“Human genius has limits, but stupidity does not.”  — Alexandre Dumas, fils

Everyone we have ever worked with is accepting, if not downright enthusiastic, about managing process safety. Likewise, everyone we have ever worked with is no more enthusiastic about complying with Process Safety Management than they are about getting a root canal.

In part, this is because of their deep-seated fear that despite their best efforts, they will never be able to do it well enough to satisfy OSHA.  Like a root canal, they are sure that PSM compliance is good for them, but if possible, they would really like to minimize its impact.

Is There a Covered Process?

Deciding whether a process is covered by the PSM standard is not usually controversial.  Does the process contain, in aggregate, more than 10,000 lbs of liquids with a flash point below 100°F?  This is the most common reason we have encountered for a process to be covered by the PSM standard.  Does it contain, in aggregate, more than 10,000 lbs of Category 1 flammable gases?  Does it contain, by chemical, more than the specific threshold quantity (TQ) of one of the 131 toxic or reactive chemicals or chemical families listed in Appendix A.  (Yes, there are 137 rows in Appendix A, but there are six pairs of synonyms.)

If there are questions, they are often about the exceptions—hydrocarbon fuels used solely for workplace consumption as fuels, and storage of flammable liquids in atmospheric tanks—or the exemptions—retail facilities, oil or gas-well drilling or servicing operations, or normally unoccupied remote facilities (NURFs).  More often, though, questions are about what constitutes “a process,” and the boundaries of a process.

Sometimes there is a temptation to split a facility up into many little “processes” in an attempt to keep the amount of covered “Highly Hazardous Chemical” (HHC) below the TQ in each process.  As you might imagine, OSHA has very little patience with this.  On the other hand, OSHA never intended that an entire facility be automatically treated as a single process.  Otherwise, the guidance in Appendix C would not recommend that “the employer might consider dispersing inventory to several locations on site. Dispersing storage into locations where a release in one location will not cause a release in another location is a practical method to also reduce the risk or potential for catastrophic incidents.”

The PSM Standard defines a single process as “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”.  This definition prevents dividing an operation into such small pieces that no one piece contains a TQ.

The definition of a process from the PSM Standard differs from the definition that a process engineer would use.  Most process engineers would define a process along the lines of “An equipment train into which raw materials or intermediates are introduced and through which this material flows, being transformed into a different intermediate or product with different chemical or physical properties and characteristics and with increased value.” Other names that a process engineer would use for a process are a “unit” or an “operation”.

Why Does It Matter?

The PSM standard has 14 elements for covered processes. For many of those elements, the effort to comply does not increase in proportion to the size of the process. For these elements of PSM, once the question of “covered” or “not covered” is settled, there is not much difference in the effort to comply, regardless of the boundaries of the process.

However, even OSHA acknowledges that the boundaries of the process will determine which equipment must be “evaluated, designed, installed, operated (training and procedures), changed, and inspected/tested/maintained per OSHA PSM requirements.”  In other words, the extent to which .119(d) – process safety information, .119(e) – process hazard analysis, 119(f) – operating procedures, .119(g) – training, .119(j) – mechanical integrity, and .119 (l) – management of change impose regulatory liabilities on a facility has a direct relationship to the extent of the boundaries.

It is reasonable, moral, and ethical for a facility to seek to be as good at managing process safety as it can be while at the same time trying to minimize its regulatory liabilities as much as possible.  After all, time used proving to a regulator that they are safe is time that cannot be used for improving safety, and resources spent addressing fines and citations are resources not spent on improving safety.

Where Does It End?

Let’s begin by assuming that a facility isn’t trying to game the standard.  If they had two distinct units on site, far enough removed from one another that they wouldn’t be considered “co-located” and in no way connected to one another, it would be straight-forward to make the case that they were two processes by the definition of the PSM Standard. Each would need a separate evaluation to determine whether they were covered under the PSM Standard.

Consider a facility with five units, A, B, C, D, and E.  Unit A feeds Unit B.  Unit B feeds Unit C, which feeds Unit D, which in turn feeds Unit E.  The feed flows from one unit to the next through interconnected piping. By the PSM Standard definition of a process, these five units comprise a single process. If the contents of this singe process included an HHC in excess of its TQ, all of the equipment and procedures associated with any of these units would be covered by the PSM Standard.

If each of the units contained more than the TQ of an HHC, each unit would be individually covered, so it would make no practical difference whether this was considered five processes or one process.

As a second example, consider a similar facility, but one where only Unit A and Unit B contain an HHC. Unit A converts the HHC into a benign intermediate, e.g. the polymerization of a toxic monomer into an inert polymer, and contains less than the TQ of the HHC. Unit B modifies the output from Unit A into another benign intermediate using more of the same HHC, and again, it contains less than the TQ of the HHC. Together, though, they contain more than the TQ. Neither Unit C, Unit D, nor Unit E contain any of the HHC, or any other HHC.

While neither Unit A nor Unit B would, by themselves, be covered under the PSM Standard, they are part of the same process because they are interconnected, and because together they exceed the TQ, they are part of covered process.  But what of Unit C, Unit D, and Unit E, each of which contain no HHC? How far downstream does the covered process extend?

