Consistency Analysis of Pretightening Force in Multistage Flange Connection Bolts

Uniform bolt load plays an important role in the sealing performance of bolted flange connection structures. To explore the relationship between bolt loading technology and the consistency of pretightening, a finite element model was established based on a bolted flange connection structure, the contact characteristics and pretightening force variations of the joint surface between the flange and gasket were analyzed, and a bolt loading experiment was conducted using a multistage six-bolt flange structure of a certain pressure vessel. In the experiment, variable factors such as loading sequence, loading rounds, reloading rounds, number of reloaded bolts, and reloading mode were set as control groups and compared with the results obtained using the loading method recommended by ASME PCC-1—2022. The mean value and dispersion coefficient of the bolt group preload were analyzed to identify the optimal loading technology applicable to real-world conditions. A high-precision digital torque wrench was used to apply the bolt load, and ultrasonic equipment was employed to monitor the pretightening force in real time. The results show that adopting an appropriate loading sequence and increasing the number of loading rounds can effectively improve the consistency of the pretightening force within bolt groups. The introduction of a reloading process further enhances the consistency of the pretightening force compared with the ASME PCC-1 method, with greater improvement observed as the number of reloading rounds and reloaded bolts increased. Moreover, applying the torque-angle method for single bolts during the final reloading process further reduces the dispersion coefficient of the bolt group pretightening force.