Larry Lyons, P.E., PMP, Plant Engineer
Continental Carbon Co., Sunray, Texas
Harmonics and overheating
VFDs are reliable and energy efficient. You need to make sure your wiring and motor are VF ready (voltages can be higher and harmonics can impact motor life.) Also, if the anticipated speed is too low, motor cooling can be a problem (the fan runs too slow to move enough air), so auxiliary air moving can be a problem, especially on larger motors.
Overall, we have found using VFDs for flow or pressure control to be very effective and they save money both in power costs and in maintenance.
Michael Mattox, Operations Manager
Atofina Chemicals Inc., Blooming Prairie, Minn.
Watch out for low flows
A drives solution is no different than controlling the speed of a steam turbine in order to regulate flow from a compressor. It's just becoming more common, with advances in electronics and with the increased availability of variable speed drives and motors for this service. Things to be aware of:
1) The drive may have a minimum speed, so don't look for it to control well at extremely low flows.
2) Pumps with long shafts (especially vertical pumps) can have a natural (critical) frequency that the variable-speed drive could allow the pump to run at. This will lead to a number of reliability problems in these pumps.
3) If a dual-gas seal is used, it has a minimum speed that it needs to run at in order for the seal faces to lift off.' This minimum speed will depend on seal size and design, but will be on the order of a few hundred rpm.
Van Richard, P.E., CMRP, Sr. Reliability Engineer
Georgia Gulf Corp., Plaquemine, La.
Many advantages with VFD
Much of whether to use a control valve vs. an AC VFD has to do with what the product is, the type of pump, and the entire piping scheme. The advantages of the VFD are energy savings, maintenance, information (feedback), and future control flexibility, if the rest of the system changes. Some people would argue cost savings as well but that may well depend on sizing, etc.
Typically, I prefer the VFD approach over control valves, but they have limitations such as ambient conditions, etc., that must be taken into account. In most cases, I feel it is definitely "worth it", and you should not need any other method of flow control unless you are feeding a multiple piping loop system such as chilled water to multiple HVAC units or heat exchangers, etc. If you are going to use a VFD in a remote location, it can be an advantage because most will be able to provide the localized PID control for closed-loop performance without having to purchase other control equipment or long cable runs from a PLC.
Kurt Lemman, Maintenance Engineering Manager
Schreiber Foods Inc., Tempe, Ariz.
We use VFDs for flow control as much as possible. A pump pumping against a valve wears both of them out. Make sure that the motor is for inverter duty and install the drive in a "nice" place. Under conduit entries or HVAC ducts are not nice places. We have between 100-200 drives installed and go weeks without a drive problem. Drives are intelligent and some can be viewed or downloaded from any Ethernet-networked computer. They can act as remote I/O and give percent load and Hz, and be started and stopped with only a communication cable from a PLC, which lowers installation cost. They have stall and overload protection built in.
Bill Potter, Control Equipment Specialist
PCS Phosphate, Aurora, N.C.
Beats buying stainless
A steam turbine or a variable-speed electric drive/motor will work very well for flow control. Applications to centrifugal pumps and positive displacement pumps can be successful. Since the power required for a centrifugal pump varies with the cube of RPM, overheating at low RPM (diminished cooling fan performance) is usually not a problem. We usually include a low limit on the speed to prevent motor overheating that might occur if the pump is run near stall conditions. Constant-torque loads need to be evaluated more carefully. For modest-sized 480 V motors, the installed cost of a drive is usually less than the cost of a control valve when stainless steel valves are required. If the pump (not the drive) is in an explosion hazard area, pay careful attention to heating and make sure the NEC or applicable code requirements are met.
Donald Wheeler, P.E., Principal Engineer
Rhodia Inc., Baton Rouge, La.
Back in the day
Allow me to refer to a 50-year-old technique I used that performed extremely well for the control of feed flow to large filters. It consisted of a U.S. Vari-Drive with a Conoflow air operator responding from a mag flowmeter. The Vari-Drive consisted of the constant-speed motor driving two variable-speed sheaves positioned by the pneumatic operator. Crude and simple, but it did an excellent job. Certainly the precision was not as exacting as a solid-state electronic motor control system, but those were the good old days.
Also in those ancient days in the paper industry, we used DC motors driven by Thyratron tubes for speed control of paper rewinders, in which role diameter was constantly changing although sheet feed was constant. Of course, the thyratrons are now replaced by solid-state electronic output signals. They too worked very well.
Ah, those good old days when simplicity prevailed.
Don Westermann, P.E., Chemical Engineer
Chemical Process Corp., Edmonds, Wash.
Works great. Less money
The big advantage to controlling flow with a drive instead of a control valve is not improved control, but energy savings because you use only as much horsepower as you need instead of burning excess across the valve.
Chad Schaffer, P.E., Instrumentation & Controls Engineer
Burns & McDonnell, Kansas City, Mo.
We do it without drives
Our process deals with a chemical that crystallizes out easily when the temperature is reduced or when flow is restricted. We tried control valves, but the restriction in the pipe caused more crystallization and made control very unreliable or uncontrollable. The solution was to use the same pumps we've been using to give a constant pressure, but now to regulate the pressure they operate at, thus controlling the flow. The final setup for liquid control has worked very well in a PLC-based system. An electromagnetic actuator controls the pressure at which the pneumatic diaphragm pumps to pump the liquid through the system and a nonintrusive magmeter monitors the flow.
Brad Stanley, Chemical Engineer
Purafil Inc., Doraville, Ga.