The butterfly valves in the coker I worked had a mechanical stop in the actuator that would prevent closing the valve completely.
We did experience sticking in these valves a couple of weeks after start-up, because of the resid would get in between the shaft and the bushing and coke up.
To correct this we had to drill the valve body and weld 1/2″ pipe with flanges to inyect steam into the bearing to keep them clean.
This was done back in 2001, and my friends there tell me the valves are still OK.
Regards, CG
