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:
- When the pump is installed high above the liquid level.
- When the pump takes a suction from a vessel under vacuum, like the hotwell of a condenser.
- Anytime you have installed a low specific speed impeller.(the vanes are almost straight)
- When the liquid you are pumping has a high vapor point.
- At high pump rpms.
- Any time you are pumping on the high capacity side of the pump curve
- When the suction line is long, or if there are valves, fittings and a strainer between the source and the pump suction.
- Any time the pump system is located at a high altitude
You can calculate the available net positive suction head (NPSHA), at the suction of a pump, if you know:
- The flow rate of the liquid (gpm)
- The suction gauge pressure (psi)
- Vapor pressure of the liquid (psi)
To make the calculation, use the following equation:
hsv = hgs + ha + hvs – hv
- hsv = available net positive suction head, in feet of liquid
- hgs = suction gauge pressure, in feet of liquid, gauge
- ha = atmospheric pressure, in feet of liquid, absolute
- hvs = suction velocity head, in feet of liquid
- hvpa = vapor pressure of the liquid, in feet of head, absolute
As an example, we will begin by connecting a mercury manometer to the 2 inch pipe at the suction nozzle of your pump
- Fluid you are pumping = 100 gpm of acid
- Specific gravity = 1.7
- Vapor pressure = 0.20 psia
- Manometer reads = -14.7 inches of vacuum (from a potential of 29.9 inches)
- Barometric pressure = 736 mm of mercury
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
- On February 18, 2018