The use of compression joints in ASME Nuclear Class 2 and 3 small diameter piping systems has become increasingly popular because their installation does not require welding, and therefore saves time, money, and radiation dose. An important question is whether these types of joints can be a practical alternative to socket welds in piping systems subject to vibration. There have been numerous operating experience events where socket welds have developed cracks due to high cycle fatigue. On the other hand, parts of compression joints plastically deform the pipe to grip and create their sealing connection; the application of a high cycle vibration load to an already plastically strained pipe might lead to premature failure. It is desired to know whether at least one type of compression joint would perform better or worse than socket welds in such an environment.

In this paper, a testing methodology is described, in which one supplier’s coupling joint design was tested for vibration loading in tubing assemblies of varying sizes. The intended application for these joints is in an Electro-Hydraulic Control system at a northeastern Boiling Water Reactor plant. Industry experience reports have identified past vibration problems in this system at other plants. A test setup was devised, in which multiple specimens could be tested simultaneously by adjusting specimen natural frequency, shake table speed, and input acceleration. Fatigue Strength Reduction Factors were derived, allowing the resistance to fatigue failure to be quantified. Both compression joints and socket welds were tested using the same procedures, in order that the fatigue damage resistance could be compared between the two types of joints.

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