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Re: If you were a Judge Some 921 citations
Posted by: PMK140 (IP Logged)
Date: June 05, 2007 08:01AM
Here are some pertinent citations from NFPA 921:
21.3.1 Low-Order Damage. Low-order damage is characterized by walls bulged out or laid down, virtually intact, next to the structure. Roofs may be lifted slightly and returned to their approximate original position. Windows may be dislodged, sometimes without glass being broken. Debris produced is generally large and is thrown short distances. Low-order damage is produced by slow rates of pressure rise.
21.8.2 Interpretation of Explosion Damage. The explosion damage to structures (low-order and high-order) is related to a number of factors. These include the fuel–air ratio, vapor density of the fuel, turbulence effects, volume of the confining space, location and magnitude of the ignition source, venting, and the characteristic strength of the structure.
21.8.2.1* Fuel–Air Ratio. Often the nature of damage to the confining structure can be an indicator of the fuel–air mixture at the time of ignition.
21.8.2.1.6 Explosions of mixtures near the LEL do not tend to produce large quantities of postexplosion fire, as nearly all of the available fuel is consumed during the explosive propagation.
21.8.2.1.7 Explosions of mixtures near the UEL tend to produce postexplosion fires because of the fuel-rich mixtures. The delayed combustion of the remaining fuel produces the postexplosion fire. Often, a portion of the mixture over the UEL has fuel that does not burn until it is mixed with air during the explosion’s venting phase or negative pressure phase, thereby producing the characteristic following fire.
21.8.2.1.8 When optimum (i.e., most violent) explosions occur, it is almost always at mixtures near or just above the stoichiometric mixture (i.e., slightly fuel rich). This is the optimum mixture. These mixtures produce the most efficient combustion and, therefore, the highest flame speeds, rates of pressure rise, maximum pressures, and consequently the most damage. Postexplosion fires can occur if there are pockets of overly rich mixture.
Table 21.13.3.1 Typical Explosion Characteristics
21.13.3.2 Identify Explosion or Fire. The first task in the initial assessment is to determine whether the incident was a fire, explosion, or both, and which came first.
21.13.4.2 Identify Preblast and Postblast Fire Damage. Fire or heat damage should be identified as having been caused by a pre-existing fire or by the thermal effect of the explosion. Debris that has been propelled away from the point of origin should be examined to determine whether it has been burned. Debris of this nature that is burned may be an indicator that a fire preceded the explosion.
21.13.4.2.1 Probably the most common sign of an overpressure condition is window glass thrown some distance from the windows of the structure. The residue of smoke or soot on fragments of window glass or other structural debris reveals that the explosion followed a fire by some time, whereas perfectly clean pieces of glass or debris thrown large distances from the structure indicate an explosion preceding the fire.
Pat Kennedy, CFEI, CFPS, MIFireE
Fire and Explosion Analyst
Sarasota, Florida
[www.kennedy-fire.com]
21.3.1 Low-Order Damage. Low-order damage is characterized by walls bulged out or laid down, virtually intact, next to the structure. Roofs may be lifted slightly and returned to their approximate original position. Windows may be dislodged, sometimes without glass being broken. Debris produced is generally large and is thrown short distances. Low-order damage is produced by slow rates of pressure rise.
21.8.2 Interpretation of Explosion Damage. The explosion damage to structures (low-order and high-order) is related to a number of factors. These include the fuel–air ratio, vapor density of the fuel, turbulence effects, volume of the confining space, location and magnitude of the ignition source, venting, and the characteristic strength of the structure.
21.8.2.1* Fuel–Air Ratio. Often the nature of damage to the confining structure can be an indicator of the fuel–air mixture at the time of ignition.
21.8.2.1.6 Explosions of mixtures near the LEL do not tend to produce large quantities of postexplosion fire, as nearly all of the available fuel is consumed during the explosive propagation.
21.8.2.1.7 Explosions of mixtures near the UEL tend to produce postexplosion fires because of the fuel-rich mixtures. The delayed combustion of the remaining fuel produces the postexplosion fire. Often, a portion of the mixture over the UEL has fuel that does not burn until it is mixed with air during the explosion’s venting phase or negative pressure phase, thereby producing the characteristic following fire.
21.8.2.1.8 When optimum (i.e., most violent) explosions occur, it is almost always at mixtures near or just above the stoichiometric mixture (i.e., slightly fuel rich). This is the optimum mixture. These mixtures produce the most efficient combustion and, therefore, the highest flame speeds, rates of pressure rise, maximum pressures, and consequently the most damage. Postexplosion fires can occur if there are pockets of overly rich mixture.
Table 21.13.3.1 Typical Explosion Characteristics
21.13.3.2 Identify Explosion or Fire. The first task in the initial assessment is to determine whether the incident was a fire, explosion, or both, and which came first.
21.13.4.2 Identify Preblast and Postblast Fire Damage. Fire or heat damage should be identified as having been caused by a pre-existing fire or by the thermal effect of the explosion. Debris that has been propelled away from the point of origin should be examined to determine whether it has been burned. Debris of this nature that is burned may be an indicator that a fire preceded the explosion.
21.13.4.2.1 Probably the most common sign of an overpressure condition is window glass thrown some distance from the windows of the structure. The residue of smoke or soot on fragments of window glass or other structural debris reveals that the explosion followed a fire by some time, whereas perfectly clean pieces of glass or debris thrown large distances from the structure indicate an explosion preceding the fire.
Pat Kennedy, CFEI, CFPS, MIFireE
Fire and Explosion Analyst
Sarasota, Florida
[www.kennedy-fire.com]
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