Effects of Nut thinning due to Corrosion on the Strength Characteristic and the Sealing Performance in Bolted Flange Joints under Internal Pressure

Abstract

In the depreciation evaluations for pressure equipment, it is important for engineers to evaluate the reduction in strength and the sealing performance in bolted flange joints. In this study the effects of nut thinning due to corrosion on the strength characteristic and the sealing performance in 3B bolted flange joints under internal pressure are examined from both FEM calculations and experiments. The following results are obtained. When the bolted flange joint has been tightened by the bolt force of 50% yield stress, the tolerance yield force of the screws will not be surpassed, but when tightened by the 90% of yield stress, a small-scale yield region occurred from the nut washers to the three threads. During the tests, when the nut height decreased, the fracture position of the bolts and nuts changed from the free-threaded part of a bolt screw to the thread engagement. While verifying this phenomenon using FEM, it was newly discovered that when the height of the nut was reduced to 1⁄3 (equivalent to approximately two threads) of the original dimensions according to the standards, the sealing performance could no longer be assured.