The scoop-tube actuators on a plant's feed-water pumps and forced/induced draft fans were Rotork units, and one problem kept recurring: at one particular valve opening, the actuator hunted (oscillated) frequently, disturbing feed-water automatic control and unit load. This "hunts only near one opening" fault is very common, and the troubleshooting approach is worth standardizing.
Transfer the valve control to local and watch:
- Hunting disappears — the problem is in the command loop: most likely the 4-20 mA command signal is disturbed and the DCS output itself is jittering;
- Hunting continues — the problem is on the actuator or the mechanical side; move on to dead band and mechanics.
A handy rule: if the fault occurs only at one specific opening while automatic control works fine at all others, interference is unlikely — mechanical sticking and dead-band settings become the prime suspects.
Oscillation comes from overshoot. For regular periodic hunting, increase the dead band (not decrease it) — a wider dead band makes the actuator less sensitive to small deviations and the oscillation decays. The price is some control accuracy, and field work must balance the two.
Real case: at one plant, widening the dead band eliminated the hunting. Rotork actuators carry the FD (dead band) parameter in the second-level menu — try increasing it step by step. If hunting persists with a wider dead band, suspect the proportional control board.
One plant's case is a classic: feed-pump actuator hunting, first suspected the actuator and "conservatively" replaced the whole unit — the same problem reappeared within days. Later, when high oil temperature forced a teardown of the feed pump, they discovered poor engagement between the linkage and the hydraulic coupling — the real root cause was in the mechanical connection, and the actuator had been wrongly blamed all along. Also remember to test both cold and hot — some sticking only appears when hot.
Where available, hook up a disturbance recorder to view command and feedback waveforms: internal vs. external causes become obvious immediately, much faster than trial and error.
For "hunts only at one opening," the mnemonic is: switch to local, widen the dead band, check the mechanics. Actuator hunting does not mean a broken actuator — replacing the actuator is the most expensive form of guessing. Separate internal from external causes with signal observation and mechanical checks first, then decide which layer to touch.
