Why has coke drum inside diameter yet to exceed 9,800 millimeters (32 feet)?
Increasing coke drum diameter is not a major design or fabrication issue. Economic incentives are attractive, led by minimizing the total number of drums and maximizing the throughput of a coke drum pair. Multiple operation-related roadblocks are responsible for setting the current diameter limit:
Coke Bed Quench
Obtaining uniform quench becomes more difficult in larger coke drum cross sectional area. Quench water flow through random coke bed channels (created during fill cycle) tends to leave pockets of unquenched coke and oil/coke “hot spots”, both of which are safety issues and also extend the quenching portion of the off-line drum cycle. Proof of safe and efficient operation is required prior to increasing the diameter beyond current known limits.
Coke Bed Drilling (Jet Pump, Cutting Bits)
High-pressure water jets must reach the coke drum wall and “cut” coke in these outermost areas. Proof of efficient coke cutting is required prior to increasing the diameter beyond current successful limits. For 9,800 millimeter coke drum diameter, total drill stem-in-drum time may be 3-4 hours. Beyond this diameter, coke-cutting times are likely to be longer and difficult to accurately predict.