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Figure 1.2 Applicability of the PSM Rule - doe-hdbk-1101-20040023
Process Safety Management for Highly Hazardous Chemicals - index
Questions cont'd - doe-hdbk-1101-20040025

If a plant has only one process covered by the Rule, must the PSM Rule be applied
to all process areas in the plant?
No. A plant may have some process areas that are covered by the PSM Rule and others
that are not. PSM provisions are invoked for covered processes only. However, DOE
contractors may choose to implement certain PSM elements in areas not covered by the
Rule if they believe activities in these areas pose a significant hazard. To avoid
regulatory confusion, contractors should document those parts of their plants that are
covered processes.
Can a facility contain more than one process?
A facility can contain several processes. If multiple processes are interconnected, they
may be considered a single process under the PSM Rule.
To determine whether a process is covered, should DOE contractors compare the
OSHA TQ for an HHC to the amount of the chemical annually consumed by the
process, or to the maximum intended inventory?
DOE contractors should determine whether a process is covered based on the maximum
intended inventory that the process can contain (tank or vessel capacity plus
interconnected piping capacity), rather than on its cumulative, annual use. Contractors
who wish to maintain the quantity of an HHC below its TQ limit, in a process capable of
containing over the TQ, must be able to demonstrate that they have effective measures in
place to ensure that the inventory in the process cannot exceed the TQ.
Several HHCs are listed in Appendix A along with a minimum threshold
concentration. What is the technical basis for the list and these thresholds?
The Appendix A list was drawn from several sources, including lists prepared by States
that have enacted similar regulations. In determining TQs and threshold concentrations
for the Appendix A HHCs, OSHA used an approach similar to that used by the State of
Delaware. For example, in evaluating toxic materials, OSHA assumed a ground-level,
continuous, steady state release for 1 hour. They also used neutrally buoyant Gaussian
dispersion modeling with a 4.3 m/s (14.1 ft/sec) wind speed, D-class stability, and urban
dispersion coefficients. The preamble to the PSM Rule discusses all of the specific
assumptions used by OSHA for its technical basis for toxic and reactive materials in
Appendix A.
In developing its Recommendation for Process Hazards Management of Substances with
Catastrophic Potential, the Organization Resources Counselors, Inc. (ORC) compiled a
list of dangerous toxic substances from a number of hazardous substance lists. This
compilation was then ranked by calculating the substance hazard index (SHI) for specific
concentrations. The SHI is defined as the substance vapor pressure (in mm Hg) at 20 oC
multiplied by 1,000,000, divided by 760, and divided by an acute toxicity concentration
for the substance. The priority cutoff for the list of toxins in ORC's recommendations
was 5,000. To be consistent with the technical basis for toxic materials submitted by
ORC to OSHA, the threshold concentration listed in Appendix A should correspond to an
SHI of 5,000. Where the concentrations specified for HHCs do not correspond, they
reflect judgments made by OSHA during the rulemaking process.

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