Piping & Instrumentation Diagram (P&ID)

  Feature image: a stiction limit cycle from a simulated loop. The process variable traces a square-ish wave while the controller output ramps and the valve sticks, then jumps. ILLUSTRATIVE simulation. Control valve hunting is the sustained, unwanted movement of a control valve back and forth around the position the loop is asking for, with the process variable swinging around its setpoint in step. It is one of the most common complaints raised about a control loop, and one of the most often misdiagnosed. The instinctive reaction is to retune the PID (proportional-integral-derivative) controller. That is the right answer some of the time. The rest of the time the loop is hunting because of something the controller settings cannot reach: a sticking valve, backlash in a linkage, an unstable positioner, a valve that is too big for the service, or another loop pushing on the same process. This guide shows how to troubleshoot control valve hunting in a way that finds the real cause fir...