PSM coverage depends on how much highly hazardous chemical is in a process, and 1910.119(b) defines that broadly: any activity involving an HHC, where "any group of vessels 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." Two questions decide most coverage disputes: what is inside the boundary, and what gets added together toward the threshold quantity. OSHA's letters on both are consistent and strict.

What's Inside the Process Boundary

Apr 28, 1994
Storage tanks: piped, valved, hosed, or just nearby
A storage tank hard-piped to a covered process is part of the process, and it stays part of the process even if a valve can isolate it. A tank only occasionally connected by hose is part of the process because it is occasionally connected. Even a permanently disconnected tank nearby, or one downwind, is part of the process if an incident in the covered process could affect it.
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Feb 15, 2017
Two employers, one process: 'on site in one location'
Air Products asked about a hydrogen generator it operated on another company's property, feeding that company's process. Neither unit alone held 10,000 lb. OSHA said it was one covered process because all the inventory sat within the host's contiguous property, interpreting "on site in one location" as contiguous areas controlled by an employer or affiliated employers, and said either or both employers could have PSM duties. OSHA added that meeting NFPA separation distances does not make a facility "geographically remote."
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Feb 7, 1996 · Archived
Terminals: vapor recovery and adjacent worksites
For GATX's bulk terminals, OSHA said vapor recovery and incineration equipment interconnected with an atmospheric tank is part of a single process, and that blending or adding additives removes the atmospheric storage exception. Nearby tanks join a covered process if an explosion there could reasonably affect them, assuming the event happens despite PSM controls. For adjacent worksites, OSHA said applicability is decided by employer, not by worksite.
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What Gets Added Together

Jul 18, 1994
At any one point in time, and by chemical category
The threshold is based on the quantity in the process at any one point in time, not aggregated over time. Flammable liquids are aggregated together, flammable gases are aggregated together, and the two are treated separately. Appendix A toxics and reactives are aggregated individually. Where an Appendix A chemical is also flammable, coverage starts at the lower of its Appendix A threshold or 10,000 lb.
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Jun 1, 1994
Actual inventory, not vessel capacity, with proof
An ammonia system able to hold 20,000 lb but run at 5,000 lb is not covered, because coverage depends on the amount in the process, not its capacity. But an employer relying on that must have an effective control program and be ready to demonstrate to OSHA, with documentation of current and past operations, that the process never reaches the threshold.
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Feb 15, 1994
Remote distance, dikes, and mixed chemicals
Processes close enough that an incident in one could release an HHC from another are a single process. A dike that fully contains a liquid release can make two vessels "remote" from each other. Different Appendix A chemicals are not added together. Chemicals still on a DOT-regulated delivery vehicle don't count toward the threshold, and small containers can be dispersed so no one location holds a threshold quantity.
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⚡ Key Compliance Takeaway

Draw your process boundary generously: anything piped, hosed, or close enough to be involved in the same release is in, and valves, sprinklers, and procedures don't shrink it. Count what is actually in the process at any moment, separately for flammable liquids, flammable gases, and each toxic or reactive chemical. If you are relying on staying below the threshold, write down how you control inventory and keep records that prove it.