Pump-NPSHa Calculations

From the Field;

If you intend to empty your pit you will have 0 NPSHa. I would lower an air driven diaphragm pump into the pit, it will empty the pit without damaging itself .

Your NPSHa=5.25X3 METERS OR APPROX 48 FEET OF HEAD 0R APPROX 20 PSI. THIS DOES NOT ACCOUNT FOR WATER TEMP AND YOU ALTTIUDE

To little info for a good Pump determination and regarding the calculation... what is the pressure on the pit etc. However... why to calculate/determine it by yourself... incorporate your problem with a good pump vendor. They will calculate it for you together with all other necessary requirements.

Kind regards

Daniel Claessens

www.dancla.be

NPSHa is essentially the pressure available at the inlet of the pump minus the vapor pressure of the fluid being pumped. If it is not high enough, the fluid will vaporize inside the pump causing cavitation.

NPSHa=origin pressure - acceleration losses - frictional losses due to pipeline flow - change in pressure due to head - vapor presure of the fluid at its flowing temperature.

Any pump manufacturer or vendor should be able to provide you with the detailed calculation.

What is the diameter of the pit (how many liters or gallons) and how fast do you wish to drain it? That will determine the size of the pump.

Is this something that will need to be done often? Helps to determine quality of the pump.

What is your power source? Single phase, 3-phase, air, other?

Is there any debris such as birds, rodents, sludge, etc. or is it clean water? This will help you select the correct type of pump.

These are a few questions your vendor should ask you and you should be ready to answer.

Simply put, the available Net Positive Suction Head (NPSHa) is the difference between the barometric pressure and the total dynamic suction lift. Nothing more, nothing less. Therefore, the NPSHa can never be more than 32 feet at 0 dynamic suction lift since the maximum atmospheric pressure is 32 feet.

Let me expound on this a little:

Let's say your suction center line is at elevation 100.00 and the bottom of the pit is at elevation 92.00. That means that you will have to lift the liquid 8 feet to get to the eye of the pump impeller. If the suction pipe you are drawing through is a 4" diameter schedule 40 PVC pipe, and you are pumping at the rate of 100 gallons per minute, the friction loss through that pipe is .627 feet per 100 feet of pipe or .0502 feet of friction loss. The total dynamic suction head then would be 8.0502 feet. The atmospheric pressure is 34' (I used 32 feet in my first response out of habit - a little safety factor). Subtracting the 8.-502 from the 34 feet leaves an available NPSHa of 24.9498 feet. If, however, the barometric pressure at the pump site is 31.5 feet, then the NPSHa would be reduced by 2.5 feet, which would make our example 23.4498.

This means that the pump you are using must not have a NPSHr (required NPSH) of more than that number. The NPSHr of the pump can usually be found on the bottom of the pump curve. Some pumps don't show it. Most water pumps, however, do.

I think you left out the acceleration term. The water has to be accellerated from a standstill to its flowing velocity in the suction line. 1/2 rho V**2 is the acceleration term. Units must match the remainder of the equation. Add that to the sample calculation you provided and you've got it.

Putting it simply and assuming you are using a vertical turbine or submersible pump. The NPSHa is the distance between the centerline of the pump volute and the top of the liquid (this will of course vary) plus atmospheric pressure which will be dependant on the location ie distance above sea level. If you are using a dry mounted pump it is atmospheric pressure minus the the distance between the pump centreline and the top of the liquid.

I believe that you have failed to account for the friction loss in the suction line leading to the centerline of the pump volute. Further, I indicated in my example that the difference was from the centerline of the volute to the bottom of the pump pit. This is a more realistic distance since the pump suction lift will increase as the level in the pit goes down. Of course, it is not physically possible to pump all the way down to the pit bottom, but the NPSHa at or near the bottom is the worst case scenario. Using the liquid level only disregards the friction loss. As the level goes down, the friction loss will increase.