Combination Tower
A delayed coker combination tower is similar in design and function to an ordinary crude distillation tower; heat is removed and products are fractionated. Referring to Figure 3-1, four products are made: wet gas, unstabilized naphtha or wild gasoline, furnace oil, and heavy gas oil.
The most common problem encountered in the combination tower is tray damage. On start-up or during short unit outages, pockets of water can form. When this water contacts hot oil, it flashes, and the resulting pressure surge will upend trays. Corrosion damage to the furnace oil drawoff trays is also a possibility. The diagnosis of these problems as well as other fractionation difficulties is identical to those found on crude fractionators.
On one coker, the combination tower trays were repeatedly upset due to water accumulating in the tower during short unit outages. After considerable investigation, the source of this water was found. Purge steam was being used under vessel relief valves to prevent the relief valve inlet lines from coking up. While the unit was out of service for several hours, it would cool off. The steam condensed and formed potentially explosive pockets of water.
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