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f EXPLOSIONS 921-85� ';"- Table 13-8 Combustion Properties of Common Flammable Gases .1 071. Limits of Flammability Percent by Air Needed Air Needed Btu Volume in Air Specific to Burn to Burn Ignition Temp per 113 MJ/m3 Gravity 1 ft;of Gas 1 m3 of Gas Gas (gross) (gross) Lower Upper (air = 1.0) (ft) (m3) °F °C Natural gas High inert type' 958-1051 35.7-39.2 4.5 14.0 0.660-0.708 9.2 9.2 - - High methane type 1008-1071 37.6-39.9 4.7 15.0 0.590-0.614 10.2 10.2 900-1170 482-632 High Btu type` 1071-1124 39.9-41.9 4.7 14.5 0.620-0.719 9.4 9.4 - - Blast furnace gas 81-111 3.0-4.1 32.2 71.3 1.04-1.00 0.8 0.8 Coke oven gas 575 21.4 4.4 34.0 0.38 4.7 4.7 - - Propane (commercial) 2516 93.7 2.15 9.6 1.52 24.0 24.0 01-100 ' 493-604 (k.,.' Butane (commercial) 3300 122.9 1.9 8.5 2.0 31.0 31.0 900-1000 482-538 Sewage gas 670 24.9 6.0 17.0 0.79 6.5 6.5 - - Acetylenei 1499 208.1 2.5 81.0 0.91 11.9 11.9 581 305 Hydrogen 325 12.1 4.0 75.0 0.07 2.4 2.4 932 500 ' Anhydrous ammonia 386 14.4 16.0 25.0 0.60 8.3 8.3 1204 651 Carbon monoxide 314 11.7 12.5 74.0 0.97 2.4 2.4 1128 609 ethylene 1600 59.6 2.7 36.0 0.98 14.3 14.3 914 490 Methyl acetylene, 2450 91.3 3.4 10.8 1.48 - - 850 454 propadiene,stabilized`[ 'Typical composition C114 71.9-83.2%;N2 6.3-16.20% ['Typical composition (''114 87.6-95.79o;N2 0.1-2.39% `Typical composition CH4 85.0-90.1%;N2 1.2-7.5% dMAPP®Gas from the NFPA Fire Protection Handbook, l7th edition,Table 3-7C does not burn until it is mixed with air during the explosion's A natural gas leak in the first story of a multistory structure venting phase or negative pressure phase, thereby producing may well be manifested in an explosion with an epicenter in the characteristic following fire. an upper story. The natural gas, being lighter than air, will When optimum (i.e., most violent) explosions occur, it is have a tendency to rise through natural openings and may almost always at mixtures near or just above the stoichiometric even migrate inside walls. The gas will continue to disperse mixture (i.e.,slightly fuel rich).This is the optimum mixture. in the structure until an ignition source is encountered. These mixtures produce the most efficient combustion and, An LP-Gas leak on the first story of a house, if it is not,. therefore, the highest flame speeds, rates of pressure rise, ignited there, can travel away from the source and,due to its maximum pressures,and consequently the most damage.Post- density, will tend to migrate downward. The gas may collect ,- explosion fires can occur if there are pockets of overly rich in lower areas of the house and concentrate.. mixture. Ignition of the gas will only occur if the concentration is For common lighter-than-air gases in residential buildings, within the flammable limits and in contact with a competent an explosion involving an optimum concentration will some- ignition source (one with enough energy). times result in some destructive shattering effects of wooden Whether lighter-or heavier-than-air gases are involved,there[ structural materials. may be evidence of the passage of flame where the fuel air layer was.Scorching,blistering of paintwork,and showing of 13-8.2.2* Vapor Density. The vapor density of the gas or "tidemarks" are indicators of this type of phenomena The: ~ vapor fuel can have a marked effect on the nature of the operation of heating and air-conditioning systems,temper;( explosion damage to the confining structure.This is especially ture gradients,and the effects of wind on a building can cause true in dwellings and other buildings. mixing and movement that can reduce the effects,of vapof I leavier-than-air gases and vapors(i.e.,vapor density greater density.Vapor density effects are greatest in still-air conditions,: than 1.0), such as from ignitible liquids and LP-Gases, tend Full-scale testing of the distribution of flammable gas cone, to settle to lower areas.Lighter-than-air gases,such as natural centrations in rooms has shown that near stoichiometric coh' gas,tend to rise and collect in upper areas.For example,signs centrations of gas wou'dc d dev the location of tht< of postblast burning in pocketed areas between ceiling joists leak and either(1) the cel an-air gases or(2)11;' may be indicative of a lighter-than-air fuel rather than heavier- the floor for heavier-thar3' so reported that than-air gases or vapors.(See 4-17.9.)Due to their higher mobil- a heavier-than-air gas tha. evel would create ity and tendency to escape upward, lighter-than-air gases are a greater concentration at the gas would. less likely to produce hazardous situations than heavier-than- slowly diffuse upward. t e relationship is' air gases, which can pool in basements, crawl spaces, wells, true for a lighter-than-air g height.VentilaL: and tanks.' tion,both natural and mechanical,can ch'atige the movement 1998 Edltlo y, a ,