Page 75 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 57

is large, it will radiate at a constant flux; for most hydrocarbons and combustible
chemicals, the radiant flux averages close to 30,000 Btu/h-ft’.
Fires are classified into four groups: Class A fires are those burning
ordinary solids; Class B fires are those burning liquids or gases; Class C fires are
those that burn either Class A or Class B fuels in the presence of live electrical
circuits; and Class D fires consume metals. Fire-protection systems can be
divided into two large categories: passive and active. Active systems include
such agents as water sprays, foam, and dry chemicals; these require that some
action be taken, either by plant personnel or as a response by an automatic
fire-protection system. Passive fire-protection systems do not require any action
at the time of the fire. They are designed and installed at the time the plant is
built and remain passively in place until needed.
One example of passive fire protection is insulating material (called
fireproofing) that is applied to steel structural members and equipment supports
in the plant. The time required for unprotected steel supports to fail during a
fire is rather short. Fireproofing can significantly extend the failure time and
provide additional time for fire fighters to reach the scene, apply cooling water
to the supports, and bring the fire under control.
An explosion is a sudden and generally catastrophic release of energy,
causing a pressure wave. An explosion can occur without a fire, such as the
failure through overpressure of a steam boiler. It is necessary to distinguish
between detonation and deflagration when describing the explosion of a
flammable mixture. In a detonation, the chemical reaction propagates at super-
sonic velocity and the principal heating mechanism is shock compression. In a
deflagration, the combustion process is the same as in the normal burning of a
flammable mixture with the reaction propagating at subsonic velocity and
experiencing a slow pressure buildup. Whether detonation or deflagration
occurs in a flammable mixture depends on such factors as the concentration of
the mixture and the source of ignition. Unless confined or ignited by a high-
intensity source, most materials will not detonate. However, the pressure wave
caused by a deflagration can still cause considerable damage.
An explosion can result from a purely physical reaction, from a chemical
reaction, or from a nuclear reaction. A physical explosion is one in which a
container fails, releasing its contents to the surroundings. The damage to the
surroundings from the sudden expansion of the confined gas can be approxi-
mated by determining the maximum energy released from an isentropic expan-
sion of the gas and converting this energy quantity to a TNT equivalent. (The
energy released by an explosion of TNT is 4.52 MI/kg or about 2000 Btu/lb.) A
useful relation for this estimation is given by

where E is the maximum energy release, V is the volume of the ,gas in the