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.

September 21, 1992

Mr. Gary Stubblefield
Senior Technician
Petrocon Engineering, Inc.
P.O. Box 20397
Beaumont, Texas 77720-0397

Dear Mr. Stubblefield:

This is in response to your letter of June 18 to Mr. Gilbert J. Saulter, Dallas Regional Administrator for the Occupational Safety and Health Administration (OSHA). Your letter was forwarded to the Directorate of Compliance Programs for response. Please accept our apology for the delay in responding.

In your letter you requested an interpretation on the Final Rule on Process Safety Management (PSM) of Highly Hazardous Chemicals published in Volume 57, Number 36, of the Federal Register on Monday, February 24, 1992. In particular you sought clarification on the need for information concerning computerized process control systems and safety interlock system software used in the petrochemical, refining and pulp and paper industries. You asked whether simplified loop diagrams or narrative descriptions could be used to describe the logic of software and the relationship between the equipment and computerized process control systems, to meet the requirements for written operating procedures at 1910.119(f)(1). Also, you asked whether system logic flow charts or narrative descriptions of the computerized safety interlock systems could be used to meet these same requirements.

It is anticipated that employers would include loop diagrams, flow charts and narrative descriptions of control and interlock systems in their compilations of written process safety information required by 1910.119(d) before they conduct any process hazard analyses (PHA's) required by 1910.119(c). After the PHA's are completed, written operating procedures must be developed to provide clear instructions for safely conducting activities involved in each covered process, consistent with the process safety information and with the associated PHA. Simplified diagrams, flow charts and narratives could be used in conjunction with instructions to meet the requirements for written operating procedures at 1910.119(f)(1). Flexibility such as this is provided to employers to comply with the performance- oriented PSM Standard.

Thank you for your interest in occupational safety and health. If we may be of further assistance, please contact us.


Patricia K. Clark, Director
Directorate of Compliance Programs