Relief valves coke drums

Vapors from the coke drum must pressure their way through to the combination tower reflux drum. Any restriction to their flow will increase the operating pressure of the coke drum. To avoid exceeding the coke drum relief valve pressure, some operators vent the reflux drum to a flare. This makes it appear as if the wet gas compressor is limiting the resid feed rate. Most often, though, the problem lies with upstream pressure drop. 

It was standard practice to protect refinery delayed coker, coke drums, from overpressure by locating a relief valve on the overhead vapor line. However, the main reason the coke drums would overpressure was coke accumulation in the inlet of the vapor line, immediately downstream of the coke drum itself and typically just upstream of the relief valve. Thus, it was a common occurrence (for example, at the... 

Suncor Coker at Ft. McMurray and the Citgo Coker in Lake Charles) for the coke drums to overpressure without the relief valves opening. In one instance, the drum pressure reached over 100 psig, even though the relief valve was set at 60 psig. 

Now, it is standard practice to site all coke drum relief valves on the coke drum itself, and never on the vapor outlet line. 

---