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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