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A recent ControlGlobal.com article explored the energy savings of variable frequency drive (VFD)-based centrifugal pumps versus control valves in flow control applications.The article, Eliminating the Control Valve sums its position in the subtitle, Valves AreEnergy Hogs; They Stick; Theyre Prone to Mechanical Failure. So Why the Reluctance

to Replace Them With VFDs?

The comparison is between controlling the rate of flow through a pipe by adjusting thespeed of the centrifugal pump versus using a control valve to throttle the flow ratedownstream of a fixed speed pump. As the articles author, Dick Caro, hi ghlights, thereare clear energy savings possible:

We know that flow rate is linear with centrifugal pump rotational speed from the pumpaffinity laws We also know that the electric power needed to drive the pump at lower

speeds to produce the reduced head then required is proportional to the cube of thespeed ratio. Therefore, by removing the control valve completely from the flow path, theamount of energy saved is much larger than one would expect.

Beyond the energy savings, he highlights improved control response, elimination of asource of fugitive emissions from the control valve, and electrical energy savings fromimproved power factor . Barriers include tradition, different buyer within plant, history of unreliability from VFD components, existing pump motors not designed for VFD, andnever considered VFDs as an alternative in flow control applications.

I turned to Emersons George Gassman , a senior principal engineer with the Fisher Valve & Instruments team. He generally agrees with the assessment of energy savingsand improved control offered by VFDs.

When looking at a flow control application, George highlighted some issues that need to be part of a plant engineers consideration set. The first is flow shutoff. Centrifugalpumps need an additional valve to guarantee tight shutoff. Also, a flow check or anautomated block valve may be required to prevent back flow in many applications.

The location and available room where the electrical power equipment needs to beexamined. The VFD inverters take up valuable floor space and large copper wires ratedfor the electrical current need to be run from the pump to the inverter, and from theinverter to the electrical supply main. And, unlike control valves with explosion proof enclosures or intrinsically safe circuits, these VFDs are not rated for and cannot belocated in hazardous areas.

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Another issue to consider is the electrical harmonics that reflect back on the power grid.The effects of heating and vibration on other plant equipment, caused by inverter noise,can be a problem, especially in applications such as offshore oil and gas platforms whereoverall power generation capability is limited. Isolation transformers are sometimes

required to dampen the adverse harmonics to keep them from affecting other plantequipment.

George pointed out to me that some throttling needs are better met using the dissipationtechnique with the control valve. For example, where the upstream head pressurealready exists, such as boiler steam pressure, gas storage pressure, turbine speedcontrol, or turbine bypass these are the prime candidates for control valves.

He also shared that the plants that he has visited are familiar with VFDs and that they

tend to use them where they feel comfo rtable with the application. Its possible thatmost of the easy applications for VFD pumps have been converted by now.

The more sophisticated applications that require a significant amount of plant re-engineering, safety review, and liability analysis must be weighed against the energy savings and improved controllability. The hurdle from this analysis is often notovercome and as a result, VFDs have found their way into few critical throttlingapplications. VFDs have been used for re-circulation flow on some boilers.

To get the full benefit of VFD for throttling service may require process (and plant)redesign. The critical processes in todays plant are designed around pressure reduction(i.e. from the rail down) where a high-pressure reservoir is available and used, through aseries of intermediate pressure reductions, to drive the process to acceptable throughputand efficiency.

The capital investment in piping and process technology focused on changing thecurrent pressure reduction design can be considerable. In contrasting the VFD economic benefits, the power expenditure is based on raising pressure from a lower level to a

higher one (i.e. from the ground up), to achieve acceptable throughput and betterefficiency. Much of the valve piping and a lot of the process design may need to bereworked to make the VFD economic benefits overcome the economic benefits of theplants designed around the control valve.