Good morning,

I am analyzing a “piping system” which encountered large plastic deformation during operation.

The piping system does not conform to B31.3 para. 319.2.2-b-3 due to a construction error. In other words, the piping system became unbalanced.

The B31.3 mentions:
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319.2.2 Displacement stresses:
a) Elastic Behaviour. Stresses may be considered proportional to the total displacement strains in a piping system in which the strains are well-distributed and not excessive at any point (a balanced system). Layout of systems should aim for such a condition, which is assumed in flexibility analysis methods provided in this Code.
b) Overstrained Behaviour. Stresses cannot be considered proportional to displacement strains throughout a piping system in which an excessive amount of strain may occur in localized portions of the system (an unbalanced system). Operation of an unbalanced system in the creep range may aggravate the deleterious effects due to creep strain accumulation in the most susceptible regions of the system. Unbalance may result from one or more of the following:
...
3. A line configuration in a system of uniform size in which the expansion or contraction must be absorbed largely in a short offset from the major portion of the run.
...
If unbalance cannot be avoided, the designer shall use appropriate analytical methods in accordance with para. 319.4 to assure adequate flexibility as defined in para. 319.1.
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Questions:
1. Is a correctly applied CaesarII an “appropriate analytical method”?

2. The CaesarII calculated elastic exp-stress levels are below the hot Sy although the hot shape is similar (not identical, only the shape is the same) to the encountered plastic deformation. This contradicts with the encountered plastic deformation. The system was operated outside the creep range. The encountered plastic deformation would require the system to exceed the hot Sy. Is the overstrained behaviour a possible cause for the difference between the calculated exp-stress levels and the encountered plastic deformation?

Best regards,

Timo Stek