This paper covers work carried out on the design of disposal systems for runaway chemical reactor relief, specifically horizontal gravity separators. Existing design methods are based on removing all but the very small liquid droplets (<300 mu m) from the two-phase vented mixture. Experimentally obtained data regarding the masses of liquid retained by the system and the maximum droplet size at the exit to the separator have been compared to those expected using the existing design methods. The design methods indicate that there can be a significant disagreement on the required separator size.