Page 76 - Plant design and economics for chemical engineers

5 8 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

container, &, is the burst pressure of the container, pa is the pressure of the
surrounding air, and y is the ratio of the specific heats.
The amount of energy that is released from a chemical reaction involving a
flammable fuel and oxidizer can be estimated from the heat of combustion of
the fuel. The damage expected from the resulting explosion may be approxi-
mated by comparison with a similar energy release from a known charge of
TNT.
There are two special kinds of explosions of particular importance to the
chemical industry, namely, the boiling-liquid-expanding-vapor explosion
(BLEVE) and the unconfined-vapor-cloud explosion (UVCE). In the former,
heat leak into a container filled with a boiling liquid results in an excessive
vaporization accompanied with a steady pressure buildup that ruptures the
tank. The sudden depressurization causes very rapid vaporization with a sub-
stantial explosive force. An unconfined-vapor-cloud explosion, on the other
hand, can result when a large cloud of gas or vapor forms following release of a
flammable material. If ignition occurs, the cloud may either deflagrate, burning
with a relatively low burning speed, or the burning speed may accelerate until
the flame front reaches detonation velocities. Substantial destruction will occur
if the flame front reaches high velocities. A method for approximating the
potential for probable loss caused by a vapor-cloud explosion consists of
estimating the quantity of combustible that can be released during an accident
and then estimating the fraction of the material that is vaporized immediately
after the spill. The explosive load is then considered to be 2 percent of the heat
of combustion of the material vaporized.?
It is important to recognize that dusts and mists may also explode when
ignited. A large number of solids can form explosive mixtures in air if they are
sufficiently pulverized to remain well dispersed and suspended over a period of
time. Some dusts are more sensitive than others to ignition whereas some dusts
cause more severe explosions than others when ignited. The ignition sensitivity
depends on the ignition temperature, the minimum ignition energy, and the
minimum explosion concentration. The explosion severity, on the other hand, is
a function of the maximum pressure measured during a test explosion and the
maximum rate of pressure rise during the test. Since small dust particles are
usually easier to ignite and burn more rapidly than larger particles, both the
ignition sensitivity and explosion severity appear to be a function of particle
size. Extensive data on the explosion characteristics of dusts can be found in the
Fire Protection Hand6ook.S
If an explosion occurs, whether it is from a physical reaction or a chemical
reaction, an overpressure will be generated. Data are available to estimate the

tJ. A, Davenport, Loss Prevention, 11:39 (1977).
$G. P. McKinnon and K. Tower, “Fire Protection Handbook,” National Fire Protection Associa-
tion, Boston, MA, 1986. 4