Page 38 - Know and Understand Centrifugal Pumps

NPSH, Net Positive Suction Head

of less than 8 feet at the duty point. And, to be sure that problems
don’t arise during the process, we could calculate the NPSHa at the
end of the process:
NPSHa = Hs2 - Hf - Hi
NPSHa = 6.0 - 1.5 - 2.0
NPSHa = 2.5 feet

Now, it’s one thing to say to use a pump with an NPSHr less than 2
feet. It’s another thing to find a pump with this design parameter, that
at the same time complies with the demands of the operation. Perhaps
it will be necessary to modify the system to increase the Hs2, reduce the
Hf, or modify the pump to reduce the Hi. Other possible options are:
1. Pressurize the tank with air or a gas compatible with the liquid and
process.

2. Turn off the pump and drain the tank by gravity.
3. Install a small booster pump that feeds the principal pump.
4. Operate the pump at a slower speed.
5. Survive the cavitation. (There’s a discussion on this later in the
book.)
As we’ve said numerous times before in this chapter, the important
thing is that the NPSHa of the system is above the NPSHr of the pump
by a sufficient amount to avoid stress and possible cavitation. If the
NPSHa should be inadequate, there are ways to elevate it. Remember
from the formula that five elements compose the NPSHa. Two of those
elements, the Ha and the Hs, add energy to the fluid. And three
elements, the Hvp, the Hf, and the Hi, subtract energy from the fluid.
We must either increase the elements that add energy, or decrease the
elements that subtract energy. To increase the NPSHa:
1. Raise the level in the tank if possible. This adds Hs.
2. Elevate the tank maybe with stilts. This adds Hs.
3. Maybe you can lower the pump. For example in many
thermoelectric plants, the fuel oil pumps (#6 bunker fuel) are in a
pit. This would permit draining the tanks down to the ground and
still maintain 15 or 20 feet of NPSHa on the fuel oil pumps. This
adds Hs.

4. Pressurize the tank if possible. This adds Ha.
5. Reduce the drag (Hf) in the suction piping. Change to larger
diameter suction piping, or reduce the pipe schedule (change from
‘schedule 40 pipe’ to ‘schedule 20 pipe’ on the suction side).
Investigate changing the pipe material. For example PVC pipe, and
food grade Stainless, is rather slick on the ID. This reduces Hf.

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