The Research And Optimization On The Tightening Scheme Of Bolted Flange Connections

Adequate and uniform bolt preload is a crucial factor of the bolt flange sealing system. Based on the experiment and numerical simulation, the rules of the elastic interaction during the bolts tightening process were researched in this paper. To provide appropriate references for the bolt-up process of bolted flange connections, an optimization tightening scheme was therefore developed using the elastic interaction coefficients method. The main work and conclusions are as follows:(1) The experimental apparatus for comprehensive performance of bolted flange connections was set up, which can monitor the load of bolts during the bolt-up process. It also can measure the leakage rate of the device, hence making the research on the rules of the elastic interaction during the bolt-up process possible.(2) Experiments were conducted on the ordinary tightening patterns, such as the cross and the sequential pattern, in order to research the rules of the elastic interaction. Then it was divided into the positive interaction and the negative interaction. The relationships between both interactions and gasket stiffness, loading increment and tightening pattern were acquired. Experimental results reveal that the intension of positive interaction in the sequential pattern is more obvious than in the cross pattern. However, the intension of negative interaction is the same in both the patterns. With lower gasket stiffness and higher loading increment, the intension of both interactions is weakened.(3) The FE model of bolt-up process of bolted flange connections was established. Comparison between the results of experiment and simulation shows that the model is accurate. Based upon this, the following conclusions were obtained.Bolt tightening has slightly influence on the flange deformation. Therefore the negative interaction only affects the adjacent bolts. Bigger flange nominal diameter and more bolts make the negative interaction stronger, but change the positive interaction unsignificantly. Furthermore, bolt load distribution of each pass depends on the times of negative interaction. More times of negative interaction makes the bolt load become smaller.Under the same condition, comparing with the graphite filled metal spiral wound gasket, the stress of the PTFE gasket is more uniform with lower stress level. Because the gasket stress distribution depends on the distribution of bolt load, therefore a lower gasket stress can be found in both cross and sequential tightening patterns.(4) According to the experiment and simulation result, an elastic interaction coefficient defined by bolt load increment is presented. It is found that the negative interaction coefficient is constant during the bolt-up process. According to this, the modified bolt load calculation method is developed, which can be used to offset the effect of negative interaction. At the same time, the effect of positive interaction is weakened by the cross and sequential combined tightening sequence. Further more, the criterion of bolt target load in multi-pass tightening scheme is presented. Thus, the design method of the optimization tightening scheme of bolted flange connections is established.(5) Experimental and numerical simulation methods were used to evaluate the effect of optimization tightening scheme. Results indicate that, comparing to the target load, the application of the above methods can achieve a higher average load with a lower dispersion. Therefore, a more uniform and sufficient gasket stress can be provided. In addition, the optimization tightening scheme is also applicable to flanges with different nominal diametesr and different bolt numbers.