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.

September 7, 1995

Mr. William J. Kessack, Jr.
Project Manager
IES Incorporated
10 East 6th Avenue
Conshohocken, PA 19428

Dear Mr. Kessack:

This is in response to your letter of April 19, addressed to Mr. Ron Davies with respect to the Ciba Self-Medication, Inc. (CSM) facility and the application of OSHA's Process Safety Management (PSM) of Highly Hazardous Chemicals Standard, 29 CFR 1910.119.

It appears that this facility has the capacity to store flammable liquids exceeding or approximating 21,000 pounds (plus undefined quantities of n-butanol, isopropanol and ethanol). This amount was calculated from the data supplied to us in your letter [(50 x 55gal)+(100 x 5 gal)] [8.3lb/gal x 0.789sp.gr.]=21,283 lb.

You stated that increased production may require CSM to store greater than 10,000 pounds of alcohol. However, you are of the opinion that the storage and use of flammable liquids in excess of the threshold quantity (10,000 lbs) would be exempted. This opinion relied on the belief that the storage containers at your facility are NOT designed for pressures other than atmospheric operations and are stored at room temperature.

In this regard, please note that 1910.119(b) defines an atmospheric tank as a storage tank which has been designed to operate at pressures from atmospheric through 0.5 PSIG.

As you may be aware, numerous mathematical formulas relating to the temperature and vapor pressure of the gas phase in equilibrium with the condensed phase have been proposed. The Antoine equation was applied for the calculation of vapor pressures of ethyl alcohol (CH(3)CH(2)OH) at expected ambient temperatures, since it gives a good correlation with experimental values.

Since the vapor pressures of ethyl alcohol (the substance that you cited in your letter) at various room temperatures of 70, 80, 90, 100, and 110 degrees F were estimated to be 0.92, 1.27, 1.73, 2.33 and 2.93 PSIG, respectively, it seems that the storage containers used at your facility are designed to operate at pressures higher than 0.5 PSIG.

The obtained vapor pressures calculated in mm Hg at the various temperatures were converted to PSIG [(14.7 PSIA)/(760 mmHg)]. As it can be seen, even at the lowest expected temperature in the summer (70-80 degrees F), the pressure in the containers would be almost twice the exempted value of 0.5 PSIG. In fact, if these containers were designed to operate at exempted pressure criteria, the risk of bursting of the containers would become imminent.

Therefore, based on the limited information that you have provided in your subject inquiry, whenever the volume of the flammables exceeds the threshold quantity of 10,000 pounds, your facility would be subject to the Process Safety Management Standard, 1910.119.

Thank you for your interest in occupational safety and health. If you need further assistance, please contact Alcmene Haloftis of my staff at 202-219-8031.

Sincerely yours,



John B. Miles, Jr., Director
Directorate of Compliance Programs