by Mark T. Coughran ^[*]

Control valve response strongly affects the performance of many process control loops. For throttling control, key performance issues are (1) the ability of the control valve to respond consistently to small input changes and (2) the installed loop gain resulting from the match of the valve to the other loop components. The definition of small and the required valve gain depend on the process and other loop components, as shown in Fig. 1. Optimal selection of the control valve requires knowledge of the control objectives of the loop; static characteristics of the process; dynamics of the process, transmitter, and controller sampling; controller tuning practices; source of the disturbances (load or set point); and the frequency distribution of the disturbances. Specification of control performance is distinct from the mechanical configuration requirements typically seen in purchasing documents, such as materials compatibility, seat leakage, pressure class, and end connections. However, the effect of the control valve on process variability should be weighed equally with these other factors, since it may dominate the life cycle cost of the control valve.

FIGURE 1: Most control valves are used in closed loop control systems. The control valve performance requirements and the appropriate open loop test methods depend on the other components of the loop.

In linear mechanical systems, errors between the percentage of input and the percentage of output vary with the history of the input in a predictable manner. Control valves, on the other hand, exhibit...