Tray Failure Due to Relief Valves

Relief valves are often placed toward the bottom of distillation towers. In particular on crude oil fractionators, relief valves are commonly located in the flash zone (i.e., at the feed elevation). 

Many such columns are equipped with movable valve caps. If the relief valves open, the vapor flow through the trays will be reversed. This will seal the valve caps up against the tray panels. The large downward force exerted by the trapped vapors could then collapse the trays. 

For such a tower, tray decks should be sieve or grid trays, and never valve cap trays. I will typically use a grid tray with a 1 -inch lift. Also, using 10-gauge rather than 14-gauge trays (which is the industry standard) will double the tray resistance to collapse, due to the relief valves lifting at the bottom of the tower. 

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