Common Mistakes in PSV Sizing: Lessons Learnt and Case Studies From Years of Operation

Abstract Pressure safety valves (PSVs) are important for oil and gas facilities as they usually act as the final protection layers to prevent loss of containment events. Despite its significance, many sizing mistakes have been found from thorough design verification process. This paper analyzes the common mistakes found and discusses the misinterpretation of PSV sizing criteria. Real-world examples and case studies are used to demonstrate the impact of these errors on safety and equipment integrity. This study conducts a comprehensive analysis of safety valve sizing methodologies, utilizing field data and historical case studies. Key focus areas include incorrect assumptions about flow conditions, misapplication of double jeopardy scenarios, and improper consideration of operational effects like backpressure, as well as neglecting other flow conditions beyond governing cases. The study examines the impact of deviations from industry standards like API 520 and 521 on sizing mistakes. Computational simulations model many errors and assess their impacts on process safety. Manufacturer guidelines are analyzed to identify inconsistencies with field practices, and recommendations for best practices in confusing areas are suggested. The investigation found that overlooking backpressure in safety valve selection leads to insufficient valve sizing and lower valve capacity. Another common mistake is using inaccurate relief load calculations from simplified flow models that don't represent actual flowing conditions, especially in multiphase flow situations or when liquid passes through upstream vessels followed by gas phases that may cause liquid swell or gas-induced liquid displacement. Incorrect assumptions regarding inlet and outlet piping pressure losses often led to an underestimation of the necessary valve size, thereby increasing the risk of overpressure events. This is typically observed when comparing the relief load calculations for each case, with the governing case being solely used for sizing the pressure drop in inlet and outlet piping. The fluid properties of other non-governing cases may produce much worse results and should be considered when assessing. The study also found that ignorance of other flow characteristics such as assuming a fixed pressure drop in the flare tip could result in a 50 - 60% reduction in valve performance, significantly compromising system safety. To avoid these errors, it is essential to validate relief load calculations thoroughly, evaluate backpressure more carefully, and adhere to updated API standards. This work presents an extensive analysis of common mistakes in sizing safety valves, supported by actual case studies and simulations. Furthermore, it provides practical suggestions for preventing these errors, therefore promoting safer operations and improved equipment reliability in the oil and gas industry. The results are essential especially for engineers engaged in the design of pressure relief systems and the selection of safety valves.