Single knife gate valve for confined-space isolation

November 12, 2021

Brian Monistere, P.E., CSP

Professional Safety Services

[Recipient address omitted]

Dear Mr. Monistere:

Thank you for your letter to the Occupational Safety and Health Administration (OSHA), regarding permit required confined space (PRCS) entry requirements. In your letter, you seek clarification from OSHA on PRCS isolation methods. This letter constitutes OSHA's interpretation only of the requirements herein, and may not be applicable to any questions not delineated in your original correspondence. Your paraphrased questions, and our responses, follow.

Background: An employer has installed a single knife gate valve equipped with a bleed valve at the bottom of the assembly. Sleeves or gaskets within the knife gate valve form a tight seal when the valve is closed.

Two relevant definitions include:

"Isolation" (29 C.F.R. § 1910.146(b)) means 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.

"Energy isolating device" (29 C.F.R § 1910.147(b)). A mechanical device that physically prevents the transmission or release of energy, including but not limited to the following: A manually operated electrical circuit breaker; a disconnect switch; a manually operated switch by which the conductors of a circuit can be disconnected from all ungrounded supply conductors, and, in addition, no pole can be operated independently; a line valve; a block; and any similar device used to block or isolate energy. Push buttons, selector switches and other control circuit type devices are not energy isolating devices.

Question: In reading these two definitions, can it be interpreted that a single line valve is not permitted for isolation of hazards during entry into a confined space?

Response: Yes. 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." Under 1910.146(b), "blanking or blinding" means the absolute closure of a pipe, line, or duct by the fastening of a solid plate (such as a spectacle blind or a skillet blind) that completely covers the bore and that is capable of withstanding the maximum pressure of the pipe, line, or duct with no leakage beyond the plate. Under 1910.146(b), "double block and bleed" means "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."

The purpose of isolation is to prevent the potential passage of toxic, flammable, or other dangerous materials or energy into the permit space during occupancy. When chemical or gas lines are connected to a permit space, they must be isolated by such means as blanking or blinding, misaligning or removing section of lines, pipes, or ducts, or a double block and bleed system. A single knife gate valve is an isolation device, but used singly with a bleed valve at the bottom of the assembly, would not meet the OSHA's requirements under 1910.146 for isolation of the permit space.

Thank you for your interest in occupational safety and health. We hope you find this information helpful. OSHA's requirements are set by statute, standards, and regulations. Our letters of interpretation do not create new or additional requirements but rather explain these requirements and how they apply to particular circumstances. This letter constitutes OSHA's interpretation of the requirements discussed. From time to time, letters are affected when the Agency updates a standard, a legal decision impacts a standard, or changes in technology affect the interpretation. To assure that you are using the correct information and guidance, please consult OSHA's website at http://www.osha.gov. If you have any questions, please feel free to contact the Directorate of Enforcement Programs at (202) 693-2100.

Sincerely,

Kimberly Stille, Acting Director

Directorate of Enforcement Programs