Page 73 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 55

protection against exposure by inhalation can be accomplished by respirators.
Such devices are capable of providing considerable protection when selected
and used properly.
Various control options or combinations of options need to be selected to
reduce the evaluated exposure level to an acceptable one. The best option or
combination of options is then selected by means of a cost analysis. The latter is
most useful when comparing two or more options that have approximately an
equal probability of reducing the exposure below an appropriate occupational
exposure limit. Costs, including capital and expense, of the various options may
then be compared using such economic parameters as present net worth or
annualized cost.
An engineering system or work procedure that is utilized to eliminate a
health effect should be evaluated to determine the degree to which it reduces
the occurrence of the health effect. Measurements of exposure, for use in
comparison with occupational-exposure limits, need to be made over the averag-
ing time appropriate to the standard.

Fire and Explosion Hazards

Besides toxic emissions, fire and explosion are the two most dangerous events
likely to occur in a chemical plant. Considerable resources are expended to
prevent both of these hazards or control them when they do occur because of an
accident. These two hazards account for the major loss of life and property in
the chemical and petroleum industry.
For a fire to occur, there must be a fuel, an oxidizer, and an ignition
source. In addition, the combustion reaction must be self-sustaining. If air is the
oxidizer, a certain minimum concentration of fuel is necessary for the flame to
be ignited. While the minimum concentration required depends on the temper-
ature of the mixture and to a lesser extent on the pressure, most interest
generally is focused on the ignition conditions necessary at ambient tempera-
ture. The minimum concentration of fuel in air required for ignition at ambient
temperature is known as the lower fEammable limit (LFL). Any mixture of fuel
and air below the LFL is too lean to burn. Conversely, the concentration above
which ignition will not occur is labeled as the upperflammable limit (UFL). Both
limits of flammability are published in various literature sources? for many
hydrocarbons and chemicals. It should be noted that there is also a concentra-
tion of oxidizer that must be present for ignition, called the limiting oxygen index
(LOI) with a meaning analogous to the LFL.

TN. I. Sax and R. J. Lewis, Sr., “Hazardous Chemicals Desk Reference,” Van Nostrand Reinhold,
New York, 1987; C. E. Grant and P. J. Pagni, “Fire Safety Science,” Hemisphere Publishing
Corporation, New York, 1986; N. I. Sax and R. J. Lewis, Sr., “Dangerous Properties of Industrial
Materials,” 7th ed., Van Nostrand Reinhold, New York, 1989.