Page 241 - CHEF Guide
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Impact Evaluation
Substituting the height to the center of the flame as ½ the flame height and assuming the typical values noted for
2
Equation 14-9, the vertical distance, XThermal, to the receptor for thermal flux greater than 21.9 A 0.305 kw/m shown in Equation
14-10 is approximately:
]
XThermal = [ 58.5 A / Er – 2.67 A 0.695 1/2 Equation 14-10
14.7 “De Minimis” Consequence
If the hazard distance or impact area is extremely small, a potential fatality or severe injury is nearly impossible. The
cross-sectional area of a toxic cloud less than a few meters distance to Level of Concern concentration is so small that a
person would not be able to take a few breaths while remaining within the cloud. The cross-sectional area of a flammable
cloud less than a few meters distance to the Level of Concern is so small that burns could not cover a sufficient fraction of
body area to be a fatality or severe consequence.
14.8 Estimating Number of People Impacted for On-site Incidents
A simple approach is to assume a constant population density within the effect zone times the impact area to obtain
an approximation. Use of a constant population density times the impact area is essentially assuming a “random” distribution
of people. For example, outdoor operations personnel could be anywhere with the process area at any time.
Any personnel who could routinely be in the immediate area should be added to the approximation to obtain a total
number of people impacted.Onsite Outdoor Population Density accounts for maintenance and other personnel who may
occasionally be in a nearby outdoor process area.
Personnel in the Immediate Vicinity include those associated with procedures requiring operator attendance such as
unloading a tank truck, sampling, etc. in addition to personnel using nearby walkways, at a nearby elevated work area, etc.
Personnel in the Immediate Vicinity are considered.
On-site Population Density should account for maintenance and other personnel who may occasionally be in a nearby
outdoor process area. A typical value is 200 people/km (0.0002 people/m ) for a large, well-automated facility. Personnel
2
2
in the Immediate Vicinity includes those associated with procedures requiring operator attendance such as unloading a
tank truck, sampling, etc. in addition to personnel using nearby walkways, at a nearby elevated work area, etc.
Off-site Population Density may vary considerably between day and night, in addition to the fraction of population
indoors versus outdoors. A rural location may contain a few hundred people/km while a densely populated residential area
2
may contain 500 (for single family homes on large lots) to greater than 5000 people/km for multi-unit apartments or mobile
2
home parks.
14.9 Estimating the On-site Probability of Exposure
When the Effect Zone is small such that the number of people impacted is less than 1, the value representing a
“random” probability of a single severe impact or fatality is used. These simplifying assumptions that are used in CHEF for
small on-site exposure probabilities are depicted in Table 14-5.
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