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Standard Interpretations - Table of Contents
• Standard Number: 1910.146; 1910.146(c)(7) ; 1910.146(d)(3)

August 6, 2007

Mr. Ken Wilcoxson
945 Calle Del Encanto
Las Cruces, NM 88005

Dear Mr. Wilcoxson:

Thank you for your September 19 letter to the Occupational Safety and Health Administration's (OSHA) Directorate of Enforcement Programs (DEP). Your letter has been referred to DEP's Office of General Industry Enforcement (GIE) regarding an interpretation of 29 CFR 1910.146, the Permit-required confined spaces (PRCS) standard. Your scenario, diagram, and questions have been restated below for clarity.

Scenario: A permit-required confined space consists of a combustion turbine (CT) and the exhaust duct of a GE Frame 7FA commercial combustion turbine. See diagram below. The permit space contains the following hazards: hydraulically operated diverter damper blade, rotating third-stage turbine blades, no lighting, tripping hazards from exposed studs and bolts, high pressure natural gas fuel supply, and #2 liquid fuel oil to supply the combustion turbine.


Question 1: Since the CT and exhaust duct have an actual hazardous atmosphere during normal operation, is the combustion turbine/exhaust duct/exhaust stack eligible for reclassification per §1910.146(c)(7)1 when the CT is shut down?

Response: Reclassification under §1910.146(c)(7) is not available for permit spaces with potentially hazardous atmospheres, as appears to be the case here. (As the manufacturer's technical manual states, the valves at issue are not isolation valves and have the potential to leak.)

Question 2: When the CT is in shut-down mode, can the exhaust duct be reclassified per §1910.146(c)(7)? The National Fire Protection Association, Electric Power Research Institute, and the Combustion turbine manufacturer all indicate that there is a significant possibility of a potential hazardous atmosphere (atmosphere greater than the Lower Explosive Limit (LEL), enough to warrant that the control system and design standard mandate a purge of the CT and exhaust system.

Response: Since the permit space has a potential to contain an atmospheric hazard, reclassification per §1910.146(c)(7) would not be available.

Question 3: Would the natural gas and liquid fuel supplies be considered "flowable" sources of a potential hazardous atmosphere?

Response: The term "flowable" is found within the definition of "engulfment." The PRCS standard states, "Engulfment means the surrounding and effective capture of a person by a liquid or finely divided (flowable) solid substance that can be aspirated to cause death by filling or plugging the respiratory system or that can exert enough force on the body to cause death by strangulation, constriction, or crushing." Therefore, the liquid fuel supply may pose an engulfment hazard, but the natural gas would not be considered an engulfment hazard. However, natural gas supplies within a confined space may still trigger the permit space classification. Natural gas and liquid fuel can each create a hazardous atmosphere or potential hazardous atmosphere.

Question 4: When the natural gas and liquid fuel supplies are isolated2 in accordance with §1910.146, would the space be eligible for reclassification in accordance with §1910.146(c)(7)?

Response: No, the space could not be reclassified to a non-permit space, since the space would still pose potential atmospheric hazards because, as you note, the valves are not isolation valves and may leak. In addition, the physical hazards (e.g., rotating parts, tripping hazards) in the space have not been eliminated.

Question 5: If the natural gas and liquid fuel supplies were not isolated in accordance with §1910.146, could the space be entered using the permit system?

Response: As part of the §1910.146 permit program and permit system, isolation measures must be implemented and documented. If an employer is unable to maintain acceptable entry conditions to protect the entrant from hazards or potential hazards during entry, then entry cannot be authorized.

Question 6: With regard to "isolation" and "lockout/tagout" of a flowable material, is it permissible to use solenoid valves if these devices are not capable of being locked out i.e., no provision for attaching a lock is provided?3

Response: Under §1910.146, isolating a fluid flowing through a pipe by double block and bleed4 does not specifically require that the valves be capable of being locked out. Double block and bleed is "the closure of a line, duct, or pipe by closing and locking or tagging [emphasis added] two in-line valves and by opening and locking or tagging a drain or vent valve in the line." Therefore, the valves and drain/vent could be tagged in accordance with the standard assuming the valves are proper for the intended application.

However, pursuant to §1910.146(d)(3), the means, procedures, and practices necessary for safe permit space entry operations must be developed and implemented. It is not clear in your scenario and diagram5 whether the valves are opened and closed by a controller located in a different area than the valve(s). If remote operation of the valve is possible, clearly tagging the in-line valves alone would not protect an entrant from the valve being actuated remotely. Under this circumstance, additional means, procedures or practices must be employed to assure that these valves cannot be opened (or closed as in the case of the drain) remotely.

In addition, the only appropriate isolation means that address a fluid flowing through pipes are blanking or blinding; misaligning or removing sections of lines, pipes, or ducts; or a double block and bleed system as described. Simply locking or tagging out a piping system, pursuant to §1910.147, is not appropriate for fluid isolation purposes.

As you may be aware, the State of New Mexico operates its own occupational safety and health program under a plan approved by Federal OSHA. Under this plan, the New Mexico Environment Department promulgates and enforces occupational safety and health standards under authority of State law, and posts them on its website at New Mexico generally adopts standards that are identical to the Federal, and any different standards must be at least as effective as OSHA's. State Plans must interpret an identical state standard in a manner consistent with OSHA's interpretation (and/or appropriate appellate court decisions), and a different standard in a manner at least as effective as OSHA's. For information on the New Mexico permit-required confined spaces standard and its enforcement, we suggest that you contact:
Mr. Butch Tongate, Bureau Chief
Occupational Health and Safety
New Mexico Environment Department
525 Camino de los Marquez, Suite 3
Santa Fe, New Mexico 87505
Telephone: 505-876-8700
Thank you for your interest in occupational safety and health. We hope you find this information helpful. 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 may consult OSHA's website at If you have any further questions, please feel free to contact the OSHA Office of General Industry Enforcement at (202) 693-1850.


Richard E. Fairfax, Director
Directorate of Enforcement Programs

1 29 CFR 1910.146(c)(7)(i) states, "If the permit space poses no actual or potential atmospheric hazards [emphasis added] and if all hazards within the space are eliminated without entry into the space, the permit space may be reclassified as a non-permit confined space for as long as the non-atmospheric hazards remain eliminated." [ back to text ]

2 29 CFR 1910.146(b) defines isolation as "the process by which a permit space is removed from service and completely protected against the release of energy and material into the space by such means as: blanking or blinding; misaligning or removing sections of lines, pipes, or ducts; a double block and bleed system; lockout or tagout of all sources of energy; or blocking or disconnecting all mechanical linkages." [ back to text ]

3 An energy isolating device, such as a valve, is considered "capable of being locked out" if it meets one of the following requirements: is designed with a hasp or other part to which a lock can be attached; has a locking mechanism built into it; or can be locked without dismantling, rebuilding, or replacing the energy isolating device or permanently altering its energy control capability (such as using a lock/chain assembly on a pipeline valve or a lockable valve cover). Please note, pursuant to §1910.147, push buttons, selector switches and other control circuit type devices are not energy isolating devices. [ back to text ]

4 29 CFR 1910.146(b) defines double block and bleed as "the closure of a line, duct, or pipe by closing and locking or tagging two in-line valves and by opening and locking or tagging a drain or vent valve in the line between the two closed valves." [ back to text ]

5 We note that the provided diagram does not indicate that a double block and bleed system is available for the liquid fuel line. [ back to text ]

Standard Interpretations - Table of Contents

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