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Hazard zone calculations 14/307
                Vapor cloud,fire-in  which a cloud encounters an ignition   Thermal radiation damage levels
                source  and the entire  cloud combusts as  air  and fuel  are
                drawn together in a flash fire situation.   Thermal  radiation  levels are typically  measured  in  units  of
                Liquidpooljires-a   liquid pool of flammable material could   kW/mZ or Btu/hr-ft2. Thresholds of thermal radiation can be
                form and create radiant heat hazards.     chosen to represent specific potential damages that are of inter-
                Fireballs-not   thought  to  be  a  potential  for  subsurface   est. These  can then  be  used  to calculate  distances  from the
                pipeline facilities, this is normally caused by boiling liquid   pipeline  at  which  that  level  of  thermal  radiation  would  be
                expanding vapor explosion (BLEVE)  episodes in which a   expected.
                vessel, usually engulfed in flames, violently explodes, creat-   Recognized  “thermal  load versus  effect” models  estimate
                ing a large fireball (but not blast effects of other types of   that a bum injury will occur within 30 seconds of exposure at a
                explosions) with the generation of intense radiant heat.   heat flux of  1600 to 2000 Btu/hr-ft2 (5.0 to 6.3 kW/m2). At a
                Vapor cloud explosion-potentially   occurs as a vapor cloud   radiant heat intensity of 5000 Btu/hr-ft2  (15.8 kW/m2) the like-
                combusts in such a rapid manner that a blast wave is gener-   lihood  of  a  fatal  bum  injury  within  this  exposure  period
                ated. The  transition  from normal  burning  in a  cloud to a   becomes  significant  (I%),  where  1  in  100  people  exposed
                rapid, explosive event is not fully understood. Deflagration is   would not  survive. Various wood ignition models have been
                the more common event. A confined vapor cloud explosion   used to estimate the steady-state effects of thermal radiation on
                is more common than unconfined, but note that even in an   property based on the duration of exposure required to cause
                atmospheric release, trees, buildings, terrain, etc., can create   piloted and spontaneous ignition. These models conservatively
                partial confinement conditions. Any explosive event can also   establish a radiant heat intensity threshold of 4000 Btu/hr-ft*
                have  associated  missiles  and  high-velocity  debris  whose   (12.6 kW/m2) for piloted wood ignition and a 10,000 Btu/hr-ft2
                damage potentials have been dramatically demonstrated, but   (3 1.6 kW/m2) threshold  for  spontaneous  wood  ignition.  At
                are very difficult to accurately model.   8000 Btuihr-ft2  (25.2 kW/m2)  spontaneous  ignition  is  very
                                                          unlikely, but after 38 seconds in the presence of a pilot source,
                The hazard scenario is deDendent on the DiDeline’s Droduct.   piloted wood ignition will occur [83].
                                          .I
               as noted in Table 14.25. Mosi damage state or hazard zone cal:   Some representative thermal radiation levels of interest are
               culations result in an estimated threat distance from a source,   shown inTables 14.26 through 14.28.
               such as a burning liquid pool or a vapor cloud centroid. It is   The U.S. Department of Housing and Urban Development
               important  to  recognize  that  the  source  might  not  be  at  the   (HUD) published a guidebook in  1987 titled Siting ofHUD-
               pipeline failure location. The location of the source can actually   Assisted  Projects  Near  Hazardous  Facilities:  Acceptable
               be some distance from the leak site and this must be considered   Separation  Distances  from  Explosive  and  Flammable
               when  assessing  potential  receptor  impacts. Note  also that  a   Hazards.  The  guidebook  was  developed  specifically  for
               receptor can be very close to a leak site and not suffer any dam-   implementing the technical requirements of 24 CFR Part 5 I,
               ages, depending on variables such as wind direction, topogra-   Subpart C, of the Code of Federal Regulations. The guidebook
               phy, or the presence of barriers.          presents a method  for  calculating  a level ground separation
                Another potential hazard for pipelines containing HVLs is a   distance  (ASD) from pool  fires that  is based  on  simplified
               BLEVE episode described earlier. This is a rare phenomenon   radiation heat flux modeling. The ASD is determined  using
               for most buried pipelines. For surface facilities, where a vessel   nomographs relating the area of the fire to the following levels
               can become engulfed in flames, the BLEVE scenario should   of thermal radiation flux:
               be evaluated.
                                                           Thermal  radiation-buildings.   The  standard  of  10,000
                                                           Btu/hr-ft2 is based on the thermal radiation flux required to
               Table 14.25  Pipeline products and potential hazard scenarios   ignite a wooden structure after an exposure of approximately
                                                           15 to 20 minutes, which is assumed to be the fire department
                           Hazard   Hazard   Dominant      response time in an urban area.
               Product     I?‘pe   nafure    hazard model   Thermal radiation-people.  The standard of 450 Btu/hr-ft2
                                                           for  people  in  unprotected  outdoor  areas such  as parks  is
               Flammable gas   Acute   Thermal   Flame jet;   based on the level of exposure that can be sustained for a long
                (methane, etc.)               fireball     period of time.
               Toxic gas (chlorine,  Acute   Toxicity   Dispersion
                H,S, etc.)                    modeling
               HVL (propane,   Acute   Thermal   Dispersion   Table 14.26  Representative thermal radiation levels
                butane,           and blast   modeling;
                ethylene, etc.)               flame jet;               ~~~~~
                                              fireball; over-   Thermal mdiatron level (Btdhr-fi)   Description
                                              pressure
                                              (blast) event   12,000            100% mortality m -30  seL
               Flammable liquid   Acute and   Thermal and   Pool fire;   5,000   1% mortality in -30  sec
                (gasoline, etc.)   chronic   contamination   contamination   4,000   Eventual wood ignition
               Relatively   Chronic   Contamination   Contamination   1,600     Onset of injury after -30  sec
                nonflammable
                liquid (diesel,                           Source Stephens, M  J , “A Model for Sizing High Consequence Areas
                fuel oil. etc.)                           Associated with Natural Gas Pipelines.” C-FER Topical Report 99068
                                                          prepared for Gas Research Institute. Contract 81  74  October 2000
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