OSHA’s Interpretations

Where the language of the PSM Standard is not clear, OSHA has issued several letters of interpretation to assist in understanding what is required of the standard. An early OSHA letter of interpretation (APCI, 30-Nov-1994) made it clear that for a train of interconnected systems the boundaries extend to include all systems that contain an HHC that in aggregate result in exceeding the TQ.  Two interconnected systems, each containing less than the TQ but in aggregate containing more than the TQ are a single covered process; all the more, then, that two interconnected systems, one containing more than the TQ and one containing less than the TQ will also be a single covered process.

A memorandum of interpretation (AkzoNobel, 28-Feb-1997) laid out a three step analysis for determining whether a process contained enough material to be a covered process and to determine the limits of that process:

  1. Begin by considering all vessels that are interconnected and separate vessels that are co-located as a single process. In the examples, this includes all five units.
  2. Determine whether this single comprehensive process contains at any particular time a quantity of an HHC that exceeds the TQ. In both examples, they are covered processes.
  3. Include any aspect of the single process that contains the HHC as part of the covered process. Aspects of the single process that do not contain the HHC but could cause a release of HHC or interfere with mitigating the consequences of an HHC release are also part of the covered process. Otherwise, aspects that do not contain the HHC are outside the boundary of the covered process. In the second example, Unit C, Unit D, and Unit E would be outside the boundary of the covered process.

…But Could Cause a Release or Interfere with Mitigation

OSHA is serious about including certain aspects of a process that do not contain an HHC but could cause a release of an HHC or interfere with mitigating the consequences of an HHC release as part of the covered process. This goes so far as to include utilities.  (Yes, utilities.  Remember that OSHA’s definition of process is not the same as a process engineer’s definition of a process.) In a more recent letter of interpretation (API, 31-Jan-2008), OSHA goes to great lengths to explain why a utility may very well be part of a PSM-covered process, although it may not be. “It is OSHA’s long-standing position that utility systems are part of the PSM-covered process when employers use them to control/prevent and mitigate catastrophic releases of HHC.”

Do Not Contain the HHC

To be outside the boundaries of a PSM-covered process, equipment must not contain the HHC that resulted in the process being covered.  What does that mean? For flammable liquids, it is simple: if the liquid has a flash point below 100°F, it is an HHC.  Composition does not matter, only flash point.  Likewise, for flammable gases; any mixture that can be categorized as a Category 1 flammable gas is an HHC, regardless of its composition.

HHCs listed in Appendix A as toxic or reactive are a bit more complicated. There are nine chemicals on the list with minimum concentrations. If the concentration is above the minimum, then it is an HHC.  If the concentration is less than the minimum concentration, then it is not an HHC that contributes to the TQ under PSM.

Appendix A also lists seven anhydrous chemicals. If they become part of aqueous solutions, they are no longer HHCs that contribute to the TQ under PSM.

Finally, there are the remaining chemicals listed as HHCs in Appendix A. How low must the concentration be before the chemical can be ignored?  Parts-per-million?  Parts-per-billion? OSHA published a memorandum on 18-Jul-2016, describing its 1% test. If the concentration of an HHC in a mixture is less than 1%, the mixture is not an HHC that contributes to the TQ under PSM.  Moreover, even if the concentration is more than 1%, the mixture is not an HHC that contributes to the TQ under PSM if the partial pressure of the HHC at process conditions is less than 10 mmHg.

An Approach

This is an approach to setting boundaries we have found useful. As with the analysis described by OSHA in its 28-Feb-1997 letter of interpretation, this approach begins with a process as defined by the PSM Standard: interconnected and co-located vessels and their associated piping. It then sorts all equipment and piping into one of four groups.

Group 1: Any equipment or piping that contains HHCs that would contribute to the TQ.

Group 2: Any equipment or piping that is connected to Group 1 equipment or piping and contains HHCs that do not contribute to the TQ, i.e. mixtures with a flash point above 100°F or Category 2 flammable gas mixtures, mixtures that contain less than 1% HHC or have an HHC vapor pressure less than 10 mm Hg, mixtures with less than the listed minimum concentration, and anhydrous HHCs containing water.  Although unlikely, the failure of any of this equipment or piping could conceivably result in a catastrophic release of HHCs from Group 1 equipment.

Group 3: Any equipment or piping that does not contain an HHC, even under abnormal conditions, but serves to isolate Group 4 equipment and piping from Group 1 equipment and piping, is co-located with Group 1 equipment and piping so that its failure could cause the catastrophic release of HHCs, or the failure of which would disable a mitigation effect.

Group 4: Any equipment or piping that does not contain an HHC, even under abnormal conditions, the failure of which would not cause a catastrophic release of an HHC, and the failure of which would not disable a mitigation effect.

Group 1, Group 2, and Group 3 are included in the covered process.  Group 4 is not.  The analysis is done piece-by-piece, not unit-by-unit.

Setting Limits

Whether or not you take the approach described above or another approach, it is important to define the limits of your PSM-covered process and list the equipment that is included. This can then become the basis for assuring that all fourteen elements are being applied to the entire process, and just as important, focusing attention of the six equipment-specific elements on the equipment that needs that attention.

So, get a set of your P&IDs and take out your four different colors of highlighters, and go to it. We like pink for Group 1 and green for Group 4.


This blog is based on an earlier version, “Setting Limits: Boundaries of a PSM-Covered Process”, posted on 3-Oct-2017 by Elsevier in Chemicals & Materials Now!


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.