Why do we care? We should all be interested in saving energy whenever possible. However, energy saving cannot deprecate control loop performance. However, by using the VFD as the final control element of a process control loop, we get both significant energy savings and improved process control loop performance.
How to Overcome Tradition
VFD suppliers should recognize that use in process control loops is potentially a huge market opportunity. The VFD is a threat to the control valve market for liquid flow control. However, the threat is only potential unless the VFD vendors recognize process control as a market and treat it as such. This means that they must prepare and execute a marketing strategy to sell the VFD to the user market—process control engineers at plants and engineering contractors. They must be willing to support the VFD product as well as the control valve suppliers support their products: with application data sheets, maintenance training, expedited spare parts delivery, etc. However, this will not happen unless process control users take the lead and begin to request quotations on VFDs.
These articles have indicated that using the VFD is a "safe alternative" to the process control valve, but users have yet to be convinced, even when they know about the benefits. Most often cited as a risk to using the VFD is reliability. Many users have memories of VFD failures for past applications, usually outside of process control. Yes, the VFD is not a new technology, and many failures of the past were associated with the use of AC synchronous motors with wound rotors and brushes for transmission of power to that rotating part. Early VFDs used thyristors to implement the electrical switching required to regenerate an AC current at variable frequencies. Thyristors were not reliable enough for this service, and have subsequently been replaced by the insulated gate bipolar transistor (IGBT.)
IGBTs are usually used in parallel within the VFD to achieve the required power levels and to provide some degree of redundancy. Modern VFDs are designed to continue to operate with a number of failed IGBTs to give high mean time between failure (MTBF) ratings. Maintenance requirements usually dictate that replacement of failed IGBTs can be made quickly and easily to achieve low mean time to repair (MTTR.) As a result, modern VFDs have achieved very high reliability and service life records within those industries that traditionally have a need for variable speeds, such as metal rolling mills, paper-making machines and conveyors.
Meanwhile, process control valves have been improved only with the availability of digital (usually called "smart") positioners, but the dominant positioning mechanism remains a pneumatic motor. The pneumatic motor, usually a large diaphragm, uses air pressure to create a linear or rotary motion to move the plug of a control valve. The positioner is a device that uses a controller output signal to regulate high-pressure air applied to the pneumatic motor and a mechanical feedback of the valve's position to control that position. The controller output may be pneumatic (3 psi to 15 psi), analog electronic (4-20mA) or digital (fieldbus.)
Control valves themselves are always a movable plug operating on a mechanical valve seat. The relationship between the control valve position and the resulting flow rate is highly non-linear. A valve seal is required to keep the process fluid inside the valve from leaking through the valve stem to the atmosphere. Over the years, manufacturers of process control valves have evolved their products to become reliable devices suitable for use in process control, and they effectively market these devices with many user-oriented tools and services. However, control valves usually develop "stickiness" as the process fluids are deposited on the valve stem or shaft. Cavitation of the process fluid causes wear of the valve seat, and many times, fugitive emissions occur through the stem/shaft packing. Mechanical cams or software in the valve positioner can partially compensate for valve non-linearity. The literature is full of case histories tracing bad control loop performance to problems with the control valve.
Clearly, if there were no traditional use of control valves, the greater simplicity and reduced energy of the VFD/AC induction motor/centrifugal pump method would be preferred over the control valve/positioner/valve motor/centrifugal pump method. But, tradition remains a strong driving force. Users can have these advantages, but they will need to ask for them from their VFD suppliers.