Have you seen a loop more oscillatory in automatic than in manual? Is the problem loop interaction, aggressive controller tuning, or a sticky-sloppy valve? If you have a digital positioner with position readback on a throttle valve supplied by a control valve manufacturer, you have a chance of figuring out if the valve is the culprit. If you don’t have digital positioner and a real control valve, the valve probably is the problem.
For rotary valves, the position feedback measurement must be of stem rather than shaft position for the readback. Block or on-off valves provided by piping valve manufacturers often have position feedback on the actuator shaft. Even when the position of the stem is measured, the high friction of the rotary element (ball, plug, or disk) seal may cause shaft windup to the extent where even the stem position is not representative of the internal element position. I have seen this big time for a ball valve posing as a control valve in phosphorous service. Even for a new valve on the bench, the ball did not move for a 10% change in stem position. In these cases, the position feedback and readback are lying and the diagnostics from a smart positioner are misleading.
In the 1980s the slapping of piston actuators and positioners on piping valves became rampant. Since they were less expensive than a control valve, they were already in the piping spec, and the ISA standard for valve response testing had not been published, the process engineers were easy targets. After all, the piping valve worked well as a block and isolation valve, the leakage specs were impressive, and pluggage was often less of problem. Plus you could always blame process variability on mysterious causes since no one knew what the valve was really doing. Position readback was rare since it required a separate position transmitter and wiring. Valve tests by technicians in the field did not reveal a problem because 25% or 50% changes in signals were used. All but the biggest valves or dampers looked decent for such large changes in valve signals. The idea that enormous controller output changes (e.g. 25 to 50% per sec) did not occur except for special situations, such as surge control, didn’t seem to cross most minds.
In the next two weeks we will get into the importance of readback in actual control valve scenarios. In the mean time if you want more information on the effect of control valves on loop performance, check out the following articles: