The water and waste treatment industry is the only one that routinely uses the VFD as a final control element for new installations. The very large sizes of pumps used in water and waste often means using very large control valves. New installations have saved considerable capital cost by installing VFD-driven centrifugal pumps with the motor speed tied to the flow controller output. Not only do these save capital by avoiding expensive very large control valves, but the pumps are also smaller, since they no longer need to generate the head loss that would have been necessary to provide the pressure drop across the control valve. On the other hand, control accuracy and process optimization is not often an issue in water and waste applications.
In each case where the VFD was actually connected to the loop controller, the interface used was the 4-20 mA speed input option offered on most VFDs. The VFDs are also available with Ethernet network interfaces that often support one of the high-level protocols, such as Modbus/TCP, EtherNet/IP or Profinet, in which the speed setpoint attribute can be changed. However, no drives are currently available with either an OPC server or Foundation fieldbus interface. Additionally, while most VFDs offer a PID algorithm and ramping function blocks, none of the companies using VFD as final control elements have taken advantage of these capabilities.
Similar to the way that control valves can be eliminated, large dampers installed in fossil-fuel-powered boilers also can be eliminated by using a VFD to drive the fan motors on forced draft power boilers. The primary control of power boilers for the generation of steam is the fuel flow rate. The fuel/air ratio controller, which usually positions the dampers to control the ratio is slaved to that fuel flow. Elimination of the dampers would save capital expense on new installations, reduce the cost of power by elimination of the pressure drop across the dampers, and possibly increase the capacity of the boiler by using the increased forced air fan capacity to operate at higher fuel flows.
In the time since Part 1 was published, new information has become available on the VFD topic. A manufacturer of industrial wiring says that it is necessary to shield electrical cables and to use electrical cables that have been designed for use with VFDs. (Shuman, "Choosing the Right Cable for your Variable Frequency Drive (VFD) System," www.belden.com/pdfs/techpprs/VFD_Choosing_WP.pdf, Belden, Inc.) Failure to install the right cables can lead to many of the problems that have been attributed to the use of VFDs in the past. The problems of electric motor failure due to harmonics and excessive electrical noise are often the result of replacing a motor starter or motor control center (MCC) with a VFD and reusing the existing power wiring. When installing a VFD to replace a motor starter or MCC, new wiring must also be installed.
The readers of Control and Motors and Drives magazines have responded to a survey related to the use of VFDs as final control elements replacing control valves. The results have been remarkably consistent with the informal interviews I have previously conducted with process control end users. In general, process control engineers are unaware that they can use a VFD driving the process pump to replace a control valve.
One strategy that might encourage end users to adopt the VFD more readily is if drive suppliers were imitate control valve/positioner suppliers, and offer fieldbus instrumentation users function blocks configured in the same way as control valve/positioner suppliers do. The trend in most new large petrochemical plants and petroleum refineries is the use of Foundation fieldbus instrumentation that is usually configured with the final control function block (PID) physically located in the control valve positioner.
At this time, none of the drive suppliers offers a Foundation fieldbus interface, either H1 or HSE, or conforming function blocks.
If drive suppliers were to offer Foundation fieldbus function blocks in their drives, as well as a suitable Foundation fieldbus interface, the use of VFDs could be more comparable to control valves.
In these articles, I have demonstrated that there is a distinct economic advantage to using a VFD on the electric motor turning a centrifugal pump in place of a constant-speed electric motor turning the centrifugal pump with the control valve/positioner combination for liquid flow loops. The resulting energy savings is quite large, but the potential savings in improved process control from improved yield, lower waste and more consistent product purity realized by operation closer to the optimal setpoint may actually be even larger. Failure to take advantage of the benefits of using the VFD and eliminating control valves when feasible is usually traceable to the lack of information and support from the suppliers of VFDs and to a lack of education on the place of the VFD in process control technology.
Dick Caro is a principal at CMC Associates, Acton, Mass.