Page 92 - Plant design and economics for chemical engineers

70 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

Magnitudes of events are typically expressed in terms of the amount of
flammable or toxic material released during an event. Release rates are esti-
mated by using appropriate single-phase and two-phase flow models. Since
release duration is directly related to the cause and context of the release, its
estimation is generally quite subjective.
The severity of the hazard usually cannot be related directly to the
magnitude of a release since this is often a function of both the proximity and
number of on- and off-site ignition sources. To determine the hazard severity
requires quantifying, with the aid of state-of-the-art hazard models, the likely
extent of toxic or flammable vapor-cloud travel under different atmospheric
conditions, the thermal-radiation fields around vapor and 1iqui.d pool-fires, the
overpressure from any anticipated explosions, and any missile or fragmentation
activity that may result from a confined explosion. These hazard events can then
be translated into hazard-zone estimates by incorporating criteria for human
injury and property damage. Finally, the results of various loss scenarios can be
combined and presented in risk profiles listing injuries, fatalities, and/or
property damage. These results can be compared with data for other risks to the
public and to workers in various related areas, and these serve as the basis for
an assessment of whether or not the risks of the facility as designed are
acceptable.

Failure Mode and Effect Analysis
The failure mode and effect analysis (FMEA) is generally applied to a specific
piece of equipment in a process or a particularly hazardous part of a larger
process. Its primary purpose is to evaluate the frequency and consequences of
component failures on the process and surroundings. Its major shortcoming is
that it focuses only on component failure and does not consider errors in
operating procedures or those committed by operators. As a result, it has
limited use in the chemical process industry.

Safety Indexes
The safety and loss prevention guide developed by the Dow Chemical Com-
pany? provides a method for evaluating the potential hazards of a process and
assessing the safety and loss-prevention measures needed. In this procedure, a
numerical “Fire and Explosion Index” is calculated, based on the nature of the
process and the properties of the materials. The index can be used two different
ways. In the preliminary design, the Dow index will indicate whether alternative,

tDow’s Fire and Explosion Index Hazard Classification Guide, 5th ed. AIChE, New York, 1981.