Objective Flange leakage verification is a critical component of stress analysis in piping. Evaluating flange leakage risks under complex conditions using appropriate methods and controlling them within limits specified by relevant codes is essential for the safe and stable operation of oil and gas storage and transportation systems. However, numerous flange leakage verification methods exist domestically and internationally, differing significantly in application scope, focus, computational complexity, and conservatism. For LNG cryogenic piping, no unified consensus has been reached on the preferred leakage verification method, and the potential leakage hazards caused by the thermal arching effect are often overlooked.

Methods Based on a review of international mainstream flange leakage verification methods, a piping analysis model was established using CAESAR II software, with the DN300 low-pressure export reserved flange of an LNG terminal as an example. Leakage verification under normal operation and thermal arching conditions was conducted using the equivalent pressure method, the NC 3658.3 method, and the ASME Section VIII Division 2 method. The applicability of each method to LNG cryogenic piping was analyzed by comparing the verification results, and effective measures to mitigate flange leakage risks were discussed.

Results The results indicated that under normal operating conditions, the equivalent pressure method yielded the most conservative results, the NC 3658.3 method was relatively less conservative, and the ASME Section VIII Division 2 method offered moderate results with a broad application scope and convenient calculation. Under thermal arching conditions, the flange bending moment increased sharply, leading to equivalent pressure far exceeding allowable limits and significantly elevated leakage risk. The study confirmed that axial force and bending moment at the flange could be effectively reduced by installing natural compensators, optimizing support layouts, and controlling the temperature difference between the pipe’s top and bottom. Additionally, increasing the flange rating raised the allowable pressure, ensuring calculated pressures remained within allowable limits.

Conclusion The ASME Section VIII Division 2 method is recommended as the preferred approach for flange leakage verification in LNG piping during the engineering design stage, with particular emphasis on assessing leakage risks under thermal arching conditions. Additionally, optimizing piping layout, controlling the pre-cooling rate, and increasing flange rating are three effective measures to reduce leakage risks. Flanges of at least Class 300 are advised for LNG cryogenic piping.