PUMP GUY MAILBAG: Minimum Flow with Variable Speed
(a.k.a. "The Pump Guy")
Hello Pump Guy,
We have communicated before. I am the pump system specialist at a water and wastewater authority for a major municipality. I need to pick your brain.
I specify the pumps and deal with the sales reps and pump companies that supply the municipality. Most of the sales reps use a standard form that makes a pump and motor selection based on the requirements of flow and head I provide. Normally, the factory pump performance curve accompanies the completed form with price and delivery quote.
Most of the provided pump curves are based on fixed-speed motors. Yet, we are mating more and more pumps with variable-speed motors.
Sometimes, a minimum acceptable flow is shown on the pump curve. Yet other pump curves don’t mention the minimum flow. Is there a way to establish the minimum flow a pump can withstand during long operating hours without causing excessive vibrations and other problems, such as internal flow recirculation?
Also, when a minimum flow is given at nominal speed, should there be a different minimum flow at lower speeds if the pump has a variable-speed motor?
And finally, is it safe to predict that the new BEP at a lower speed will be equal to the ratio of the reduced speed to the nominal speed?
Your answers will help demystify some issues that have bugged me for years.
Thanks.
Graham T.
Hello Graham,
Every established mass-marketed product (cars, picture frames, cell phones, boats, TVs) has certain common design parameters. An airplane folded from a piece of photocopy paper has certain design constants in common with a large passenger jet, especially if I want the paper airplane to glide through the air around my office.
These design constants or parameters are functional and physical characteristics of the product that facilitate the performance of the product. These parameters serve as guides to develop a new product that never existed before. The same parameters can be manipulated to alter an existing product so that it can perform a different task.
Likewise, industrial pumps have certain design parameters. This brings me to your first question: Is there a way to establish the minimum flow a pump can withstand during long operating hours without causing excessive vibrations and other problems such as internal flow recirculation?
Yes! One such design parameter pertaining to centrifugal pumps is the “Suction Specific Speed.” The suction specific speed employs three tidbits of information on the pump performance curve to suggest an operating window of the pump. This operating window incorporates the minimum acceptable flow as a percentage of best efficiency flow. The pump will go into degeneration (excessive vibrations and internal recirculation) if the pump is operated (restricted) below this minimum acceptable flow (GPM).
Now, I’ll answer your second question: When a minimum flow is given at nominal speed, should there be a different minimum flow at lower speeds if the pump has a variable-speed motor?
Yes! The answer takes us again to the Suction Specific Speed. The three tidbits of info on the pump curve that compose the Suction Specific Speed are:
- Shaft speed in RPM
- Best efficiency flow in GPM
- NPSHr at best efficiency
The formula for SSS is:
Because of the Affinity Laws, a reduction in the shaft speed (RPM) will bring about a proportional reduction in the best efficiency flow (GPM). The third element, the NPSHr will reduce by the square of the change in the speed. Therefore, the SSS will be proportionally lower with a reduction in the RPM.
But because the SSS suggests the minimum flow as a percentage of best efficiency flow, the percentage might vary slightly. I say “slightly” because the NPSHr is a delinquent stepchild of the Affinity Laws.
Officially the NPSHr is a head. Therefore it should follow the second Affinity Law…“Heads change by the square of the change in the speed (RPM).”
But the NPSHr doesn’t precisely follow the second Affinity Law. The new NPSHr at a lower speed is “somewhat-but-not exactly” equal to the ratio of the reduced speed to the nominal speed. Therefore, the percentage of best efficiency flow might vary slightly.
It’s not a big issue in a practical world. But, it makes good conversation when standing next to a philosophy professor at a Christmas cocktail party. For once, you can bore the philosophy professor to death. The philosophy professor will declare, “Nice Weather Today!” and move away to bother someone else, leaving you alone to devour the smoked oysters and spiked punch.
Now, let’s go to your third question: Is the pump’s BEP at a lower speed equal to the ratio of the reduced speed to the nominal speed?
Yes, within limits. Let’s identify “reduced speed” and “nominal speed.” Let’s say the pump/motor nominal speed is 100-percent rated speed for the motor. Let’s say that our reduced speed is 95 percent of nominal speed. Then, the BEP at 95 percent of nominal speed is equal to the ratio of the reduced speed to the nominal speed.
Let’s say the pump/motor reduced speed is 75 percent of nominal speed. Then the BEP at 75 percent of nominal speed is approximately equal to the ratio of the reduced speed to the nominal speed.
Let’s say the pump/motor reduced speed is 15 percent of nominal speed. Then, the BEP at 15 percent of nominal speed is not equal to the ratio of the reduced speed to the nominal speed.
Regards,
The Pump Guy
Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at [email protected].
