2003

Andstén, Tauno; Keski-Rahkonen, Olavi; Myllymäki, Jukka

Pressurised powder and water based extinguishers have traditionally not been equipped with safety devices against an incidental rise of the internal pressure. Such safety devices have, on the other hand, always been mandatory for carbon dioxide and halon extinguishers. Dissenting opinions have always been raised, particularly by ministries responsibly for technical safety issues and by pressure vessel authorities. The Finnish Ministry of the Interior requested VTT already in the 1980's a to carry out a study on the behaviour of pressurised portable fire extinguishers without safety devices when subjected to fire. In the present study, a set of equations for calculation of the pressure development inside a closed extinguisher as a function of temperature has been derived based on basic physical chemistry. The influence of the geometry of the extinguisher body on the strength of the cylinder has been calculated by applying equations for strength calculations related to internal pressure of pressure vessels. By combining the yield stress of steel with the temperature dependence of the strength of the extinguisher body a diagram has been designed showing the maximum allowed pressure as a function of the dimensions of the extinguisher. Three methods have been applied for determining the decomposition pressure of the fire extinguishing media as a function of temperature. Measurements of pressure vs. temperature showed a good correspondence with the presented theory. The total pressure build-up in the extinguisher could be explained as the sum of the partial pressures of the propellant and the thermally decomposing extinguishing media. This study does not consider the question on the necessity of safety devices of portable fire extinguishers. Pressurised powder and water based extinguishers have traditionally not been equipped with safety devices against an incidental rise of the internal pressure. Such safety devices have, on the other hand, always been mandatory for carbon dioxide and halon extinguishers. Dissenting opinions have always been raised, particularly by ministries responsibly for technical safety issues and by pressure vessel authorities. The Finnish Ministry of the Interior requested VTT already in the 1980's a to carry out a study on the behaviour of pressurised portable fire extinguishers without safety devices when subjected to fire. In the present study, a set of equations for calculation of the pressure development inside a closed extinguisher as a function of temperature has been derived based on basic physical chemistry. The influence of the geometry of the extinguisher body on the strength of the cylinder has been calculated by applying equations for strength calculations related to internal pressure of pressure vessels. By combining the yield stress of steel with the temperature dependence of the strength of the extinguisher body a diagram has been designed showing the maximum allowed pressure as a function of the dimensions of the extinguisher. Three methods have been applied for determining the decomposition pressure of the fire extinguishing media as a function of temperature. Measurements of pressure vs. temperature showed a good correspondence with the presented theory. The total pressure build-up in the extinguisher could be explained as the sum of the partial pressures of the propellant and the thermally decomposing extinguishing media. This study does not consider the question on the necessity of safety devices of portable fire extinguishers.