Throttling the discharge of a centrifugal pump is a common method of stopping a cavitation problem. Why does it work? Take a look at the pump curve shown bellow:
As you can see, the NPSH (net positive suction head) required increases with capacity. Take the 13-inch impeller as an example:
- At 300 gallons per minute an NPSH (net positive suction head) of 10 feet is required
- At 200 gallons per minute only 6 feet is required
Reducing the capacity by throttling the pumps discharge will lower the NPSHR (net positive suction head required) and stop the cavitation. Some maintenance shops stop cavitation by introducing air into the suction side of the pump. The air takes up space, the capacity reduces, and the NPSHR reduces.
The same thing will happen if you throttle the suction side of the pump, but it is a little trickier. Restricting the suction of a pump can cause a cavitation problem in its self. So why bring the subject up at all?
We use suction recirculation if the product you are sealing will react negatively with an increase in temperature. Discharge throttling is not an option in this instance because the discharge head will increase, the internal recirculation will increase and the pump’s internal temperature will rise increasing the fluid temperature.
With a volatile liquid, suction throttling may be the wisest choice.
Some operators have been known to throttle the suction of a pump to reduce capacity. If you have excessive NPSH available, this could be a way to save power.
The capacity of condensate pumps is often submergence controlled which is the same as suction throttling.
The best solution to eliminate suction throttling, is to use the correct size pump, running at its BEP (best efficiency point) and then cavitation is seldom ever a problem.
- On February 17, 2018