OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.



November 30, 1994

Mr. L.W. Creitz Air Products and Chemicals, Inc.
7201 Hamilton Boulevard
Allentown, PA 18195-1501

Dear Mr. Creitz:

This is in response to your May 10 letter requesting interpretation of the Process Safety Management (PSM) of Highly Hazardous Chemicals (HHCs) standard, 29 CFR 1910.119. Please accept our apology for the delay in responding. Your cases and associated questions and our reply follows.

Case 1: This site involves three systems each of which provide distinct functions. They are connected in series, with System A feeding System B, which feeds System C. Only System C contains an inventory of highly hazardous material in excess of the defined threshold quantity and is clearly a PSM "covered process." System A is separated from System B. System B is separated from System C. The distances between the systems are such that an upset or emergency in either A or B would not affect C.

Question 1: Is System B a covered process since it is directly coupled to System C.

Question 2: Is System A a covered process since it is indirectly coupled, through System B, to System C?

Question 3: Other than system inventory and connections to other systems which exceed the threshold inventory, are there any factors which affect the decision on where the PSM boundary limits are drawn? If a defensible argument can be made that there is no reasonable scenario where an incident in System A or System B could result in a catastrophic release of highly hazardous chemicals (in excess of the threshold quantity), would System A and System B be non-covered?

Case 2: This site involves three systems each of which provide distinct functions. Systems X and Y are in parallel, and feed System Z. Only System Y contains an inventory of a HHC in excess of the threshold quantity and is clearly a covered process. System Z is a distribution system in the plant to collect and distribute the products of System X and System Y to a pipeline network.

Question 4: Are Systems X and Z covered processes since they are directly coupled to System Y?

Question 5: Are there any other factors, not mentioned in your response to Question 3 which might affect where the boundary limits are drawn for this case? Again, if there is no reasonable scenario where an incident in System X or System Z could result in a catastrophic release of a HHC, would these systems be non-covered?

Reply: Compliance with the PSM standard is determined on a work site by work site basis. [During a July 13 telecon between you and Mr. Ron Davies of my staff, you clarified that "System A which is separated from System B which in turn is separated from System C (and similarly, Systems X, Y, and Z)" means that although interconnected, these systems are located, distance wise, away from each other on the same worksite.] The boundaries of a process on a work site are determined by the definition of "process" at 1910.119(b). In response to your first inquiry in question 3 above, this definition (as explained in the preamble, page 6372, of the enclosed PSM Final Rule) includes other factors, for example, proximity, in addition to interconnection and inventory as described in your letter, which determine where the process boundaries are drawn. A process must contain at any one point in time a threshold quantity (TQ) or greater amount of a specified HHC to be covered by the PSM standard. In your letter, you indicated that each system, that is, A, B, and C and X, Y, and Z, contain a HHC specified by the PSM standard. For the purpose of determining whether there is a TQ or greater amount at any one point in time, flammable liquids contained in a process are considered in the aggregate. The same criteria applies to flammable gases. From an aggregate standpoint, flammable liquids and flammable gases are treated separately. Also, the HHCs, listed in Appendix A as toxics and reactives, are treated separately and aggregated individually for coverage purposes.

In cases where an Appendix A-listed HHC is a flammable liquid or gas, the coverage threshold is the lower of the following amounts: the threshold quantity specified in Appendix A or 10,000 pounds (4535.9 kg).

As described in your letter and depicted in the attached block flow diagrams, the interconnected systems A, B, and C in Case 1 and the interconnected systems X, Y, and Z in Case 2 are considered to be single processes. Since these processes contain threshold quantities or greater amounts of a specified HHC, both are covered by the PSM standard.

The following clarification is provided in response to your second inquiry in question 3 above. The processes you describe are covered because they are interconnected and therefore are considered a single process which contains a threshold quantity or greater amount of a highly hazardous chemical. The manner in which PSM standard coverage translates into compliance obligations varies from process to process. For example, "the goal of the mechanical integrity provisions (1910.119(j) is to ensure that highly hazardous chemicals covered by the (PSM) standard are contained within the process and not released in an uncontrolled manner." To this end, process equipment identified in 1910.119(j)(1) are subject to the mechanical integrity requirements of the PSM standard. This process equipment, which is deemed critical to process safety, is common to all processes. Otherwise, process safety equipment is not subject to 1910.119(j) requirements with the following exception. Additional process equipment which the employer deems critical to a particular process, should be considered equipment covered by 1910.119(j)(2) through 1910.119(j)(6) and treated accordingly by the employer. This clarification is addressed in the preamble on page 6389 of the PSM Final Rule, a copy of which is enclosed for your use.

The employer must determine to what extent (if any) it is required to comply with the performance-oriented Process Safety Management (emphasis added) standards as the PSM requirements may apply to a process or processes at a particular work site. The process safety analysis (PHA) can serve as a means by which the PHA team independently verifies the extent of coverage and other PSM determinations made by the employer.

We appreciate your interest in employee safety and health. If we may be of further assistance, please contact [the Office of General Industry Enforcement at (202) 693-1850].


John B. Miles, Jr., Director
Directorate of Compliance Programs

[Corrected on 10/20/06]