Tightening method for flange joints proposed by ASME PCC-1 specifies that bolts are tightened by the cross-pattern sequence and tightening torque is raised with several steps. On the other hand, new tightening method has been proposed by Japan BFC committee, in which bolts are tightened in the clockwise-pattern sequence and tightening torque is 100% of the target torque in all steps after an install step with some snug torque. Tightening tests using flanges with various nominal sizes, performed in this study, show that the new tightening method achieves comparable uniformity in bolt preloads and in flange alignment with ASME PCC-1 procedure. This new tightening method is able to reduce both the work volume in the tightening operation and the possibility of human errors like missing the tightening order in it. Step-like increment of the bolt tension under the repeated tightening with small increment of the tightening torque is also discussed. Experimental results show that continuous control of the bolt tension by the repeated tightening is impossible. The step-like increment of the bolt tension is observed in the tightening process of the actual bolted flange joints so that the excessive iteration of the repeated tightening round hardly improves the uniformity of the bolt preloads.

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