Page 72 - Plant design and economics for chemical engineers

54 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS
achieve the desired risk reduction. This choice of options is a judgemental
decision since the precise degree of risk assessment achievable by a specific
strategy is usually not known in advance. Furthermore, the strategy selected
must meet company safety standards, comply with regulatory requirements,
receive worker acceptability, and not adversely impact production and operabil-
ity.
These are three general control principles utilized in reducing the expo-
sure of workers to occupational health hazards. These involve source controls,
transmission barriers, and personal protection. In the first strategy, measures
are taken to prevent the release of the toxic contaminant to the air. The second
strategy provides means for capturing or blocking the contaminant before it
reaches the worker. The final strategy assumes the first two were unsuccessful
and requires workers to wear some protective device to prevent contact with the
toxic contaminant.
Containment eliminates most opportunities for exposure and is the pre-
ferred method of control in chemical manufacturing. Actually, containment in
many chemical plants is dictated by pressure, temperature, fire, or product-loss
requirements and is really not a health-hazard control option. However, it must
be recognized that containment is never perfect, releases and exposure opportu-
nities will still occur, and additional control will probably be required.
Basic or detailed changes in the way the process is permitted to operate
can eliminate or reduce exposure. For example, rather than handling a material
as a dry powder, it might be handled as a slurry or in solution. A special case of
process change involves the substitution of a less hazardous material in the
process for a more hazardous one. If such a substitution is not possible, then it
may be necessary to completely isolate the process from the worker, as has been
done in the manufacture of HCN (prussic acid).
The primary purpose of local exhaust ventilation is to control contaminant
exposure by establishing a control surface or barrier between the emission
source and the worker so that the contaminant is captured and does not reach
the worker’s breathing zone. Local exhaust ventilation is cumbersome, inconve-
nient, and requires considerable maintenance. It is an effective form of control
that can be retrofitted to an existing plant and thus minimize a problem that was
not anticipated in the original design. However, local exhaust ventilation is
rarely completely effective since capture is not complete and not all release
points are adequately covered.
Dilution ventilation, on the other hand, removes air containing a contami-
nant from the workplace after it has become mixed and been inhaled by the
workers. The objective of dilution ventilation is not to prevent any exposure, but
to keep the exposure to acceptable levels by dilution. This strategy should only
be used in low release rate, low toxicity (low hazard) situations.
There are also procedures and precautions that can be taken by workers
themselves to minimize exposure while on the job. Such practices do not
generally eliminate a hazard by themselves but are necessary to prevent overex- I
posure by emission sources not controlled by engineering design. Personal