In the first of these articles, I introduced the technology of the VFD—an established device that is capable of creating a type of alternating current (AC) that can be used to power AC induction electric motors used to turn centrifugal pumps. Since the VFD generates AC at different frequencies, the induction motor rotates at speeds proportional to the frequency.
In that article, I pointed out that the laws of physics relating to centrifugal pumps, called the affinity laws, show that pump rotation speed and the flow rate of the pumped fluid are directly proportional, whereas the power required varies as the cube of the speed and the square of the pressure head. For a process control flow loop, this means that the output of a PID loop controller can be directly applied to the speed setpoint of the VFD in order to control flow. Of course, the PID output classically has been connected to the position setpoint of a process control valve.
The clear advantage of using the VFD to control pump speed, and therefore, the flow rate, is that it is no longer required to pump with enough head to overcome the pressure drop across the control valve. The energy savings caused by the reduced pump pressure head is significant because of the pump affinity law. In addition, as long as the desired flow rate remains within the speed control range of the VFD with a particular pump, the control loop response will be much faster than can be obtained using an electromechanical control valve. The few users who have actually tried this configuration have reported large energy savings and adequate control loop performance. Unfortunately, there have been no experiments reported to measure the actual performance of such a control loop where the VFD provides the final control element.
Don't Ask, Don't Tell?
In my market research for a VFD supplier, I found that users were generally not aware of the option to use a VFD as the final control element for a process control loop. Use of VFD is rarely if ever specified, except in water and waste applications, where a control valve would be exceedingly expensive because of its size. As it turns out, this is no surprise, since the VFD manufacturers generally do not market their products to process control users, which would require that they supply users with an application data sheet for the use of VFDs as the final control element of a process control loop. Users generally learn about the equipment they specify in their plants from the college classroom, data sheets supplied by their vendors and from direct sales efforts of suppliers. College textbooks, as well as ISA's own textbooks on process control, currently do not show the use of a VFD as the final control element for a control loop. Without the informative and educational
application data sheets, it continues to be unlikely for a user to specify the use of VFD as the final control element of any process control loop.
The suppliers of VFDs tell me that they do not market the VFD as a final control element because users do not ask about it. The supply chain for equipment in most process industry plants is usually divided into these categories:
- Process equipment (tanks, shut-off valves, piping, pumps, etc.)
- Electrical equipment (motors, transformers, drives, motor control centers, switchgear, etc.)
- Instrumentation and control systems (field instruments, control valves, DCSs, PLCs, etc.)
The organization of the end user's company and its engineering contractors usually separate purchasing, engineering and maintenance along these same lines as well. The result is that VFD suppliers sell to the electrical equipment interests, but not to the control systems interests. While the process industries buyers think of VFD in terms of replacement for DC and AC synchronous motors, they really don't know much about their use in process control systems. Process engineers and control systems engineers generally are unfamiliar with VFDs and their applications. This series of articles published in a process control-oriented publication hopes to change this situation and make process control engineers aware that the VFD/AC induction motor/centrifugal pump is an energy-saving and accurate alternative to the more traditional control valve.
However, not all control engineers read this publication. It seems that the VFD manufacturers would recognize the potential market for VFDs in process control. In fact, most of the VFD manufacturers have been contacted in my survey and are at least somewhat aware that VFDs can be used as the final control element of a process control loop. However, they are currently not trying to develop this market that seems to be "out of their comfort zone." They are not willing to staff their marketing effort with the necessary people to support bids into process control applications, or to create the sales collateral materials (application data sheets) necessary to support active sales into process control applications. Even companies who make both VFDs and process control systems are not willing to cooperate to offer the VFD as an alternative to the process control valve that they often cannot supply themselves. When the VFD drives vendors were asked, they simply state that there is no market for the VFD in process control systems. Likewise, when control systems suppliers were asked if they would quote VFDs as final control elements with their process control systems, they prefer to respond to the user RFQ that specifies only control valves.
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.
