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 https://www.osha.gov.

July 19, 1977

Mr. Lawrence W. Bierlein
Attorney at Law
910 Seventeenth Street, N.W.
Washington, D.C. 20006

Dear Mr. Bierlein:

This is in response to your letter of May 12, 1977, to Mr. Thomas H. Seymour concerning your interpretations of 29 CFR 1910.106(e)(3)(vi) and 29 CFR 1910.106(e)(7).

In order to address your interpretation of the fire wall requirements in 1910.106(h)(4)(i)(c) we reviewed NFPA 30-1969 the national consensus standard upon which our standard is based and other technical fire protection literature. Paragraph 8313 of the NFPA standard was adopted, in part, by OSHA as our paragraph 1910.106(h)(4)(i)(c) and it reads as follows:

"8313. Storage tanks inside of buildings shall be permitted only in areas at or above grade which have adequate drainage and are separated from the processing area by construction having a fire resistance rating of at least two hours.

Note: This is not intended to prevent processing equipment from containing flammable or combustible liquids in such quantities as are essential to continuity of daily operations. Day tanks, running tanks, surge tanks, and feed tanks are permitted in process areas."

I say adopted in part because OSHA deleted the explanatory note found at the end of paragraph 8313 of the NFPA standards because it is non-regulatory in nature.

In light of our review and in our professional opinion, the tanks described in your letter can be considered as processing equipment and as part of a process line. They would not be considered as storage tanks if they are used in the manner you describe. Therefore, because paragraph (h)(4)(i)(c) applies only to storage tanks, there is no need for compliance with 29 CFR 1910.106(h)(4)(i)(c).

Your interpretation of the electrical requirements causes some concern. The measurements of vapor concentrations inside of tanks, along the process line and above spill areas presents impressive values. A workplace area with vapor concentrations this low is not likely to suffer a fire or explosion. While specific values in terms of percent of flammable vapor concentrations for determination of Class 1, Division 1 or 2 locations, have not been developed by OSHA, surely any concentration kept below 5 or 10% of the lower flammable limit would not expose employees to a fire or explosion hazard. Generally, concentrations in the area of 15 to 25% say, would probably necessitate Class 1, Division 2, wiring and we believe anything exceeding 60% or so of the lower flammable limit would need Class 1, Division 1, wiring. These values are based on professional opinion and are not intended to set any specific limits at this time.

Our concern revolves about the problems associated with maintaining the concentration of flammable vapors at those levels which were measured throughout the plant or exposed area. If there is positive assurance that these values are not exceeded, we could concur with your interpretation. However, what would happen if the system failed would flammable vapors develop without ventilation? Is there any control of the vapors being released from the containers holding the flammable liquid? What safeguards would be provided to assure that employees would not be trapped in a hazardous workplace which is rapidly filling with flammable vapors?

Unless there is a fail-safe means to provide for employee safety, such as vapor concentration measuring device interconnected with an employee evacuation alarm and an employee evacuation plan or fail-safe ventilation, we would have to enforce the standards for the 20 and 3 foot distances.


John K. Barto,
Chief Division of Occupational Safety Programming