Commissioning Control Systems – Valve Performance

As addressed in my July 18th blog, when a valve positioner is installed on a valve actuator, as long as it is functioning properly it will automatically increase the actuator pressure until the valve moves to a position that matches that requested by the control system. Feedback of the stem position is used by the positioner to sense if the valve has responded. Thus, it is possible for the valve position to eliminate overshoot or offset. When a control loop is placed in automatic control, it is easy to detect if a valve is not responding to the control system by observing the response of the controlled parameter to control system changes in the PID output. An example of the response that may be observed with a sticky valve is shown below.

As this example illustrate, in the absence of a valve positioner a significant change in the setpoint may cause a change in the control system output, as shown by the PID output, that is large enough to generate the actuator pressure needed to move the valve stem but result in the valve overshooting the target position and introduce cycling in the control. Small output changes made by the control system for small deviations in the controlled parameter from setpoint must accumulate before enough force is developed to move the valve. When the valve moves in response to accumulated changes in PID output, then a large change in the controlled
parameter may be observed. The change in the control system output in response to this large change in the controlled parameter may be enough to again cause the valve to move. Thus, a sustained, characteristic cyclic pattern will develop in the controlled parameter and the manipulated parameter. The amplitude and period of the cyclic pattern will vary depending on the controller tuning and the force needed to cause the valve to move. As address in some detail in Chapter 12 of Control Loop Foundation – Batch and Continuous Processes, the cyclic behavior caused by a sticky valve cannot be eliminated through tuning. Changes in tuning will only impact the period of the cycle that develops. The only way to eliminate this type of behavior is to install a valve positioner.

Cyclic behavior in automatic control may also be observed if deadband is introduced by a mechanical linkage between the actuator and the final control element. However, an important difference is that the cyclic pattern caused by deadband may die down over time. Deadband is characterized by the final control element responding in a linear fashion to small control system output changes that are made in one direction. However, when changes are made in the opposite direction, the final control element will respond only after a sufficient amount of change has been made in this new direction. This may extend the time required to get to setpoint or for the PID to compensate for changes in disturbance inputs. By examining the mechanical linkage between the actuator and the final control element, it is often possible to identify and correct the source of deadband. For example, wear in the pins that connect linkage arms is often the cause of deadband and can easily be corrected.