| Larry Bachus
(a.k.a. “Pump Guy”)
I read your article “Chaos on Steroids” in the September 2011 edition of Flow Control (page 40). I have a question. You did not address the use of a variable-speed motor in lieu of a nearly closed valve. If I use a variable-speed motor and start the pump slowly, will this do the same thing that a closed valve will do? Or do I still need to install a control valve?
That article was about imprecise language in the engineering profession, specifically confusion regarding the word “throttle” used with industrial pumps and valves. The word throttle has a variety of interpretations from control, obstruct, reduce, choke, strangle, or suffocate. “Control” and “reduce” evoke certain meanings and images. “Choke,” “obstruct,” “strangle” and “suffocate” evoke totally different images.
An engineer or supervisor might say, “Throttle that control valve and then start the pump!” The operator might understand, “Reduce (the flow through) that control valve and then start the pump!” However, another operator might understand, “Close that control valve and then start the pump!” The misunderstanding can be significant, maybe disastrous.
Having said that, it is desirable to restrict a control valve about 80 percent (meaning about 20 percent open) as the pump is started in a system with no backpressure. The restricted valve provides artificial resistance on the pump to prevent damage.
Using a pump to fill a vessel from the bottom is a classic example. While the discharge vessel is mostly empty, there is no backpressure on the pump.
|This system features negative static head and very little resistance on the pump. The control valve (throttled about 80 percent) can provide the artificial resistance to prevent stressing the pump.|
Another occasion to provide artificial resistance (backpressure) is when the pipes are empty and the pump’s first duty is to fill the discharge pipes with liquid. There is no backpressure on the pump while the pipes are filling with liquid.
And now, your question, “If I start a pump slowly with a variable speed motor, is it the same as starting the pump against a restricted valve?”
The answer is: It really depends on the design of the system and the profile of the system curve. I’ll explain with the following two examples:
• Example #1: Let’s say you need a pump to recirculate a liquid through a filter and heat exchanger at 500-gpm for the next 3-hours.
The system design (horizontal & vertical pipe runs, elbows, flanges, the filter, valves, instrumentation and other process devices) contains 70-ft of energy.
The system curve is composed of:
0-ft of static head (Hs);
0-ft of pressure head (Hp); and
70-ft of friction (Hf) and velocity head (Hv) at 500 GPM.
The pump must overcome this energy. The pump is designed to develop 70-ft of head at 500 GPM rotating at 1,750 RPM on a variable-speed motor (VSM).
In this case the “pump-with-VSM” will perform the duties of a control valve or a soft start. And, you can operate the pump-with-VSM at any speed as long as the RPM is above the minimum performance velocity of the pump and the VSM.
• Example #2: Let’s say you need to complete another operation. You want to transfer a liquid from a constant level holding tank and pump into a constant level elevated tank at 500-gpm over the next 3-hours.
This system design also contains 70-ft of energy. In this example, the system curve is composed of:
60-ft of static head (Hs);
0-ft of pressure head (Hp); and
10-ft of friction (Hf) and velocity head (Hv) at 500 GPM.
The pump must overcome this energy. Let’s say the pump is designed to develop 70-ft of head at 500 GPM rotating at 1,750 RPM on a VSM. It could even be the exact same pump from the previous example.
At 90 percent speed on the VSM (1,575 RPM), the pump will only develop 56-ft of head. The system static head is 60-ft. This pump is deadheaded (0-flow). You’re pumping nothing, and the pump is under severe stress.
This pump is deadheaded at any speed less than 1,633 RPM, or 94 percent max. velocity because of the design of the system. If you want to complete this operation in three hours, you’ll need maximum speed from the beginning. Is this the same as a throttled control valve or soft start? No.
In this case, you cannot use the pump-with-VSM to perform the duties of a throttled control valve or a soft start. And there is no reason to purchase a VSM if the effective operating range is 98-percent to 100-percent maximum speed.
Strangely, the second example facetiously answers your original question. Starting a pump slowly with a VSM will act precisely like a closed valve. They will both deadhead the pump (0-flow), depending on the design of the system.
This is why I say that it depends on the design of the system and the profile of the system curve. Does this answer your question? Is it the answer you expected?
Larry (the Pump Guy”)
P.S. A pipe wrench is a mechanical tool. A screwdriver is also a mechanical tool. The mechanic must know the difference, and he must know when and where to apply each tool. For a successful dinner party, the hostess must understand when and where to use a steak knife and a serving spoon.
Likewise, pump curves and system curves are important tools for the engineers and technicians to resolve pump problems. These tools are as important to pump reliability as vibration analysis or laser alignment. You will graduate from the Pump Guy Seminar with a complete understanding of pump performance curves and dynamic system curves.
As you incorporate the curves into your reliability program, you’ll watch the unplanned pump maintenance disappear. Your pumps will behave. I guarantee it. Yes, I stole that line from the “Men’s Warehouse” commercial. Everything I know, I learned from someone else. We’ll be in Chicago in March and New Orleans in June. Come if you can … FlowControlNetwork.com/PumpGuy.
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 firstname.lastname@example.org.