A Realistic Approach to Relief Header and Flare System Design

Most existing pressure relief header and flare systems were originally designed by taking little or no credit for any of the multitude of mitigating measures, commonly referred to as safeguards, which are present in a typical operating facility. However, as plants have increased throughput and added process units, the relief header and flare systems are usually no longer adequate given the same conservative methods that were used in the original design. As such, operating companies are faced with the decision of either installing additional relief headers and flare capacity to bring the system into compliance with the original design methods or evaluating the relief header and flare capacity using more pragmatic design methods. To further complicate this decision, experience during flaring events is often not consistent with the theoretical calculations indicating that the system is undersized. It is imperative to distinguish between design or capacity modifications with a real safety benefit and those with a theoretical benefit.

With this in mind, a more pragmatic approach to the analysis of pressure relief header and flare systems that is consistent with recognized and generally accepted good engineering practices (RAGAGEP) is required. Quantitative risk analysis (QRA) can be applied to the problem to better understand the risk profile associated with these systems. The application of QRA renders it possible to evaluate the relief header and flare system by accounting for all relevant safe-guards as well as the frequency of all the relieving scenarios of interest. QRA method will be presented which has been utilized by several major operating companies …. Some fundamental principles of Risk Assessment methods will be discussed.