Control valve SE - How big an issue? Sub-standard control valve performance is probably the biggest contributor to process variability

Survival in today's competitive world markets depends on reducing cost and maximizing production. An active program to find and fix process control problems is a key ingredient. This paper has shown that the ability of the process control system to achieve optimum control is dependent to a large degree on valve selection. Oversizing the control valve is a common problem, which results in increased process variability and potential for limit cycling. Fitting the control valve characteristic to the hydraulic characteristic of the process system will make the control loop dynamics more linear and will enable a single set of tuning parameters to provide optimum control over the range of operating conditions. This is especially important in process systems with interactive control loops where the success of the tuning strategy depends on uniform control dynamics. Finally, minimizing the valve tracking non-linearities will reduce vulnerability to limit cycling and compromised control performance. It is important to realize that all of the loops in the case study system interact, and that deficient performance of any one of these can affect the process variability of the other loops. The first step in improving the performance of the process control system is to recognize the problems. There are some key indicators that signal something may be wrong with the control loop and costs are probably increasing. Is the loop in manual? Is the loop difficult to tune? Does the process variable cycle? Is the chemical consumption rate higher than expected? Is the process variable filtered to cover up a problem? A "Yes" answer to any of these questions signals a control problem. In many cases the control valve is the source of the problem, and the solution is simple and economical.