DEGADIS is an EPA approved
dense-gas model that predicts downwind vapor concentrations for explosion
hazards, developed specifically to model heavier-than-air gaseous releases. A
cloud from a dense gas release behaves very differently than a plume from a
lighter-than-air release. Since the gas is heavier than air, the cloud
characteristics are primarily gravity-driven. Negative buoyancy and stable
density stratification are among the factors that prevent the application of a
Gaussian dispersion model from accurately simulating a dense gas release.
DEGADIS can model a mixture of release type and duration scenarios:
- Ground-level release type: continuous, finite, and transient durations
- Vertical Jet release type: continuous and finite durations
- Evaporating liquid spill release type: continuous, finite, and
instantaneous durations
Release Durations
- Continuous Release: A continuous release is a steady-state release
of dense gas at a constant rate into the atmosphere over a long period of time.
As a rule, any release that is greater than 1,000 minutes is modeled as a
continuous release. The output from modeling a steady-state release is
concentration estimates at various downwind distances determined by the
model.
- Finite Duration: A finite duration release is a
steady-state release of dense gas at a constant rate into the atmosphere over a
short period of time. As a rule, any release that occurs over a period of less
than 1,000 minutes is modeled as a finite duration release. Finite duration
model output is organized either by time or distance, depending on which
parameter is of greater interest.
- Transient Release: A
transient release varies over time; for example, if a liquid pool boils off or
a container of gas depressurizes. As the pool decreases in size, the emission
rate and radius change. Other transient releases include near-instantaneous
releases such as container ruptures. Transient modeling output is organized
either by time or distance, depending on which parameter is of most
interest.
Release Method
- Vertical Jet Release: A jet
release is a vertical release of a dense gas or aerosol. The simulation uses
the Ooms mathematical model for a dense gas jet plume. The jet plume model
requires that the jet be vertical, with a definable exit velocity. If the jet
release is such that the plume centerline does not reach the ground before
dispersing, the jet plume model is run alone. If this is unclear, or if the
plume centerline does reach the ground, the jet plume model is run in
conjunction with the regular DEGADIS model as either a continuous or finite
duration release.
- Liquid Spill: A liquid spill is the release
of a chemical in its liquid state. The liquid is assumed to form a pool at
ground level, with the evaporation rate calculated using one of three
different evaporation models incorporated into DEGADIS. The results from the
evaporation model are run in the DEGADIS model as either a continuous or
finite duration release.
A graph of the centerline concentration at the height of interest, which users can generate after successfully running the DEGADIS model.
For plotting results on the base map, users can specify the concentration contours and output file.