Can I Use a Drive to Control Flow?

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I'm hearing about using a drive with my pump and motor setup for better flow control instead of control valves. Is it worth it? Do I still need some measure of flow control besides a shut-off valve?

,"from July 2003 Chemical Processing

Good points and bad points
I think a VFD control can offer better efficiency, but diminished control accuracy, response time and shut-off performance. It is not as reliable as a regular control valve scheme. If we need efficiency and performance, we can consider using a drive system as a core control and a valve (maybe a ball valve) as a fine control. The valve should always be 90% open to keep the throttling loss down. The valve can also be used for shut-off purposes to improve the response time and leakage performance.
Siu Ki So, Plant Technician
Air Products, sosk@apci.com

Need both for key applications
If the fluid being pumped is a critical fluid, a key raw material process intermediate with flow parameters that directly affect product quality or safety of the process, I would say that controlling the fluid flow via both VFD control and a flow-control valve/loop makes sense (for redundancy protection).

In my estimation, the costs associated with a VFD are quite a bit less. We use VFDs to control many of our processes here, and they have shown themselves to be reliable.
David Todd, Process/Project Engineer
Norac Inc., Helena, Ark.

Not for everywhere, yet
We have experience using the variable frequency drives to control the combustion air flow of Boilers FD fans and cooling tower water supply pumps. They all worked fine. We are also aware that one of our clients is using variable-speed drives for ground well pumps without any complaints.

However, we advise cautious review of lower range conditions for case-by-case application, since the discharge pressure also goes down faster than flow when we try to control the flow. We still do not feel comfortable and confident using variable speed drives to control flow in critical areas like boiler feed water pumps.
Ven Venkatesan, Director of Process Engineering
Armstrong Service Inc., Orlando, Fla.

Holds reflux pump prime
We have been quite pleased with VFDs replacing control valves in reflux pump service. Control is excellent, a source of leakage is eliminated and, during upsets the reflux pumps do not lose prime, an important safety consideration. In services pumping against lower heads, once flow is established, siphoning can set a minimum flow rate. If you need to hold lower flows then you have to put up with control valves.

Substituting VFDs for control valves has other advantages: energy is conserved, the power factor is improved, pump design is simpler, impeller sizing is more uniform, seal life is longer, and smaller installations cost less.

VFDs are not as robust as electric motors during power flickers and lightning strikes, so isolation transformers are desirable and sparing policy needs to be reviewed. No problem with harmonics has ever come to our attention, but much has been written on the subject. The short distances we have between pumps and VFDs are thought to minimize this problem.

Control is excellent as long as siphoning is avoided and the suction pressure never exceeds the discharge pressure. If it does, control is lost. For this reason, we use VFDs to control reflux pumps, but not bottoms or tank transfer pumps.

VFDs are so much of an improvement that we would no longer take a reflux control valve if it were given to us.
Andy Schwartz
Keeshan & Bost Chemical Co., Manvel, Texas

Holds pressure setpoints
For the system I have described, there are no control valves used other than shut-off valves. One significant advantage to using a variable-speed controller is that the power consumed by the pump is typically less for a unit operating at a reduced speed, as compared to a 60 Hz operating speed with a control valve. A flow control valve is turning a lot of energy into wasted heat. The additional cost of a VFD is often recaptured in a very short time from the reduced power costs and simplified operation.
Robert de Long, Manager of Downhole Pump Engineering
Baker Hughes Centrilift, Claremore, Okla.

Might cost more, but worth it
A VFD to control flow may require more capital than a control valve and a normal motor on your pump. However, it does save energy instead of burning the pump energy across the control valve. Additionally, a control valve and its associated problems with leakage and sticking stem are eliminated. The control parts are all electronic and not wetted, except for the pump. This is especially important in handling corrosive materials. Furthermore, because wear is related to some higher power of speed, the bearings and seal on a pump driven at less than its normal speed should last longer.

Of course, there is no free lunch. There are safety considerations for your particular process. The price you pay is that there is no emergency shut-off. In a power failure, the pump stops pumping. You may need to automate a block valve if you require positive shut-off.
Jim Burton, Director of Development
Cincinnati Specialties, Cincinnati

Mind the harmonics
There is another factor to consider. If a facility is considering this replacement for multiple large AC drive circuits, the power distribution system should be evaluated for the possible detrimental effects of too much harmonic distortion. VFDs are known to cause changes in the sine waveform because of the way power electronics draw current, and these changes (distortions) are known to occur in integer multiples of the electrical frequency (or harmonics). In a typical three-phase system, if the phases are balanced, there is no (or not much) current load on the neutral. The addition of lighting circuits and electronic drives add harmonic distortion which, if the system impedance is high enough and the power is distorted enough, can affect other equipment,"especially electronic equipment, including computer systems, electronic instruments, and even VFDs themselves. It is known that feedback due to harmonics can be additive on the neutral, causing significant current where there shouldn't be any, and problems are sometimes seen, such as breakers that trip when the measured power draw doesn't exceed their setpoint, premature motor failures, and transient effects that can be very difficult to troubleshoot. The secondary cost of filters and other apparatus to clean up the power to distortion-sensitive equipment on the same power system as the VFDs may need to be considered, especially if the VFD(s) power consumption is a significant percentage of total usage.

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