Technical documentation and software quality assurance for project ...

• radiation levels of concern (LOC)
MODEL DESCRIPTION
Figure 2 shows the flow diagram for the module. The model assumes complete tank
failure while the tank is surrounded by fire. This is a common situation for a BLEVE
occurrence. A limit of 5000 metric tones is employed as the maximum amount of
chemical that will be modeled. This limit is of the order of the largest historical single
BLEVE incident.
The model calculates only the thermal radiation hazard. The overpressure wave
and any fragment hazard are not calculated although warning messages are provided. The
reasons why they are not calculated are several. First, estimates of release energy by
standard methods can only be given to an order of magnitude. Second, the main hazard
posed by BLEVE of a flammable liquid is the radiation from the resulting fireball
(AICHE Guidelines, 1994). Third, estimates of shrapnel mass, velocity, and number are
apt to contain high uncertainties and are currently not estimated in the existing ALOHA
code. Although non-flammable pressurized liquid containers can BLEVE, the model will
not calculate effects from these scenarios, since overpressure and fragmentation effects
are not calculated
Depending upon the chemical and the tank pressure and temperature at failure,
not all of the product will flash and participate in the fireball. The fraction, f , that
flashes is given by
f = Cp T − T ( b)
Hv where T is the chemical temperature at tank failure, Tb is the ambient boiling point, Cp is the specific heat capacity at constant pressure, and H
€
v is the latent heat of
vaporization. If the fraction is more than one third, then the entire tank contents
€ participate in the fireball. If it is less than one
€
third, then following Hasegawa and Sato
(1977), three times the amount flashed is used in the fireball calculation. The
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amount not
contributing to the fireball is used for a pool
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fire scenario.
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