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.

October 25, 1995

Sam Mannan, Ph.D, PE, CSP
Division Director
Process Safety and Risk Assessment
RMT/Jones & Neuse, Inc.
912 Capital of Texas
Highway South
Suite 300
Austin, Texas 78746-6163

Dear Dr. Mannan:

This is in response to your letter of July 24, addressed to Mr. Michael A. Marshall of the Office of Construction and Engineering, regarding the Process Safety Management (PSM) for Highly Hazardous Chemicals Standard, 29 CFR 1910.119 and its applicability to the specific scenarios that you have outlined. Your letter was forwarded to our office for response.

Following are the scenarios outlined in your letter and our response to each inquiry, respectively.

Scenario 1: Does simple blending or mixing (without a chemical reaction) of a flammable mixture (quantity less than 10,000 pounds) in proximity of multiple storage containers of flammable liquid with an aggregate in excess of 10,000 pounds require coverage under PSM?

Response: According to the information that you have provided, 1910.119 would apply to this scenario. Paragraph (a)(1)(ii) of the referenced Standard states that this Section is applicable to, "A process which involves a flammable liquid or gas (as defined in 1910.1200 of this part) on site in one location, in a quantity of 10,000 pounds or more. Please note that a "process" according to 1910.119, "means any activity involving a highly hazardous chemical including any use, storage, manufacturing handling, or the on-site movement of such activities. For purposes of this definition, any group of vessels which are interconnected an separate vessels which are located such that a highly hazardous chemical could be involved in a potential release shall be considered a single process."

Scenario 2(a): Does simple blending or mixing (without a chemical reaction) of a flammable mixture which is connected to a storage quantity greater than 10,000 pounds require coverage under PSM?

2(b): Please define blending, mixing and agitation if they are viewed differently with regard to PSM.

Response 2(a): Please note that the definition of a "process" does not include a chemical reaction as a necessary condition to determine the applicability of the Standard. Therefore, due to the vessel interconnectivity and the definition of a "process" as quoted in the previous response, this scenario would also be covered under 1910.119.

Response 2(b): Blending, Mixing and Agitation are not viewed differently with regard to PSM. That is, the method of achieving a desirable product or intermediate, etc., is not a condition of the Standard's applicability, according to 1910.119(a). As you may be aware, during the process of gas-liquid mixing in agitators, the materials involved in an air-gas mixture are capable of forming an explosive mixture (due to the simple process of evaporation), and due care must be exercised to eliminate possible sources of ignition. In addition, hazards of possible combustion could be expected during liquid-liquid mixing. Currently, OSHA reviewed an accident involving a blender. No chemical reaction was intended in this process. This particular blend, however, resulted in a major explosion including a number of fatalities. In this event, a leaking line resulted in the formation of a reaction between the water and the water reactive chemicals in the blender. This process was designed as a simple blending process, but due to the inadvertent leak, a massive explosion occurred. Therefore, the applicability of PSM is determined by the scope of 1910.119 which is under paragraph (a) and not by the method utilized to mix or blend chemicals.

Scenario 3: How is connectivity in general interpreted? For example, consider a system having a supply line disconnect such as a removable section of pipe which creates a non-connection to a process during process operation. Can we interpret the supply tanks/storage to be not covered by PSM?

Response: This would not be a correct interpretation. Again, please reference the definition of "process" above. In addition, so long as the threshold quantities listed in Appendix A are exceeded, 1910.119 would be applicable.

If you need further assistance, please contact Alcmene Haloftis of my staff at 202-219-8031.


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