An Integrated Approach to Design Hybrid Independent Protection Layers

摘要

A popular method to assess process risks semi-quantitatively and determine the performance requirements of safeguards which may prevent the materialization of the risk scenario is a fault propagation modeling technique called Layer of Protection Analysis (LOPA). It is popular because it is neither excessively complex nor time consuming, and provides better accuracy than qualitative estimates. The principle behind LOPA is to determine effective safeguards, called independent protection layers (IPL), capable of preventing and/or mitigating the consequence. The safeguards qualify as IPLs only if they are specific, independent, reliable, and auditable [1]. Companies have developed procedures to apply LOPA methodology, pre-establishing possible initiating causes and their likelihoods, as well as IPL types. IPL types are mostly mechanical (pressure relief valves, rupture disks, check valves, restrictive office, mechanical over-speed trip, etc.), or instrument related (control loops, alarms, SIF). IPL type lists are usually restrictive and a combination of mechanical and instrumented systems is not commonly addressed. Many times, during LOPA exercises, the identified safeguard cannot meet the specificity requirement because it is not able to fully prevent the consequence of the initiating event/scenario considered; therefore, it is disqualified as an IPL. This paper presents, through a guided example, a quantitative risk assessment methodology to integrate into a single protection layer, mechanical and instrumented subsystems that separately would not meet the specificity requirement or would provide a negligible RRF for the risk scenario. The methodology is of special interest for scenarios that need a risk reduction of three or more orders of magnitude, where the development of a single protection layer that prevents the scenario may not be feasible or practical [2].