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 can consult OSHA's website at http://www.osha.gov.

April 6, 1981

Jerry D. Smith, P.E.
Principal Mechanical Engineer
NUS Corporation
South Central Operations
11511 Katy Freeway, Suite 500
Houston, Texas 77079

Dear Mr. Smith:

This is in response to your inquiry regarding OSHA Standard 1910.169(b)(3)(ii), which was forwarded to us for reply from our Dallas Regional Office. Please accept our apology for the delay in response.

29 CFR 1910.169(b)(3)(ii) clearly states: "No valve of any type shall be placed between the air receiver and its safety valve or valves," while 29 CFR 1910.169(a)(2)(ii) states, in part: "All safety valves used shall be... installed... in accordance with the A.S.M.E. Boiler and Pressure Vessel Code, Section VIII Edition 1968."

Your reference to the ASME Boiler and Pressure Vessel Code, Section VIII, allowing a full-area stop value between the vessel and the relief device is contained in Appendix M, Installation and Operation, Paragraph UA-354(a)(copy enclosed). Note that the reference to Appendix M is in Paragraph UG-134(e)(2) (copy enclosed), which states the exception as: "Under conditions set forth in Appendix M.", and that Paragraph UA-350, Introduction to Appendix M, indicates that any rule contained in Appendix M is for general information only, not a mandatory part of Section VIII, but may be permitted when granted by the authority having legal jurisdiction over the installation of unfired pressure vessels.

Therefore, the jurisdiction in the state or municipality operating under the ASME Code Section VIII may authorize a stop valve between the pressure-relieving device and the unfired pressure vessel. (Note the definition of "Jurisdiction" or "Jurisdictional Authority" in Chapter I Glossary of Terms (copy enclosed) of the National Board Inspection Code - A Manual for Boiler and Pressure Vessel Inspectors.) Such an authorized stop valve should be considered a de minimis situation, provided the conditions set forth in Paragraph UA-354(a) or (b), whichever is applicable, are met.

For your information, 29 CFR 1910.170 indicates that [T]he source for 29 CFR 1910.169 is the 1938 ANSI B-19 Safety Code for Compressed Machinery, which refers to the 1937 edition of the ASME Code for Unfired Pressure Vessels. The 1937 edition did not contain the exception for a stop valve indicated above in Paragraph UG-134(e)(2) of the 1968 edition.

[This document was edited on 11/09/00 to strike information that no longer reflects current OSHA policy.]

Should you have any further questions, please feel free to call or write.


Bruce Hillenbrand
Deputy Director,
Federal Compliance and State Programs