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.

September 27, 1995

Mr. J. B. Evans Union Carbide Corporation 39 Old Ridgebury Road, K3 Danbury, CT 06817-0001

Dear Mr. Evans:

This is in response to your June 1, 1994 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 questions and our replies follow:

Question 1: Does the presence of a closed double block and vent in a pipeline (that is, a pipeline containing two closed valves, between which is a vent which is open to the air) break the connection such that vessels on either side are not "interconnected"? Note that a "block" is considered an "energy isolating device" under the lockout/tagout standard, 29 CFR 1910.147(b).

Reply: A closed double block and vent in a pipeline containing two closed valves between which is a vent open to the air is considered to be interconnected as this term is used to define a "process" at 1910.119(b). This interpretation is predicated on the assumption that there is a reasonable probability that an event such as an explosion would affect interconnected vessels notwithstanding engineering (including valve) and administrative controls required by the PSM standard to prevent release of highly hazardous chemicals. (See the preamble discussions on the middle column of page 6372 of the PSM Final Rule which is enclosed.)

Question 2: Are temporary flexible hoses running between twoessels enough to establish an interconnection?

Reply: "Interconnect" as this term is used to define a "process" at 1910.119(b) includes temporary flexible hoses between two vessels.

Question 3: In the following scenario, is the second part of the manufacturing unit considered part of the covered process?

Scenario - A polymers manufacturing unit is divided into two parts. The first part, which contains operations involving flammables and/or PSM-listed materials above threshold quantities. The second part, which does not involve any flammables or PSM listed materials, is located in a separate part of the chemical plant such that there is not likely to be any impact from any fires or explosions in the first part. The two are interconnected by a polymer pellet pneumatic conveying line, which is set up so that no flammables or PSM-listed materials can pass through it. The PHA for the first part confirms that the first part cannot adversely affect the second part.

Reply: By the definition of process at 1910.119(b), the polymer manufacturing unit you described consisting of two parts interconnected by a conveyor line is considered a single process.

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 (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) he or she is required to comply with the performance-oriented Process Safety Management (emphasis added) standard as the PSM standard (may) apply to a process or processes at a particular work site. The process safety analysis (PHA) may 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 can be of further assistance, please contact Mr. Ronald J. Davies of my staff, telephone 202-219-8031, extension 110.


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