Bellows vendors will often quote two different cycle lives for their bellows. One is in accordance with ASME B31.3, Appendix X. The other is in accordance with the Standards of The Expansion Joint Manufacturers Association. The latter provides a significantly higher cycle life.
The EJMA bellows fatigue curve is a best fit curve based on data that was available, so it provides an expected average cycle life. The ASME curve was developed also using bellows fatigue data, but it provides design margins consistent with ASME pressure equipment codes.
Historically, designers have put large factors of safety on the number of design cycles for bellows, which was in part justified since there was no design margin in the EJMA fatigue curve. This would result in a margin of safety entirely dependent upon the designer, and could vary from large, to none. This was not a satisfactory condition for an ASME Code, so I developed the fatigue curve with safety factors that was put in ASME B31.3 Appendix X. When using this curve, it should be recognized that appropriate design margins are already included, and they should not also be included in the specified number of cycles. With bellows, design for an excessive number of cycles can unnecessarily compromise other design aspects, such as design with respect to internal pressure and column stability (resistance to squirm).
There is ongoing work to potentially revise the EJMA fatigue curve to include a factor of safety, which may be variable. This would be a very good development as it will lead to more uniform design practices and less confusion. When such a fatigue design basis is provided in the EJMA Standards, then it is very likely that the fatigue curve in Appendix X of ASME B31.3 will be removed, and ASME B31.3 will simply refer to the EJMA fatigue design rules, and may specify the required design margin if the revisions to the EJMA rules include a variable design margin.
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