Page 331 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

141308Absolute Risk Estimates
Table 14.27 More sample thermal radiation levels Table 14.28 Fatality probability at various thermal radiation levels
Thermal Percent fatalities Percen tjotalities
radiation Heatflux (kWhn2) outdoors indoors
(kWhnz) Description
6.3 3 0
1.2 Received from the sun at noon in summer 8 11 0
2.1 Minimum to cause pain after 1 minute 9.5 21 0
4.7 Will cause pain in 15-20 seconds and injury 12.6 49 0
(at least second-degree burns) after 30 seconds of 15.6 70 3
exposure; intensity in areas where emergency 19 85 11
actions lasting up to several minutes may be 24 95 21
required without shielding but with protective 31.5 100 49
clothing 39 IO0 70
6.3 Intensity in areas where emergency actions lasting 47.5 100 85
up to 1 minute may be required without shielding 60 100 95
but with protective clothing
9.5 Intensity at design flare release at locations to which Source: Office of Gas Safety. "Guide to Quantitative Risk Assessment
people have access and where exposure would be (QRA)," Standards Australia ME-038-01 (Committee on Pipelines:
limited for a few seconds for escape Gas and Liquid Petroleum), Risk and Reliability Associates Pty Ltd..
12.6 Significant chance of fatality for extended April 2002.
exposure; high chance of injury; heats wood such
that it will ignite with anaked flame (piloted
ignition of wood) Release model. The release model assumes that the gas peak
15.6 Intensity on structures where operators are unlikely effective release rate feeds a steady-state fire even though
to be performing and where shelter is available
23 Likely chance of fatality for extended exposure and the rate of gas released will immediately drop to a fraction of
chance of fatality for instantaneous exposure; the initial peak rate. Therefore, the release model's calcu-
spontaneous ignition ofwood and failure of lated effective release rate is a maximum value that overesti-
unprotected steel atter long exposure mates the actual rate for the full release duration of a typical
35 Cellulosic materini will pilot ignite within 1 minute gas pipeline rupture fire.
of exposure; significant chance of fatality for Heat intensity threshold. A heat intensity threshold estab-
people exposed instantaneously
37.5 Intensity at which damage is caused to process lishes the sustained radiant heat intensity level above which
equipment the effects on people and property would be considered sig-
nificant. The degree of harm to people caused by thermal
Source: Office of Gas Safety, "Guide to Quantitative Risk Assessment radiation exposure is estimated by using an equation that
(QRA)," Standards Australia ME-038-01 (Committee on Pipelines: relates the chance of burn injury or fatality to the thermal
Gas and Liquid Petroleum), Risk and Reliability Associates Pty Ltd., load received. The degree of damage to wooden structures
April 2002. through piloted ignition and spontaneous ignition can also
be estimated as a function of the thermal load received.
Jet fire modeling
Combining the model's effective release rate equation with
The following is based on work presented in Ref. [83]. the radiant intensity versus distance equation gives a hazard
Models are available to characterize the heat intensity asso- area equation of
ciated with ignited gas releases from high-pressure natural gas
pipelines. Escaping gas is assumed to feed a fire that ignites r =4(2348 x p x dZ)/I
shortly after pipe failure. The affected ground area can be esti- where
mated by quantifying the radiant heat intensity associated with r =radius from pipe release point for given radiant heat inten-
a sustainedjet fire. sity (fit)
The relationship presented below uses a conservative and I = radiant heat intensity (Btu/hr-ft2)
simple equation that calculates the size of the affected worst p = maximum pipeline pressure @si)
case failure release area based on the pipeline's diameter and d = pipeline diameter (inches).
operating pressure. This release impact model includes the fol-
lowing elements: Reference [83] recommends the use of 5000 Btu/hr-A2 as a
heat intensity threshold for defining a "high consequence
1. Fire model. The fire model relates rate of gas release to the area."This heat intensity corresponds to apredicted 1% mortal-
heat intensity of the fire. This approach conservatively mod- ity rate for people, assuming they are exposed for 30 seconds
els releases as vertically oriented jet flame or trench fire while seeking shelter after the rupture, and a level where no
impact areas. The conservatism compensates for the possi- nonpiloted ignition of wooden structures would cccur, regard-
bility of a laterally oriented jet, delayed ignition fireball, less of the exposure time. It is chosen because it corresponds to
and/or the potential wind effect on actual fire position. a level below which
Additionai conservatism is employed because a significant
portion of the radiant heat energy will actually be absorbed Property, as represented by a typical wooden structure would
by the atmosphere. not be expected to bum