Part 2: effect of the gasket type on the behavior of NPS 4 class 150 bolted flanged joints subjected to external bending loads

摘要

PVRC Project 97-09 is an experimental study that investigates the effect of the gasket type on the behavior of bolted flanged joints subjected to external bending loads. This project is closely related to two previous pilot studies, PVRC Projects 94-26 and 96-01. In these previous studies, the effect of the initial bolt load and of the flange type on the joint behavior, was investigated by performing bending moment tests with a single type of gasket, a NBR/asbestos sheet gasket. The present project complements the findings of the previous work with the results of a series of bending moment tests conducted with four other types of gaskets: a virgin PTFE sheet, an expanded PTFE sheet, a spiral wound graphite filled gasket and a graphite coated corrugated metal gasket. The tests are performed on two different NPS 4 Class 150 bolted flanged joints, one with weld neck flanges and the other one with lap joint flanges. The four gasket types are tested in the weld neck flanges, and the two PTFE based gaskets are also tested in the lapjoint flanges. Some ofthe most important aspects of the joint behavior are measured during the tests: the leakage rate, the variation of the bolt loads, the gasket thickness change and the variation of the flange rotation. All the gaskets are tested under an initial bolt stress levels of 172 MPa (25 ksi). The spiral wound graphite filled gasket is also tested with a 345 MPa (50 ksi) initial bolt stress in the weld neck flanges. Overall, the leakage rate results of the tests performed in the weld neck flanges indicate that the bending moment effect on the joint tightness is relatively small. When a bending moment applied to the pipe assembly is increased, substantial bolt load variations occur in the joint and these variations are a function of the bolt position relative to the neutral plane. The test results also demonstrate that the magnitude of the bolt load variations is a function of the gasket rigidity. The non-uniform bolt load distribution resulting from the application of the bending moment, generates a reaction moment that reduces the moment imposed on the gasket. The study of gasket deformation has lead to the finding of a relationship between the bolt load variation and the gasket thickness change under bending moment load. This relationship appears to be the same for all the gaskets tested in the weld neck flanges. The analysis of the flange rotations has shown that the gasket type can have a strong effect on the bottom flange (compression side) rotations. When the tests performed in the lap joint flanges are compared to the tests of the weld neck flanges, significant differences are found in the joint behaviors. These differences are mainly caused by the flexibility of the lap joint flanges. Finally, several gasket stress distributions at maximum bending moment are computed with different methods to evaluate the accuracy of the ASME Code (Section III) calculation method that replaces the bending moment with an equivalent internal pressure. The report concludes by a short analysis of the advantage and the disadvantage of the equivalent pressure method.