Maloperation of Velan switch valve due to shorting of slow release relay (component failure) in the remote panel circuit.
While on routine operation of coke drum switching, operator had finished the vapour heating of drum but was yet to open the feed isolation valve to vapour heated drum.
As soon as switch valve selector switch was put on remote position (as safety interlock exists in this mode) and operator went to open feed isolation valve, switch valve rotated on its own towards the heated drum and further to bypass position sticking there at physical stopper bar (pin).
This created complete block flow of heater effluents and heater oil pressure rose to 72 kg/cm2 causing smoke at switch valve inlet and operator could not approach the selector switch again to reverse/correct the position. Heater firing however tripped due to lo-lo flow
Heater completely coked up (all coils) and caused 26 days s/d for hydrojet cleaning. production loss 3 mm usd.
Post analysis shows inherent fault in the vendor supplied remote panel relay circuit as it gave direct power to the rotor motor bypassing all safety interlock.
Moral of the Story: Proper hazops are to be done on vendor supplied safety interlock logic component failures also.
Thanks to Raj K Sharma for contributing this article.