by James F. Beall IV ^[*]

The effect of process measurements, control valve performance, and controller tuning can have a significant impact on process control. In some audits [1], improving the performance of the control valve and its related components (I/P, actuator, positioner, etc.) significantly decreased the variability of over 50% of the control loops. Case studies presented in this section show that, of the control loops studied, up to 75% need improvements on the instrumentation or control valves in order to meet process control requirements. In most cases, improvements to the instrumentation and control valves must be done before better controller tuning will improve the process control.

A plant program to analyze and correct problems with process measurement instrumentation, control valve response, and controller tuning can significantly reduce process variability and increase process availability. Reductionsin process variability of individual control loops 10:1 or more are possible in some cases. This, in turn, can result in a significant reduction in the variability of the process unit (e.g., the analysis of an intermediate or final product).

The economic benefit from these improvements is significant. One benchmark study [2] found that the average improvement of 12 companies with these typesof improvements accounted for 25% of the economic benefit from all categories of process control improvements. In addition, good basic control is required for obtaining the potential benefits from advanced control technologies.