This presentation emphasizes the unique contingencies applicable in the emergency relief system design of tray and packed columns used for mass-transfer operations. Attention is given to the internals of the column and their impact on such phenomena as flooding and obstruction to the flow required to attenuate pressure during an emergency relief.
While the relief devices should always be located at top of the column to promote unidirectional vapor relief flow and process vapor flow thereby avoiding reverse vapor flow of relief and consequential damage of internals, there are circumstances when the flow of the relief device(s) located at the top of the column is high enough to cause flooding. An unwise mode of differential pressure control with coincident double jeopardy of overhead condenser failure may also create high relief load. The flow may be obstructed due to softening of non-metallic packing material under a fire scenario, coking, or formation of pop-corn polymers. Too high a flow may cause a pressure drop through a tray that may damage the tray.
Under these circumstances the choices are consideration of metal packing, redesign of tray, tray support, and an inherent means of reducing flow through relief device by restricting lift or restricting the scenario flow, safety instrumentation, or location of a relief device at the bottom section of the column in the vapor space. If the location of the relief device at the bottom of the column is unavoidable, two-phase flow relief along with phase separation in effluent handling equipment and safe relief discharge location must be considered. This is a price that a company must pay for irresponsible project and process management in not engaging an Emergency Relief System (ERS) design Engineer in the conceptual and design phases of the engineering.
In the end, an ERS design of a mass-transfer column without the consideration of the internal is not only hollow, but also equally irresponsible.
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