This paper discusses revisions to a method previously used for sizing relief devices (rupture disks) for 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 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. Simulations for other events at diffierent operating conditions, not shown, indicate a good match for peak pressures. Therefore, the described equations provide a good estimate of ethylene decomposition peak pressures given the initial conditions. Although these methods were derived specifically for ethylene decompositions, the methods could be used for other decompositions.