This paper discusses revisions to a method previously used for sizing relief devices (rupture disks) on vessels in high pressure Polyethylene processes. The Re-evaluation was forced by an unexpectedly high peak pressure in a product separator (High Pressure Separator - HPS) when an ethylene decomposition occurred. A method, based on revised equations for calculating the rate of pressure rise, is presented for determining relief requirements for an ethylene decomposition in a confined space. The method applies to both HPSs and Polyethylene Autoclave Reactors. Causes of decompositions and effect of comonomers are also discussed.

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