Abstract
Abstract
For high hazard process facilities, management of Change (MOC) for safety critical instrumentation needs careful evaluation to ensure that the operational improvements do not affect functional safety. A framework integrating Hazard and Operability Study (HAZOP), Layer of Protection Analysis (LOPA) and risk graph is presented to assess the risk of a process and the required Safety Integrity Level (SIL) of the safety instrumented functions (SIF) during modification of a Delayed Coker Unit (DCU). The methodology provides a systematic connection between process deviations and initiating causes found in HAZOP and consequence assessment, initiating-event likelihood, independent protection layers (IPLs), residual risk gaps, and SIL requirements. The framework was used on the H-01 Coker Heater, focusing on the proposed addition of the ultraviolet (UV) scanner alarm/trip signals to the heater shutdown interlock. The protection architecture in place was also taken into account, and the effectiveness of the protection system following the proposed change was checked by considering the current hazardous scenarios. The results showed that the integrated analysis approach gave an understandable foundation for identifying risk-important scenarios and the extent to which IPLs are adequate. Three more safeguards were determined to be required to fill risk gaps that remain: maintaining minimum openings of 11-FV-131 and 11-PV-170, and adding a low pressure alarm to 11-PIC-170. After the above actions the identified risk gaps were closed and appropriate SIL requirements were determined for the evaluated SIFs, based on the risk-graph assessment. The results show that the use of the UV-scanner trip signals can only be technically justified if risk gaps have been identified and can be shown to be closed by the use of the UV-scanner, and the credited safeguards can be considered to be independent, available and properly maintained. Thus, the integrated HAZOP–LOPA framework also provides a structured and auditable basis for MOC decisions concerning safety-instrumented systems, and provides a practical means of achieving a balance between the process safety, instrumentation reliability, and operational continuity requirements in high-hazard process facilities.