Page 93 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 71

less hazardous processes should be considered in the manufacture of a specific
chemical product. In the final ,design, after the piping and instrumentation
diagrams and equipment layout have been prepared, the calculated index is
used as a guide to the selection and design of the preventive and protective
equipment needed for safe plant operation.
The Dow index applies only to main process units and does not cover
process auxiliaries. Also, only fire and explosion hazards are considered. Re-
cently the index has been expanded to include business-interruption losses. The
principles and general approach used in the Dow method of hazard evaluation
have been further developed by Mond in the United Kingdom to include
toxicity hazards. This revised Mond index is described in a paper by Lewis.?

Safety Audits
The principal function of most safety audits in the past has been to verify the
adequacy of safety equipment and safety rules. The former includes equipment
for fire protection, personnel protection, and on-site emergency responses. In
addition to reviewing the general safety rules, the audit has provided explicit
safety rules for new process areas and associated emergency response proce-
dures. However, with the greatly increased concerns for environmental health,
safety, community relations, and loss prevention, safety audits have become
significant, as well as continuous activities for all chemical process companies.
Detailed checklists have been developed that cover every aspect of health,
safety, and loss prevention. An example of such a checklist has been prepared
by WhiteheadS and is shown in greatly condensed form in Table 5. (For
complete details, the original table should be consulted.) A critical analysis of
all the items on this checklist will generally identify the major hazards in a
proposed or existing facility and assist in prescribing preventative actions.
Because of their importance, several of the items on the checklist are amplified
in later sections of this chapter.
A typical example of the steps involved in the development of a process
plant is shown in Table 6. The enormity of the task confronting the design
engineer is illustrated by the fact that each of the items in Table 5 must be
considered at each of the stages in Table 6. It becomes apparent that consider-
ing these items only at the end of the design is unwise because decisions have
been made that foreclose what might have been the optimum control option for
occupational health reasons. Experience has shown that continuous integration

tD. J. Lewis, AIChE Loss Prevenfion Series, No. 13:29 (1979).
$L. W. Whitehead, Appl. Id. HE., 2:79 (1987); the unabridged table is also reproduced by L.
Lipton and J. Lynch, “Health Hazard Control in the Chemical Process Industry,” Wiley, New York,
1987, pp. 85-96.