TL;DR
Hunting on a control valve is usually a PID gain too high or stem stiction creating a deadband. Test stem movement with the positioner in manual, then re-tune the PID loop.
What you might see
- stem moving back and forth repeatedly
- process variable oscillating around setpoint
- positioner signal cycling continuously
- audible chattering from the actuator
Likely causes
PID controller integral (I) or proportional (P) gain too high for the process lag
Stem stiction from dried packing or buildup on the stem, creating a deadband that destabilizes the loop
Positioner gain set too high causing the actuator to over-correct continuously
Process disturbances too fast for the control loop response
Required tools
- Fisher FIELDVUE positioner (if installed) with ValveLink or AMS software
- Fine abrasive cloth (320 grit or higher)
- Clean lint-free cloths
- Wrench for packing gland nuts
- LOTO kit (for valve isolation)
Safety first
- Process fluid in the valve body can be at high pressure or temperature. Follow the site valve-isolation and bleed-down procedure before opening the packing gland.
- Never loosen packing on a live valve to the point where process fluid escapes past the packing.
Procedure
- 1
Switch the process loop controller to manual. Observe whether the stem hunting stops. If it does, the fault is in the PID tuning, not the valve mechanics.
- 2
With the loop in manual, command the stem to 50%, then step in 5% increments up and down. Check that the stem moves smoothly and proportionally. Jerky or delayed steps indicate stiction.
- 3
If stiction is confirmed, isolate the valve from the process line per the plant safety procedure for this service.
Warning: This valve may carry high-pressure or hazardous process fluid. Follow the site valve-isolation and bleed procedure before touching the packing gland or stem. - 4
Inspect the packing box. If the packing follower is over-compressed, back off the packing gland nuts half a turn at a time until stem movement is smooth. Do not loosen past the point of acceptable packing leakage.
- 5
If packing is correct, clean the exposed stem with a clean rag and inspect for scale or corrosion buildup. Light corrosion can be addressed with fine abrasive cloth, heavier deposits require packing replacement.[1]
- 6
To tune the PID loop: reduce integral gain by 50%, then run a step test of the setpoint. Increase proportional gain until the process response is critically damped with minimal overshoot.[1]
- 7
If a Fisher FIELDVUE positioner is installed, use ValveLink or AMS to run a valve signature test. The test will quantify stem friction and flag tuning anomalies.[1]
Sources
Fisher easy-e ED, ET, EZ Sliding-Stem Control Valve Instruction Manual, Fisher Controls (Emerson)
Fisher easy-e ED, ET, EZ Sliding-Stem Control Valve Instruction Manual, packing adjustment and positioner tuning guidance (general)
View source
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