Application cont'd - hdbk11010024

DOE-HDBK-1101-96

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 mea-

sures 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 20C 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 speci-

fied for HHCs do not correspond, they reflect judgments made by OSHA during the

rulemaking process.

How will the list of covered toxic and reactive chemicals found in Appendix A and their

TQs be updated?

The list of HHCs can be revised through the normal OSHA rulemaking process. Other than

corrections for typographical errors, OSHA has indicated no firm plans to change the

Appendix A HHCs.

How do DOE contractors determine if a process is covered, when an Appendix A

chemical is contained in a mixture? Do you calculate the mass of the Appendix A chem-

ical in the mixture and then compare it to the TQ for the pure HHC listed in Appendix

The substances listed in Appendix A without specified concentration limits are intended to be

covered by the PSM Rule at commercial grade percentages purity. The commercial grade of

most of the HHCs is approximately 99% pure. Many of the HHCs, if not actually 99% pure,

are only one to two percent less than 99% pure. For example, the commercial grades of

acrolein and allyl chloride are 97% pure. However, some of the HHCs are considerably less

than 99% pure. For example, the commercial grade of hydrogen fluoride is 70%.