Testing for Net Positive Suction Head Required (NPSHR) 16-5
How can you tell the NPSH required for your pump? It’s easy, just ask the manufacturer!
As logical as that sounds, we still find people assuming that if the know the NPSHR for pump brand “A” and pump brand “B” is the same size, both pumps should have the same requirement. So why isn’t it true? There are a couple of reasons:
- ANSI pumps conform to the same envelope (outside) dimensions, but the internal dimensions are different. This means that the friction losses within the pump are not the same.
- The surface finish of the pump’s internals changes with materials and age. If it is a used pump and you have been pumping abrasive materials, the scored metal components will offer more resistance to the liquid flow than smooth, new metal parts.
If you do not have enough NPSH available at the suction of your pump, the pump will pump in spurts, lose some of its capacity and begin to cavitate. All of this translates into poor pump performance, wasted energy, impeller and volute damage and premature mechanical seal and bearing failure. If you are unfamiliar with how we determine the NPSH available to your pump check out the paper I wrote on that subject.
The test for NPSH required is a simple one for you to do, just as long as you remember that the NPSH required increases with capacity. The more fluid you pump, the more NPSH you need to stop your product from vaporizing. Here is how the pump manufacturer did the test at his facility. To duplicate it you will need:
- Gauges to read suction and discharge pressures.
- A gauge to read the fluid flow.
- The pumping temperature.
- Barometric pressure
- The rpm of the pump.
Using a suction valve, the manufacturer gradually reduced the fluid flow at the pump inlet. Watching his discharge gage, he kept reducing the flow until the discharge gage showed a drop in the pump’s total head.
This total head reduction occurred because the fluid vaporized in the impeller. The NPSH available at the suction flange just equals the NPSH required by the pump. A 3% drop in this total head is just about the point where cavitation will begin.
At this point you should record:
- Suction gage pressure.
- The flow rate.
- The fluid temperature.
- Barometric pressure
- Pump’s speed in rpm.
Remember that this reading is giving you the NPSH required for just one point on the pump curve. You are going to have to record and plot a series of these points against the pump’s capacity to get a clear picture of the NPSH required over the operating range or window of the pump. After you do this, you will notice that the NPSH required increases with the pump’s capacity
Let’s try an example:
We will determine the NPSH required for a 2 x 1.5 (2″ suction, 1.5″ discharge) pump moving 240 gallons per minute of fresh water at 80 degrees Fahrenheit. We throttled the suction and recorded that the suction gage read 18.2 inches of mercury vacuum when the pump was within 3% of its normal head. The barometric pressure at the test facility is 29.0 inches of mercury vacuum
Here are the formulas. First we must convert suction gage pressure to feet of head:
Next we convert barometric pressure to feet of head
= 32.81 absolute
From a pipe friction chart we learn that 240 gpm of water flowing through a 2 inch pipe has a velocity head of 8.18 feet
From a water properties chart we learned that the vapor pressure of 80 degree fresh water is 1.2 feet absolute
Putting all of that information into the NPSH available formula we get:
Hsv = hgs + ha + hvs – hvpa
- 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 liquid, abolute.
Hsv = – 20.6 + 32.81 + 8.18 – 1.2
= 19.19 feet
This number is then plotted as the required NPSH for that pump at 240 gpm when it is handling 80 degree Fahrenheit water.