“Space is something you have to define. Otherwise, it is like anxiety, which is too vague. A fear is something specific. I like claustrophobic spaces, because at least then you know your limits.”  — Louise Bourgeois

A group recently met to discuss locking out a process vessel for entry. They identified the agitator, all of the lines going into the vessel (including the nitrogen purge gas line), and the lines flowing out of the vessel (because of the potential for reverse flow from downstream equipment). After completing their thorough analysis, though, one of the team members asked, “Is this vessel even a confined space?”

The members of the team believed it was, but no one felt certain. And somehow, feeling certain didn’t seem enough.

So, what is a confined space?

Kinds of Spaces

The relevant OSHA standard for confined spaces is 29 CFR 1910.146, Permit-required confined spaces. What should be apparent from the title of the standard is that there are “permit-required” confined spaces, confined spaces that do not require permits, and other spaces.

The first distinction to make is between confined spaces and other spaces. A space is a confined space if it meets all three of the following criteria:

  1. It is large enough and so configured that an employee can bodily enter and perform assigned work.
  2. It has limited or restricted means for entry or exit.
  3. It is not designed for continuous employee occupancy.

If an employee cannot fit their entire body into the space and perform work, the space is not a confined space. A school locker is not designed for continuous occupancy, and it has limited or restricted means for entry or exit. But only if it was large enough to completely enter and then perform work, would it be a confined space. The cabinet space under a kitchen sink is not a confined space, not because employees cannot reach into it to get cleaning supplies or to work on a jammed garbage disposer, but because their entire bodies will not fit in and allow them to work. This is something OSHA clarified in its March 5, 2008 letter of interpretation.

“Limited or restricted means of entry or exit” typically means that entry or exit is not through a doorway or portal that a person can walk through. However, in its December 20, 1994 letter of interpretation on elevator pits, where technicians climb down a ladder to get to the pit door, OSHA states that “a space containing such a door or portal may still be deemed a confined space, if an entrant’s ability to escape in an emergency would be hindered.”

The final criterion for a confined space is that is not designed for continuous human occupancy. So, a toll booth, despite being a claustrophobic space for some, is still not a confined space. Neither is a prison cell for solitary confinement, despite the name.

Not All Confined Spaces are Permit-Required

A permit-required confined space is a confined space with any of the following hazards:

  1. It contains or has a potential to contain a hazardous atmosphere.
  2. It contains a material that has the potential for engulfing an entrant.
  3. It has an internal configuration such that an entrant could be trapped or asphyxiated by inwardly converging walls or a by floor which slopes downward and tapers to a smaller configuration.
  4. It contains any other recognized serious safety or health hazard.

When the only hazard in a confined space is actual or potential atmospheric hazard, it is possible to avoid the requirements of permit-required vessel entry, per paragraph (c)(5).

Per paragraph (c)(7)(i), it is possible to reclassify spaces from permit-required to non-permit spaces by eliminating all hazards, as long as none of the hazards are actual or potential atmospheric hazards. So, grain silos can be rendered non-permit spaces by removing all of the grain, and agitated vessels can be rendered non-permit spaces by locking out the agitator. On the other hand, a confined space classified as permit-required because of the atmospheric hazards remains a permit-required confined space even after the atmospheric hazards have been locked out.

A confined space with both actual and potential hazardous atmospheres AND with another hazard that can be locked out will remain a permit-required confined space, because paragraphs (c)(5) and (c)(7) are mutually exclusive.

Likewise, a vessel with anything other than a flat bottom is going to be permit-required confined space because there is nothing to be done to eliminate condition 3.


Something that sometimes causes confusion is the idea of “entry”.  OSHA defines “entry” as passing through an opening into a permit-required space and “is considered to have occurred as soon as any part of the entrant’s body breaks the plane of an opening into the space.” But just because a space has been entered (a part of the entrant’s body breaking the plane) doesn’t mean that the space is a confined space.

A space that OSHA gives as an example of a confined space is the space above a dropped ceiling. OSHA goes on to state that these spaces are not likely to be permit-required confined spaces, because of the absence of hazards. However, these spaces will only be confined spaces if there is room above the dropped ceiling for an entrant to put their entire body. When the height above the dropped ceiling is several feet, this is a confined space. On the other hand, if the height is less than a foot, it is not.  In either case, however, lifting a tile and reaching into the space is an entry.

Reaching into the cabinet under the sink, reaching into a 6” filter housing to change the cartridge, or being stuffed into a school locker all constitute entry, but not confined space entry, permit-required or otherwise.

About That Process Vessel

What about that process vessel that the group evaluated, with the gut feeling that it was a permit-required confined space? It was 6’ diameter and 8’ tangent-to-tangent, so it was big enough to enter. A 2’ dia. by 3’ t/t tank would not have been big enough to enter. It had a 20” hatch on top, so it could be entered, but with limited or restricted means. A 12” hatch would not be big enough to allow entry. And categorically, process vessels are not designed for continuous human occupancy. Therefore, it was a confined space.

Was it a permit-required confined space? It had nitrogen line, with a potential for an asphyxiating atmosphere. The other lines connected to the vessel also had the potential for creating a hazardous atmosphere. That’s enough to make it a permit-required space and would disqualify the vessel from the exceptions allowed for in 29 CFR 1910.146(c)(7).

The vessel was equipped with an agitator, a recognized serious safety or health hazard. Sure, the agitator could be locked out—and should be for vessel entry—but this hazard disqualified the vessel from the exceptions allowed for in 29 CFR 1910.146(c)(5). Even without an agitator, the bottom of the vessel was a dished head, meaning the floor sloped downward and tapered to a smaller configuration.

Clearly and unambiguously, the process vessel was a permit-required confined space and needed to be marked accordingly.

Evaluate Your Workplace

There is a requirement in the Confined Space Standard that applies to all employers: to evaluate the workplace to determine if any spaces are permit-required confined spaces, and if there are any permit-required confined spaces, to post danger signs (or using other equally effective means) warning of the spaces and their hazards. It is not possible to do this if you don’t know what permit-required confined spaces are. Even if there are none, it is wise to have evidence that the evaluation was done.


  • 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.