Environmental Engineering Reference
In-Depth Information
or a single total mass released. The thermodynamic state of the material at the exit
of the rupture is required as an input to the subsequent jet and dispersion modeling.
Also because the flow may be choked at the exit the pressure at the exit may be
above atmospheric pressure, and consequently the jet material will accelerate after
the exit. This requires the calculation of the momentum flow rate of the jet
including any acceleration due to an elevated pressure at the exit plane.
Because of the many possible release scenarios that could develop, a suite of
six model equations is suggested. These allow for gas, two phase or liquid storage
and release through ruptures of various types including sharp edged and “pipe-
like” ruptures. A major change from previous reports has been the adoption of the
ω-method as developed by Leung and Grolmes (1988).
Following a rupture of a storage vessel there will be a mass flow rate from the
vessel and this may be gas, liquid or a mixture of gas and liquid depending upon
the contents, their disposition and the rupture position. The vessel response (called
blowdown in the chemical engineering community) will depend upon the funda-
mental equations for mass, momentum, enthalpy and entropy including heat transfer
to or from the vessel contents, transfers between different phases within the storage
vessel and the thermodynamic equilibrium or not of the vessel contents. These
changes will lead to a change in the phase fractions in the vessel and the tempe-
ratures and pressure within the vessel and these will allow vapor, two-phase or
liquid material to exit the vessel. A simple model of vessel blowdown is presented.
Model equations for jet depressurization and phase change due to flashing
are summarized in Britter et al. (2009). The breakup of the jet into fine droplets
and their subsequent suspension and evaporation, or rain out, is still a significant
uncertainty in the overall modeling process. Two models for drop formation
are the most widely used - the RELEASE model (CCPS, 1999; Johnson and
Woodward, 1999) and the model by Witlox et al. (2007). The Witlox et al. (2007)
JIP model is suggested though its uncertainty must be kept in mind.
An important scenario is a tank failure that results in the formation of a liquid
pool and subsequent evaporation of the pool. The major liquid pool hazard, with
the most rapid boiling, would occur for large spills of cryogenic TICs, whose boiling
points are less than the ambient air and the ground temperature. Our review shows
that the current pool evaporation models, such in SCIPUFF (Section 8.3 of Sykes
et al., 2007), do agree fairly well with limited available data.
Some of the recommended source emission models have been evaluated with
data from TIC field experiments, in particular recent experiments with pressurized
liquefied gases. The experiments used for model evaluation include CCPS (1999)
RELEASE (flashing jets and droplet sizes), FLIE, DNV JIP flashing jet studies
(Witlox et al., 2007), and those from Richardson et al. (2006).
Figure 1
provides
an example of the comparisons of droplet diameter, D, model predictions with the
RELEASE and JIP observations. The two models are the RELEASE model and
the Witlox et al. (2007) JIP model, which are associated with the experiments with
the same names. It should be mentioned, though, that the JIP data contain direct
observations of droplet diameter, while the CCPS RELEASE data use their model
to back-calculate the droplet diameters from observations of the rate of deposition
