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.

July 11, 1994

Mr. Joseph R. Schneider
Professional Development Chairman
Central Florida Section
American Institute of Chemical Engineer
2510 Twelve Point Drive
Lakeland, Florida 33811

Dear Mr. Schneider:

This is in response to your March 22 letter, requesting interpretation of the Process Safety Management (PSM) of Highly Hazardous Chemicals (HHC) standard, 29 CFR 1910.119. Please accept our apology for the delay in response. Your questions and our responses follow.

Question #1:

Can mitigating devices such as excess flow values, etc., be used to isolate a portion of an interconnected covered process containing less than a threshold quantity of a HHC in order to avoid coverage by the PSM standard.


No. The interconnected process would be considered part of the covered process, which is defined at 1910.119(b) as follows: "For purposes of this definition, an group of vessel which are interconnected and separate vessels which are located such that a highly hazardous chemical could be involved in a potential release shall be considered a single process."

Question #2:

If it is determined during the Process Hazard Analysis that there is no risk of a release of the covered chemicals, does it follow that the specific equipment in question still has to be included under Paragraph (j) Mechanical integrity, merely because it was previously considered under PHA?


The equipment defined to be covered by the PSM standard prior to the PHA is still considered to be covered by the standard, including paragraph (j), regardless of the findings of the PHA. PHA findings must be consistent with the following mechanical integrity requirements. At least the equipment specified in 1910.119(j)(1) is subject to the mechanical integrity requirements contained in 1910.119(j). OSHA believes equipment listed in 1910.119(j)(1), which is common to many processes, is critical to process safety. If an employer deems additional equipment to be critical to a particular process, the employer should consider that equipment to be covered by 1910.119(j) and treat it accordingly. This is discussed in the preamble in the middle column of page 6389 of the Final Rule a copy of which is enclosed for your use.

Question #3:

Appendix A of the PSM standard does not list Hydrogen and Sodium Hydroxide. Will chemicals such as these be added to the list in the future?


With exceptions (See 1910.119(a)(1)(ii)), a process containing a threshold quantity, that is, 10,000 pounds (4535.9 kg) or a greater amount of flammable gases, including hydrogen, is covered by the PSM standard. OSHA intends to add highly hazardous toxic and reactive chemicals to Appendix A in the future (in accordance with rulemaking procedures) as the need arises. This is discussed in the preamble at the bottom of the left hand column of page 6366 of the Final Rule.

Question #4:

Are railroad cars and tractor trailers containing more than the threshold quantity, of a highly hazardous chemical and remaining in a facility for a period of time, covered under 1910.119? How will a company know if a hazard analysis has been performed on a rail car or tractor trailer? How will a company know if the requirements under the mechanical integrity sections have been followed by the owner of the rail car or tractor trailer rig?


Commercial railroad tank cars and commercial tank motor vehicles (CTMVs) when remaining on a worksite and used to store threshold quantities or greater amounts of specified HHCs are covered by the PSM standard. Please note that these railroad tank cars and tank motor vehicles are covered by the PSM standard to the extent that they are not covered any other regulatory authority. For example, the Hazardous Material Regulations of the Department of Transportation (DOT) (see 49 CFR Subchapter C and particularly, Part 177-Carriage by Public Highway) cover CTMVs. These DOT regulations cover cargo tank design, construction, maintenance (including repairs) and certain operations of CTMVs. Generally speaking if the cars are considered "in transit" by DOT, OSHA will defer jurisdiction to DOT.

Thank you for your interest in occupational safety health. If we maybe of further assistance please contact us.


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