Page 69 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 51

volume tanks, particularly those with floating roofs, or by scrubbing, flaring, or
recovering the vented gas stream.
Solids handling can provide considerable exposure to contaminants when-
ever the operation is performed in an open atmosphere. Where possible, such
operations should be retrofitted with a closed system. Even then, potential
release problems exist, particularly during maintenance and repair of the
system.
It should be recognized that the maintenance of any closed system can
pose a hazardous exposure problem since most maintenance is performed while
the plant is in operation and requires that workers be in close proximity to the
operating equipment for long periods of time. Under such conditions, it is
necessary to consider not only local contaminant releases but also physical
hazards that may be present, such as noise and thermal radiation.
In a closed system, equipment that must be repaired should first be
cleaned to reduce exposure before the system is opened. Where highly toxic
process materials are present, it may be necessary to flush equipment with a
low-toxicity stream, strip with steam, and then purge with nitrogen. In such
situations, the equipment design should include special fittings necessary for the
flushing and purging procedures.
Turnarounds, or major periodic overhauls of chemical plant units, are a
special case of plant maintenance. Since the units are shut down, some exposure
risks are avoided. However, since the unit is not in production, there is a time
pressure to complete the turnaround and resume production. In such an
environment, there is the potential for disorganization and misunderstanding on
the part of workers with the unanticipated release of contaminants. To conduct
a safe turnaround requires careful planning. Contingencies need to be antici-
pated to the greatest extent possible and plans made to deal with them.
It should be noted that the materials and operations used in a plant
maintenance effort may involve a new set of hazards quite separate from the
exposure hazards encountered with feedstocks, intermediates, and products for
the process plant. For example, proper maintenance often involves such opera-
tions as welding, sandblasting, painting, chemical cleaning, catalyst handling,
and insulation replacement. The maintenance of safe conditions requires exten-
sive worker training in each one of these operations.
In the same vein, certain waste-handling procedures, even those per-
formed intermittently, can result in very serious contaminant exposure without
proper precautions. Workers need to be instructed in the proper procedures for
cleaning up spills and accumulated debris. Spilled materials can become air-
borne and pose an inhalation hazard. Spills and chemical process wastes may
end up in the waste-water treatment facilities where they again can be volatilized
into the air and result in unexpected worker exposure.

Exposure Evaluation

If health hazards are to be controlled, they must be recognized and evaluated.
A logical place to initiate the process of health-hazard recognition is with a total