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:
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 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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Dr. Becht, Fellow ASME, former Chairman of the ASME B31.3, Process Piping Code, is a recognized authority in pressure vessels, piping, expansion joints, and elevated temperature design. He has more than 40 years of experience in design, design review, analysis, check-out, mechanical integrity, development, troubleshooting, and failure analysis. He has been a member of 14 Codes and Standards committees, five of which he has chaired.
He has more than 60 publications including two books (on B31.3 and B31.1 piping) and seven patents and is a frequent speaker and chairman in technical forums. He received the ASME Dedicated Service Award in 2001 and was the recipient of the 2009 ASME Pressure Vessel and Piping Medal and the 2014 J. Hall Taylor Medal. He is President of Becht Engineering Co., Inc, CEO of Helidex, LLC, and Director/Owner of Sonomatic Ltd.
Dear Mr. Becht,
I would like to ask what could be possible impact of cyclic service for the calculated pipe wall thickness under para 304.1.2? Do the reduce allowable stress factor is also applicable for thickness calculation?
Designation of a system as being in severe cyclic service does not impact the wall thickness calculations for pressure design. Note that the definition of severe cyclic service as been changed, and is now simply systems the owner designates. There no longer exists the .8SE criteria.
My understanding is that the calculation of SE (for comparing to 0.8SA in the evaluation) should include only the displacement stresses as that is the focus of the section (319) in the code. Is this understanding correct? My confusion arises as the SE formula includes the resultant bending moment etc, without identifying if it is only the bending moment due to displacement stresses or not.
SE with E as a subscript is defined as displacement stress range and includes only displacement stresses. Note that the most recent edition of B31.3 has eliminated the calculated stress range as a criteria for determining if a system is in severe cyclic service and left it to the owner to determine more subjectively what systems are more susceptible to fatigue failure and thus warrant the more fatigue resistant design/construction requirements of severe cyclic service
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