Elastic region tightening based on torque control method is conventional method of tightening a bolt. Axial force of the bolt is controlled by a torque wrench, however, it is not easy to achieve uniform bolt tightening force. When torque method is applied to flange joint assembly, the scatter of the bolt tightening foeces are large. They might cause the leakage of the internal fluid from a flange joint. Recently, plastic region tightening is remarked for critical applications, which provides good uniformity in bolt preloads and high preloads compared with the elastic region tightening. In this research, the plastic region tightening is applied to flange joint assembly and its superiority in uniformity of the bolt tightening force is demonstrated. For the tightening tests, JPI-4inch flange, spiral wound gaskets and M16 bolts were used. Axial force and elongation of all bolts in the flange were measured. Bolts were tightened by modified HPIS flange tightening procedure which incorporates the angle control method into the clockwise tightening sequence. Experimental results show that variations of the axial force in the plastic region was smaller than those in the clastic region. The influence of the elastic interaction on the axial force in the plastic region is also small. It is concluded that the application of plastic region tightening to flange joint assembly is effective for the leak-free joint and that the nominal diameter of the bolt can be reduced.

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