Severe cyclic conditions

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.  The definition as to when the rules of severe cyclic apply is in the 300.2, Definitions.  It states that it is 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, severe cyclic conditions applies to piping systems with a lot of displacement cycles, which are rare in most 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 will generally result in piping systems with greater fatigue resistance.

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 that are over 80% of the allowable stress or otherwise designated by the designer.  However, this still leaves many puzzling questions when following the rules.

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.  Severe cyclic limitations for these type of components will most likely only apply if the designer elects to designate them as in severe cyclic service.

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

Let’s take an example, say there is a system with 10,000 equivalent displacement cycles, so it is more than 7,000, and the calculated stress in one location, a 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.  But, 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 want 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.

While the rules do not always appear to make sense, the intent is that if you are close to the allowable displacement stress, fatigue is more likely to be an issue, and the rules require precautions to make the system more fatigue resistant.  You can completely avoid the severe cyclic service rules by simply making sure that, for any system with more than 7,000 equivalent displacement cycles, that the displacement stress does not exceed 80% of the allowable. 

When Should the Rules for Severe Cyclic Conditions (Service) in ASME B31.3 Be Used?

Posted by Chuck Becht on Friday, 04 October 2013 in Piping