The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #97.

There are many opportunities for process optimization by valve position control (VPC). Table 1 (below) summarizes common examples of the use of VPC for increasing process efficiency and capacity. Efficiency is increased by reducing energy and raw material costs for a given production rate. Capacity is increased by increasing feed rates for a given efficiency. Table 2 is a summary of key PID features that make the implementation of VPC easier and more effective at achieving process objectives and dealing with process disturbances.    

Table 1 - Examples of Optimization by Valve Position Control

Concept: Valve position control (VPC) can be quickly implemented for small process optimization opportunities. An integral-only structure has traditionally been used for VPC to eliminate interaction. However, an integral-only controller is difficult to tune, can get into limit cycles, and can be too slow to prevent running out of valve; that is, a valve is on the flat part of its installed characteristic and is close to an output limit. The key PID features shown in Table 2 enable the use of PI control.

Table 2 - Key PID Features for Valve Position Control

Details: Use valve position control on valves that are the constraints to improving the capacity or efficiency of a unit operation. For loops that manipulate utility flows, use a maximum throttle position for the VPC setpoint. For expensive fuels and reagents, use a minimum throttle position for the VPC setpoint. To increase efficiency, use the VPC output to maximize a less expensive or waste fuel or reagent, minimize a prime mover (e.g., compressor or pump) discharge pressure, or maximize a chiller or cooling tower outlet temperature. To increase capacity, use the VPC output to maximize a feed rate to a fedbatch or continuous operation. Use an enhanced PID with a threshold limit to ignore inconsequential changes in valve position and to stop limit cycles.

Use directional setpoint rate limits with external-reset feedback (Tip #72) to reduce interactions and provide a slow approach to an optimum and a fast getaway; movement away from the optimum for disturbances; to prevent running out of valve (valve close to output limit or on flat part of installed characteristic). Use a feedforward summer in the VPC to provide preemptive action. Use adaptive tuning to help the VPC deal with nonlinearities and changing objectives. Review the application with the process engineers. Train Operations in the use of the VPC. Work in the control room with the operators to improve tuning and make sure the VPC does not cause any operational problems.

Watch-outs: Just one abnormal condition, one bad batch, or one increase in off-spec will likely prevent the VPC from ever being used again. The VPC should start with very narrow output limits to restrict what the VPC can do. Only after the VPC is tuned and confidence is gained should the output limits be widened. The VPC will have to be retuned to deal with nonlinearities as the limits are widened. If you do not have external reset feedback (e.g., dynamic reset limit), you may have to use an integral-only structure for the VPC to prevent interaction.

Exceptions: If unmeasured disturbances are too fast and disruptive, VPC is not feasible. If the optimum setpoint is a function of operating conditions rather than simply valve position, model predictive control is a better optimization solution.

Insight: Valve position control can be implemented in a matter of hours to start optimizing the efficiency or capacity of a unit operation.

Rule of Thumb: Use valve position control with key PID features to maximize production rate and to minimize energy use and raw material costs for a unit operation.

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

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Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.

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