7.6 Risk Analysis (Step 4)
177
• W wc : worst-case cloud width [m] (assumed to equal 0.1 x, where x is distance
from the source in meters)
The source emission rate (Q) is estimated by assuming that the total mass in the
system is released over a 10-min period (i.e., 600 s) (see p. 7 in CCPS (1996)). If the
estimated air concentration for the worst-case scenario is considerably lower than
the set safe exposure level, then more detailed modeling may not be needed.
Fires and Explosions
The impacts of fires and explosions include overpressure and projectile effects, as
well as thermal radiation effects, respectively. To quantitatively model these, vapor
cloud explosion models and boiling liquid expanding vapor explosion models are
used in the chemical industry.
A vapor cloud is formed when a large amount of flammable vaporizing liquid
or gas is rapidly released into the atmosphere. This can result in a vapor cloud
explosion or a flash fire if the cloud is ignited before being diluted below its lower
flammability limit. After its release, if the vapor cloud comes into contact with an
ignition source immediately after the cloud’s release, then a flash fire will occur as
the size of the cloud is sufficiently small. Otherwise, without an immediate ignition
source, the accumulation of material in the cloud could increase to a concentration
sufficient to cause an explosion (Santamaría Ramiro and Braña Aísa, 1998).
A boiling liquid expanding vapor explosion occurs when a large mass of
pressurized superheated liquid (i.e., a liquid above its boiling point) or liquefied
gas is released into the atmosphere. Such explosions are usually caused by a fire
next to a storage container that weakens it and leads to a sudden shell rupture.
Further background information about fires and explosions is provided in
Appendix C, and detailed information about these and additional approaches for
modeling fires and explosions is available in CCPS (1999).
Effects on People, the Environment, and Property
In a final step of quantitative consequence analysis, outputs from the previous
models are used to determine endpoint effects (damages) to people, the environment, and property. While the effects on people can be expressed in terms of
fatalities and the effects on property in terms of monetary losses, the effects on the
environment are more complex to assess and to express. Environmental effects from
a loss event may involve contamination in multiple environmental compartments
and include impacts in the surface water compartment on aquatic organisms such as
algae, daphnia, fish, etc.
Consequential effects on people from exposure to toxic substances and/or fires
and explosions can be estimated using a dose-response method. This can be coupled
with a probit equation to linearize the response. Applying this probit method helps to
describe a time-dependent relationship between a variable and its outcome defined
by a normal distribution, and it results in a statistical correlation between a damage
load or toxic dose and the percentage of affected people or property lost.
177
• W wc : worst-case cloud width [m] (assumed to equal 0.1 x, where x is distance
from the source in meters)
The source emission rate (Q) is estimated by assuming that the total mass in the
system is released over a 10-min period (i.e., 600 s) (see p. 7 in CCPS (1996)). If the
estimated air concentration for the worst-case scenario is considerably lower than
the set safe exposure level, then more detailed modeling may not be needed.
Fires and Explosions
The impacts of fires and explosions include overpressure and projectile effects, as
well as thermal radiation effects, respectively. To quantitatively model these, vapor
cloud explosion models and boiling liquid expanding vapor explosion models are
used in the chemical industry.
A vapor cloud is formed when a large amount of flammable vaporizing liquid
or gas is rapidly released into the atmosphere. This can result in a vapor cloud
explosion or a flash fire if the cloud is ignited before being diluted below its lower
flammability limit. After its release, if the vapor cloud comes into contact with an
ignition source immediately after the cloud’s release, then a flash fire will occur as
the size of the cloud is sufficiently small. Otherwise, without an immediate ignition
source, the accumulation of material in the cloud could increase to a concentration
sufficient to cause an explosion (Santamaría Ramiro and Braña Aísa, 1998).
A boiling liquid expanding vapor explosion occurs when a large mass of
pressurized superheated liquid (i.e., a liquid above its boiling point) or liquefied
gas is released into the atmosphere. Such explosions are usually caused by a fire
next to a storage container that weakens it and leads to a sudden shell rupture.
Further background information about fires and explosions is provided in
Appendix C, and detailed information about these and additional approaches for
modeling fires and explosions is available in CCPS (1999).
Effects on People, the Environment, and Property
In a final step of quantitative consequence analysis, outputs from the previous
models are used to determine endpoint effects (damages) to people, the environment, and property. While the effects on people can be expressed in terms of
fatalities and the effects on property in terms of monetary losses, the effects on the
environment are more complex to assess and to express. Environmental effects from
a loss event may involve contamination in multiple environmental compartments
and include impacts in the surface water compartment on aquatic organisms such as
algae, daphnia, fish, etc.
Consequential effects on people from exposure to toxic substances and/or fires
and explosions can be estimated using a dose-response method. This can be coupled
with a probit equation to linearize the response. Applying this probit method helps to
describe a time-dependent relationship between a variable and its outcome defined
by a normal distribution, and it results in a statistical correlation between a damage
load or toxic dose and the percentage of affected people or property lost.