Fire Protection Philosophy and Design Guide
ABOUT FIRES AND FIRE PROTECTION
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| 2.0 ABOUT FIRES AND FIRE PROTECTION Generally, fires occur when a fuel vapor, an oxidizer, and energy are combined. Flammable gases are easiest to ignite flammable liquids (and solids) require the development of vapors (thus the lower the vapor pressure of a flammable liquid, the easier it is to start burning. Fires are ignited when the concentration of fuel vapor in air is within the flammability limits when in the presence of an ignition source or when above their auto-ignition temperature. Flammable vapors explode into fire the more vapors present, the bigger the explosion. Gas fires occur when gas leaks or escapes from a pressurized vessel, compressor, or line. The gas may ignite immediately. If so, the gas fires should not be extinguished. If they are extinguished, all that happens is the gas cloud gets bigger until it finds anew ignition source. Gas fires should be isolated and allowed to burnout (which occurs quickly once the supply of fuel is stopped). If the gas does not ignite immediately, it should still be isolated and allowed to dissipate. Gases that have the potential to be heavier than air require drainage away from ignition sources and towards remote locations. Liquid fires occur when vapors are released in the presence of oxygen and reach an ignition source or are heated to auto-ignition. Burning liquid maybe falling, flowing or pooled. Spilled flammable liquids should be prevented from spreading by passive fire protections, which are built into the facility, i.e. drainage away from the center of the pipe rack and away from fire hazardous equipment (especially high value equipment, spacing and containment, and fireproofing of structural members and ASME-coded vessels within the fire hazard envelope. The fuel source should be isolated. FIRE PROTECTION PHILOSOPHY AND DESIGN GUIDE PROCEDURE NO. PTD-DGS-133 REV 0 DATE Nov. 11, PAGE OF The fire should be extinguished and then the fuel and any hot debris should be cooled to avoid re-ignition. Application of copious amounts of water to a liquid fire may extinguish the fire: • By cooling the fuel • By production of steam which separates the flame from atmospheric air. • By dilution of the liquid fuel (if miscible with water). • By development of an emulsification layer over the burning surface (if not miscible with water. Note Water can also spread the fire when the fuel floats therefore good grading and adequate drainage is required.) • By mixing with foam concentrates, which is far more efficient at producing a layer over the top of the burning surface. • Application of copious amounts of water to the surfaces of ASME – coded pressure vessels or to structural members is sometimes an alternative to fireproofing and is also used to cool hot surfaces to prevent re-ignition. Explosions have the same ingredients as fire fuel (including many dusts, an oxidant, and an ignition source. Explosions occur as deflagrations, where the pressure wave expanding out from a point of ignition move at less than the speed of sound, and as detonations, where the pressure waves move in excess of the speed of sound. Unless specifically designed to contain explosive forces, confinement results in destruction of the container and the addition of shrapnel to the pressure wave. Explosions occur rapidly and any system designed to handle them must respond almost instantaneously. • Passive means of limiting damage due to explosions include blast resistant construction and explosion venting. • Venting is most often achieved using a (large) rupture disk and a vent line to safe location. (Frangible seams on storage tank roofs are an example of an explosion venting system) An active isolation system is sometimes used along with venting systems. Either physical or chemical isolation systems react rapidly to prevent the flame front from propagating down process piping to additional vessels. • Active means of limiting explosion damage are explosion suppression systems. • These systems must first detect the explosion (typically, a rapid pressure increase) (within 20 milliseconds of ignition) followed by activation of the suppression system (within 25 seconds of ignition) and suppression within 60 milliseconds of ignition. FIRE PROTECTION PHILOSOPHY AND DESIGN GUIDE PROCEDURE NO. PTD-DGS-133 REV 0 DATE Nov. 11, PAGE OF Suppressants work chemically by interfering with the chemical reaction of the explosion and thermally by removing the heat from the flame front and lowering the temperature to halt further combustion. Refer to NFPA 68 and 69 for Explosion Suppression Systems FIRE PROTECTION PHILOSOPHY AND DESIGN GUIDE PROCEDURE NO. PTD-DGS-133 REV 0 DATE Nov. 11, PAGE OF 50 Download 384.78 Kb. Share with your friends:
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