Specifying Control Valve Performance to Achieve Desired Process Performance

In the 1990’s, a study of over 5000 control loops revealed that 30% of the loops were INCREASING process variability due to poor valve performance. On a recent control foundation improvement project, over 50% of the loops had poor performance due to significant control valve performance issues. Poor control valve performance continues to be a widespread source of poor control loop performance. A control valve may be properly selected based on physical specifications, but its operational performance may not meet the requirements to achieve the required process performance. Properly specifying valve performance metrics such as dead band, resolution, process gain, step response time, and response regions are vital. Industry standard ISA/ANSI-75.25 provides definitions and test procedures for positioning and response time performance of control valve but does not provide guidance on selecting appropriate control valve performance specifications for a specific application. This presentation explains the impact of valve performance parameters and how to specify them based to achieve the desired process performance. The definition and importance of these performance metrics will be explained, and field examples will be provided. The metrics can also be used to diagnose and improve existing control valve performance when it does not meet the required control loop performance. The benefits include longer valve life, reduced process variability and improved process performance.