Anonymous
Please ensure proper purge steam to all the valves (feed line isolation , switch or vapor valve) on routine basis. Not passing of purge steam to the valves has caused coke deposition in your case finally resulting in non operation or jamming of valve. Failure of feed line flushing steam will cause coking of the spool piece between switch & isolation valve.This will result in increased pressure at switch valve inlet during switch over to the concerned coke drum & will lead to unit interruption immediately.
Mrityunjay.