When a control loop is placed in automatic control, it is easy to detect if a valve or actuator is not responding correctly to the control system. As discussed in Chapter 3 of Advanced Control Foundation – Tools, Techniques and Application, this can be done by observing the response of the controlled parameter to control system changes in the PID output as illustrated below.
The most common problems in commissioning a control system can often be traced to a valve without a positioner, or to a positioner thathas not been properly installed or has malfunctioned. The rule of thumb is that to achieve best control performance, all regulating valves should be equipped with a properly functioning positioner. Without a positioner, the level of control performance that can be achieved is very limited when a valve is sticking—which is inherent in most valves.
The cyclic behavior caused by a sticky valve (with no positioner) or a malfunctioning positioner cannot be eliminated through tuning. Changes in tuning will only impact the period of the cycle. The only way to eliminate this type of behavior is to install a valve positioner and ensure that it is functioning properly