Explosion Severity: Propane versus Natural Gas

Natural gas, composed mainly of methane, is in some ways similar to propane gas. Both fuels have similar energy densities per unit mass, and similar laminar premixed flame burning velocities. However, propane explosions have been shown to produce higher overpressures in unconfined explosion tests when compared to methane. In vapor cloud explosion modeling, methane is considered to be a “low” reactivity fuel, while propane is listed as a “medium” reactivity fuel. In closed vessel explosion testing, the maximum rate of pressure rise for propane is almost twice than that for methane (based on KG values reported in NFPA 68 (2007) Standard for Explosion Protection by Deflagration Venting, table E.1). This study provides a direct comparison of the explosion severity between commercial propane and natural gas. Empirical correlations available for vented vessel explosions and unconfined Vapor Cloud Explosions (VCEs) are used to predict the difference in overpressure expected for a commercial propane explosion versus natural gas explosion. Although the maximum laminar burning velocity associated with propane is only about 15% higher than that associated with methane, commercial propane explosions are expected to result in overpressures that are about 40% higher than that of a natural gas explosion under identical conditions with a perfectly-mixed nearstoichiometric fuel-air mixture, based on empirical correlations. In addition to the laminar burning velocity, other fundamental differences in the fuels may also play an important role in the explosion severity. Propane has a slightly higher expansion ratio than methane when undergoing combustion. The mass diffusivity of propane and methane are also quite different, making the premixed propane flame more prone to wrinkling under turbulent conditions. Future testing in the 20-L explosion chamber is suggested. INTRODUCTION Propane and natural gas are common fuels used in heating systems and appliances in residential and commercial structures throughout the country. Natural gas is composed mainly of methane; with typical compositions consisting of 72-95% methane. Commercial propane is composed of about 95% propane and propylene gas. In this paper we will assume that the deflagration properties of commercial propane and natural gas are analogous to those associated with the pure propane and methane gas constituents. As will be shown in the next section, natural gas and commercial propane have very similar properties once mixed with air. Although both fuels share many properties, propane explosions have been shown to produce higher overpressures in unconfined explosion tests when compared to natural gas (or methane). In vapor cloud explosion modeling, methane is considered to be a “low” reactivity fuel, while propane is listed as a “medium” reactivity fuel. In closed vessel explosion testing, the maximum rate of pressure rise for propane is almost twice than that for methane (based on reference deflagration index values reported in NFPA 68 (2007) Standard for Explosion Protection by Deflagration Venting, table E.1). Anecdotal evidence from fire explosion investigations also show that propane explosions tend to result in worse damage to structures, compared to natural gas. ISFI 2012 International Symposium on Fire Investigation Science and Technology