsv = hgs + ha + hvs - hvpa
= -9.8 +19.3 + 1.4 - 0.3

= 10.6 feet at 100 gpm

 

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