At one power unit, the flue gas bypass damper was driven by a pneumatic actuator, and trouble came in succession: the damper opened by itself twice, with nobody issuing any command, and worked normally again afterward. Testing revealed an air leak, but the unwanted opening persisted even after the leak was repaired; the control wiring checked out fine too. The young maintenance engineer on duty was understandably alarmed.
The troubleshooting directions suggested by peers were quite focused, because flue gas bypass dampers normally use self-maintaining pneumatic actuators (holding position on loss of air, power, or signal) and "should never open without a reason":
- Air supply: stability and quality of the instrument air come first; hunt and repair every leak thoroughly;
- Triple-loss protection: verify the "loss-of-signal, loss-of-power, loss-of-air" position-holding functions in the positioner and the loop — one plant installed an electronic switch monitoring the 4-20 mA signal and supply in the bypass damper circuit; on loss of power or signal the control circuit is cut directly and the bypass fast-opens;
- Solenoid energization mode: whether the solenoid is continuously energized or wired through a normally-open contact directly affects the misoperation risk;
- Mechanical backstop: if in doubt, add a mechanical lock on the valve stem to hold the damper in position;
- Some voices argued for switching to electric actuators, but the majority held that pneumatic actuators — fast and low-maintenance — remain the first choice for flue gas dampers, provided the air supply and the logic are properly managed.
Unable to pin down a single culprit, the engineer finally applied a combined fix and later posted the complete solution:
1. Remove the field PLC and control directly from the DCS, with some logic modifications — one less field control layer means one less source of spurious commands;
2. Change the continuously energized solenoid circuit to a normally-open contact arrangement, greatly reducing the possibility of misoperation;
3. Thoroughly re-inspect the air and electrical circuits.
The fault never returned. The engineer also gave an honest caveat: changing normally-closed contacts to normally-open is itself a risky modification to the safety logic — each plant must evaluate it against its own logic and safety requirements, not copy it blindly.
When a pneumatic actuator "opens by itself," investigate as a system, not just the actuator body:
1. Separate the three threads first — air (leaks and supply quality), electricity (solenoid and relay energization mode), and signal (control circuit) — and eliminate them one by one;
2. Position-holding is the last line of defense for a pneumatic damper: verify the triple-loss protection item by item, including the protection options inside the positioner;
3. More control layers mean more sources of spurious commands: with both a field PLC and the DCS in the loop, any slip in logic coordination can leave a misoperation path open;
4. Changing the type of holding contact (NC to NO) is a safety-logic change that must go through proper assessment and approval — it is not routine maintenance.
