(57e) PHAs Must Have Equal Focus on Non-Normal Operations, Non Routine Operations and Alternate Operating Cases

Non-normal operations include startup, maintenance, and shutdown. Non routine operations are those that form part of normal operations but are carried out less frequently, once a month, such as pipeline pigging, reactor bed or drier bed regeneration, decoking. Routine operations are those carried out more frequently such as sampling, draining once a shift. The characteristics of these operations are greater involvement of operators in executing the tasks and greater reliance on supervisor checks as the main safeguard. Alternate operating cases are more common in batch plants but not uncommon in continuous operating plants, eg lean gas case and rich gas case.

Identifying hazards from normal operations remains the primary focus of most PHAs. While this could be justifiable, the amount of time allocated in PHA studies to analyse non-normal, non-routine, routine and alternate operating cases, is typically less than 10%, which may be too little, given that these operations have significant potential for leading to major accidents, in case of any omission. This bias may be a result of the guideword structure in HAZOP which is oriented towards analysing normal operational upsets in a continuous plant. The bias may also arise from greater emphasis on design aspects and less on operational aspects including the absence of operational details and procedures during design phase PHA studies. The other major reason is that the time required for a PHA study is estimated based on a count of P&IDs without factoring the need to analyze the same system on a P&ID for both normal and non-normal operations.

The author has facilitated a number of PHA studies and would like to share his experience in this paper presenting case studies and approaches to bridge the gap in the hazard identification process. One example of a Non-normal operation that results in an Alternate Operating Case is the scenario arising from continuing with amine circulation through an absorber in a gas processing plant after the forward gas flow has stopped due to any process upset. The composition of off-gas from the regenerator changes, after some time, from a CO2 rich stream of 100 tons/ hour to a hydrocarbon rich stream of less than 100kg/hr. This non-normal operation or its impact due to the change in composition of the off-gas stream to vent stack may not be captured in a typical PHA. As it happened, this resulted in a vent stack explosion in a facility.

In order to capture the hazards from such non-normal operations, the approach to conducting PHAs will need to be revisited. The time allocated has to be substantially increased and a separate session may need to be allocated for this. Also, the approach adopted for batch processing plants in terms of analyzing each step and task in a sequence will need to be incorporated, while analysing such operations in a continuous operating plant. While a PHA study during design phase may be sufficient to identify design safety measures to be incorporated, a detailed PHA will be required before the start-up phase to ensure that hazards from such operations are fully identified.

This paper presents a few examples of non-normal and non-routine operations in continuous operating plants and outlines the approaches that need to be adopted to effectively identify hazards and ensure that controls are adequate to mitigate the risks.