I have worked in various coker units, and almost all of them have had open-top tanks, it has advantages over closed-top tank (roofed tank), as well as allowing faster cooling, it also allows a quick overview of their level from the floor or from the cutting platform, the distinction of putting the roof up or not general are associated with the potential emissions of H2S, because of their sour water condition
the designs of these units (FW) indicates that the water collected in the Blowdown drum can send the tank between 3 to 4 hours after starting the cooling of the drum with water, since the content of H2S and NH3 are low (theoretically), but some of the problems that we had was the content of hydrocarbon accumulated in the tank that subsequently created problems in the pit of coke, we did several tests with the time and deemulsifier but finally we decided not to send the water to the cutting water