An investigation of the maximum experimental safe gap anomaly

Abstract

The "safe gap" is defined as the maximum gap between two close-fitting, flat, machined metal surfaces that will just prevent an explosion within a vessel from producing a propagating combustion wave in the gases that surround the vessel. At present three distinctly different apparatuses are used throughout the world to determine experimentally the maximum safe gap for different combustible gases and vapors. Unfortunately, the maximum safe gap thus determined sometimes differs widely from vessel to vessel. This report investigates the nature of the processes by which the explosion is transmitted to the surrounding media, paying particular attention to the differences from apparatus to apparatus. It is found that the time constants associated with the combustion process and with vessel exhausting are markedly different for the different vessels and that the pressure levels developed inside a vessel during the test also vary markedly from vessel to vessel. Based on these observations, it is recommended that a more thorough theoretical and experimental investigation of the safe gap testing procedures and vessel design be made as the basis for a proper design for adequate safe gap testing of all combustible vapors and gases.