Hazard evaluations, also called process hazard analysis (PHAs) have been performed formally in gradually improving fashion for more than five decades. Methods such as HAZOP and What-If+ Analysis have been developed and honed during this time. But, one weakness identified 25 years ago still exists in the majority of PHAs performed around the world. Most PHAs do not thoroughly analyze the errors that can occur during startup, shutdown, and other non-routine (non-normal) modes of operations; sadly the commonly used approaches for PHA of continuous mode of operation only find about 5 - 10% of the accident scenarios that may occur during startup, shutdown, and online maintenance. This is despite the fact that about 70% of major accidents occur during non-routine operations. Instead of focusing on the most hazardous modes of operation, most PHAs focus on normal operations (e.g., HAZOP of equipment nodes). In a majority (perhaps more than 80%) of both older operations and new plants/projects, the non-routine modes of operations are not analyzed at all. This means that perhaps 70% of the accident scenarios during non-routine operations are being missed by those PHAs. If the hazard evaluation does not find the scenarios that can likely occur during these non-routine operations, the organization will not know what safeguards are needed against these scenarios.
Chapter 9 of “Guidelines for Hazard Evaluation”, 3rd Ed, 2008, AIChE/CCPS requires hazard evaluations of all hazards of the process during all modes of operation. The US OSHA PSM regulation requires PHA of all hazards during all modes of operation as well, and several key citations since 1990 have focused on PHA of non-normal modes. US CSB and other agencies have also recognized this weakness in PHAs.
This paper explains the business case for doing PHAs of procedure steps for non-routine modes of operation, while also describing the growing regulatory pressure from US OSHA and others. The paper recaps the practical ways to efficiently and thoroughly analyze the step-by-step procedures that are used to control non-routine operating modes, as well as those for batch and between batch operations. This paper builds upon the methods and rules provided in papers beginning in 1993 and builds upon the CCPS textbook just cited. The reader will be able to use the results of this paper to estimate the number of accident scenarios they may be missing and to estimate the time it would take to complete an efficient and thorough PHA of the non-routine modes of operation. The reader will also be able to alert their management to the growing regulatory impetus for such analyses.