Pressure loads generated by gas phase explosions in vessel-pipe geometry due to precompression and subsequent detonation

Abstract

Reactors used for chemical production plants have a couple of pipes starting or ending in the upper reactor head. For most of the reaction time these pipes are blocked by a shut-off valve which is located at a certain distance from the point where the pipe is flanged to the reactor head. In many reactors the head space is filled with a gaseous mixture which can be explosive, either under upset operating conditions or sometimes even permanently during normal operation. If an accidental ignition occurs inside the head space of the reactor, unreacted mixture can be pushed (“precompressed”) into the attached pipes. When the flame propagating inside the head space of the reactor finally arrives at the point where a pipe is flanged to the reactor head, the pressure of the yet unreacted mixture inside the pipe can have risen up to typically 10 times the initial pressure. In the course of further propagation inside the pipe the flame can undergo the transition from deflagration to detonation (DDT) and the resulting pressures inside the pipe can become extremely large, in particular in those cases when the DDT occurs at the blinded pipe end such that DDT and reflection coincide. In this paper the entire phenomenon will be comprehensively explained and formulae will be derived that allow quantifying the static equivalent pressures to be expected in the attached pipes in dependence on the geometry of the vessel-pipe system, the process parameters (temperature and pressure) and the explosion characteristics of the involved gas mixture. In this way, a detonation pressure resistant design under precompression conditions can be realized. Finally, examples from real life involving ruptured pipes due to precompression with subsequent detonation are presented.