The Importance of a Robust Assessment Procedure in Protecting against Rapid Overpressure Hazards

The identification of a suitable basis of safety to protect, or prevent, against overpressure hazards from gas, vapour or dust explosions and runaway chemical reactions or decompositions is a prerequisite for safe operation. However, the process of successfully realising and implementing the basis of safety is critical to its success. This paper examines the critical phases of selection of the most appropriate basis of safety and its specification and implementation. Some potential pitfalls at each stage of the process are highlighted with short case studies. The common, and universal, approach to safety involves material or reaction characterisation, hazard assessment, identification of a basis of safety and design of the required functional safety systems. The overall success of the functional safety system(s) relies on each stage being well executed – deficiencies in any phase of the safety lifecycle will directly impact on the end result. Within the intended layers of protection, particular attention is paid to the technical aspects of selecting a basis of safety for rapid overpressure events resulting from explosions or runaway reactions in process plant. The criticality of a thorough technical understanding of the properties of the materials and processes is highlighted. The options for safe operation including prevention and protection systems are overviewed and criteria are proposed to enable selection of the most desirable basis for safety. The application of IEC 61508/11 to the design of a safety instrumented system with selection of appropriate safety integrity levels (SIL) is discussed, as are the increasingly important aspects of ATEX and the assessment of ignition sources arising from mechanical items of equipment.