|This posting (November 2019) is an update to an October 2013 post which reflects changes to the ASME B31.3 Code relative to severe cyclic service that have occurred since the original post.|
There has been a fair amount of confusion as to when the rules for severe cyclic conditions in ASME B31.3 should be used, and the rules themselves can be somewhat confusing to apply. In the 2016 edition of ASME B31.3 the definition of when the rules for severe cyclic service are applicable were changed, which may reduce the confusion. The definition as to when the rules of severe cyclic apply is in the 300.2, Definitions. Prior to the 2016 edition, it stated that severe cyclic conditions are:
- Conditions applying to specific piping components or joints in which SE computed in accordance with para. 319.4.4 exceeds 0.8SA (as defined in para. 302.3.5); and
- The equivalent number of cycles (N in para. 302.3.5) exceeds 7000;
- Or, other conditions that the designer determines will produce an equivalent effect.
So, by this definition, severe cyclic conditions applied to piping systems with a lot of displacement cycles, which are rare in most continuous operation process plants, and the calculated displacement stress is close to the allowable displacement stress. For these piping systems, fatigue is a greater concern. Following the rules for severe cyclic conditions generally results in piping systems with greater fatigue resistance.
In the 2016 edition, consideration of calculated stress and number of cycles was eliminated in the definition, and it was changed to: conditions applying to specific piping components or joints for which the owner or the designer determines that construction to better resist fatigue loading is warranted. So, for systems where experience has shown fatigue is a greater concern, such as highly cyclic units, the owner or designer can designate severe cyclic service to get a constructed piping system that is more resistant to fatigue failure.
While it appears that some of the rules in the code are written with the idea that severe cyclic conditions may apply to an entire piping system, the rules only apply to the specific components. In the past it was those that are over 80% of the allowable stress or otherwise designated by the designer. With the change to the new definition, it only applies to components and joints designated by the owner or designer.
The prior rules led to some puzzling circumstances. For example, there are some components that are prohibited for use in severe cyclic service for which stresses are not calculated. An example is slip-on flanges. The stress in the pipe at that location could be over 80% of the allowable stress, but this is highly unlikely in most systems, as components such as elbows and tees typically govern in the flexibility analysis. And the rules called for considering the stress in the component, something that was not available for a slip-on flange. With the change in the definition, things are clearer. If the owner or designer designates a flanged joint as in severe cyclic service, slip-on flanges are prohibited.
If a component or weld is determined to be in serve cyclic conditions, the following additional requirements apply:
- The rules prohibit the use of less fatigue resistant components and joints for use with severe cyclic conditions
- There are some fabrication requirements, such as 328.5.6 which states that “A welding procedure shall be employed which provides a smooth, regular, fully penetrated inner surface.”
- 100% visual examination of fabrication is required
- 100% volumetric examination of butt and miter groove welds
- Socket welds and other branch connections welds examined by magnetic particle or liquid penetrant examination
- The acceptance criteria for welds is more stringent, for example, prohibiting any undercutting
The following example illustrates how the rules worked prior to the change in the 2016 edition:
Assume there is a system with 10,000 equivalent displacement cycles, so it is more than 7,000, and the calculated stress in one location, an unreinforced fabricated tee, exceeds 80% of the allowable stress (but as required by code, is less than the allowable stress). To start with, you probably don’t want to use an unreinforced fabricated tee in such a service anyway, so you would reinforce it or replace it with a welding tee, the calculated stress will undoubtedly drop below 80% of the allowable, and it is no longer in severe cyclic conditions. With the 2016 edition, the calculation of stress and the number of cycles is no longer relevant but assume now that the owner designated the tee as being in severe cyclic service. For the purposes of looking at the rules, let’s assume that you went ahead with the unreinforced fabricated tee.
Per 328.5.6, you are requried to use an appropriate weld procedure. Looking at 341.4.3, you can note that all threaded, bolted, and other joints shall be examined. Well, the tee doesn’t have any joints, other than the weld joint itself, which should be examined. And 341.4.3 (a) (3) requires all dimensions, alignment, springs, guides, etc be checked. It is not very clear what should be included since only the tee is considered to be in severe cyclic conditions, but it is reasonable to assume that it should be those that may affect the stress in the subject tee. The joint requires 100% magnetic particle or liquid penetrant examination, and more stringent acceptance criteria apply than for normal fluid service.
With the change in the definition of severe cyclic service in the 2016 edition of ASME B31.3, some of the confusing aspects of applying the rules have been eliminated. The intent is if there is an elevated concern for fatigue failure in a system, generally considered to be as a result of thermal displacement cycling, the owner or designer can designate specific components or joints as in severe cyclic service to get more fatigue resistant piping construction.
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