Flow Control with VFD Pumps versus Control Valves

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 Are Energy Hogs; They Stick; They’re 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 the speed of the centrifugal pump versus using a control valve to throttle the flow rate downstream of a fixed speed pump. As the article’s author, Dick Caro, highlights, there are clear energy savings possible:

We know that flow rate is linear with centrifugal pump rotational speed from the pump affinity 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 the speed ratio. Therefore, by removing the control valve completely from the flow path, the amount of energy saved is much larger than one would expect.

Beyond the energy savings, he highlights improved control response, elimination of a source of fugitive emissions from the control valve, and electrical energy savings from improved power factor. Barriers include tradition, different buyer within plant, history of unreliability from VFD components, existing pump motors not designed for VFD, and never considered VFDs as an alternative in flow control applications.

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

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

The location and available room where the electrical power equipment needs to be examined. The VFD inverters take up valuable floor space and large copper wires rated for the electrical current need to be run from the pump to the inverter, and from the inverter 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 be located in hazardous areas.

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 where overall power generation capability is limited. Isolation transformers are sometimes required to dampen the adverse harmonics to keep them from affecting other plant equipment.

George pointed out to me that some throttling needs are better met using the dissipation technique with the control valve. For example, where the upstream head pressure already exists, such as boiler steam pressure, gas storage pressure, turbine speed control, 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 comfortable with the application. It’s possible that most 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 not overcome and as a result, VFDs have found their way into few critical throttling applications. 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 today’s plant are designed around pressure reduction (i.e. from the rail down) where a high-pressure reservoir is available and used, through a series of intermediate pressure reductions, to drive the process to acceptable throughput and efficiency.

The capital investment in piping and process technology focused on changing the current 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 better efficiency. Much of the valve piping and a lot of the process design may need to be reworked to make the VFD economic benefits overcome the economic benefits of the plants designed around the control valve.

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