176
7 Risk Assessment and Management of Chemical Processes
In the case of instantaneous releases of pressurized liquid gases, such as chlorine
or ammonia, source term modeling includes modeling the evolution of the cloud
size, the concentration in the cloud after the release (before being dispersed into the
atmosphere), and the ratio of the material that remains in the air to the material that
forms the evaporating liquid pool (see p. 2.114 in CPD (1996)).
For instantaneous non-boiling liquid discharges, pool spreading models are
used to estimate source terms that are then incorporated into dispersion models
or fire pool models (see p. 3.26 in CPD (1996)). For an instantaneous release of
compressed gas, information about the release rate can be directly input into vapor
cloud dispersion models.
Dispersion Modeling
Once the source terms of the released substance have been defined, dispersion
models can be used to estimate the concentration in the atmosphere as a function of
time and distance from the source.
The relevant independent variables for this calculation include the mean wind
velocity, the released material volume (for instantaneous releases) or emission rate
(for time-varying releases), the dimensions of the source, and the densities of the
substance and the ambient air.
Particularly relevant for process risk assessment is the modeling of heavy gas
dispersion, which describes the movement of released gases with a density greater
than the ambient air. Many hazardous and volatile substances are heavier than
air, including, for example, chlorine, ammonia, and hydrogen fluoride, which are
released as cold gases (see p.4.42 in CPD (1996)).
A dense gas released into the atmosphere will descend to the ground and from
there spread radially under the effect of gravity in a self-induced flow, producing
a shallow cloud. For instantaneous releases of dense gases, wind dispersion has an
important effect on the dilution of the gas cloud.
The CCPS distinguishes the following types of dispersion models: (1) phenomenological models or empirical relations; (2) intermediate models, such as box
models; and (3) advanced models based on the Navier-Stokes equations. Detailed
information about these models can be found in Appendix D as well as in Chap. 4
of CPD (1996).
Before running a detailed model, a preliminary screening can be carried out to
estimate the concentration of a released substance near to the ground using the
following formula:
c =
Q
u wc × H wc × W wc
(7.1)
• c: concentration [kg/m 3 ]
• Q: source emission rate [kg/s]
• u wc : worst-case wind velocity [m/s] (assumed to equal 1 m/s)
• H wc : worst-case cloud height [m] (assumed to equal 50 m)
7 Risk Assessment and Management of Chemical Processes
In the case of instantaneous releases of pressurized liquid gases, such as chlorine
or ammonia, source term modeling includes modeling the evolution of the cloud
size, the concentration in the cloud after the release (before being dispersed into the
atmosphere), and the ratio of the material that remains in the air to the material that
forms the evaporating liquid pool (see p. 2.114 in CPD (1996)).
For instantaneous non-boiling liquid discharges, pool spreading models are
used to estimate source terms that are then incorporated into dispersion models
or fire pool models (see p. 3.26 in CPD (1996)). For an instantaneous release of
compressed gas, information about the release rate can be directly input into vapor
cloud dispersion models.
Dispersion Modeling
Once the source terms of the released substance have been defined, dispersion
models can be used to estimate the concentration in the atmosphere as a function of
time and distance from the source.
The relevant independent variables for this calculation include the mean wind
velocity, the released material volume (for instantaneous releases) or emission rate
(for time-varying releases), the dimensions of the source, and the densities of the
substance and the ambient air.
Particularly relevant for process risk assessment is the modeling of heavy gas
dispersion, which describes the movement of released gases with a density greater
than the ambient air. Many hazardous and volatile substances are heavier than
air, including, for example, chlorine, ammonia, and hydrogen fluoride, which are
released as cold gases (see p.4.42 in CPD (1996)).
A dense gas released into the atmosphere will descend to the ground and from
there spread radially under the effect of gravity in a self-induced flow, producing
a shallow cloud. For instantaneous releases of dense gases, wind dispersion has an
important effect on the dilution of the gas cloud.
The CCPS distinguishes the following types of dispersion models: (1) phenomenological models or empirical relations; (2) intermediate models, such as box
models; and (3) advanced models based on the Navier-Stokes equations. Detailed
information about these models can be found in Appendix D as well as in Chap. 4
of CPD (1996).
Before running a detailed model, a preliminary screening can be carried out to
estimate the concentration of a released substance near to the ground using the
following formula:
c =
Q
u wc × H wc × W wc
(7.1)
• c: concentration [kg/m 3 ]
• Q: source emission rate [kg/s]
• u wc : worst-case wind velocity [m/s] (assumed to equal 1 m/s)
• H wc : worst-case cloud height [m] (assumed to equal 50 m)