Piping thermal expansion stress limits for ASME Section III Class 2/3 piping are directly based on the original thermal expansion stress rules given in B31.1-1955. Cyclic thermal expansion stresses are limited by Equation 10 to SA where SA is a function of the expected number of full temperature cycles. The Eq. 10 limit may be exceeded, if the pressure-plus-weight stresses are below their limit. This requirement is expressed as Eq. 11. The basis for Eqs. 10 and 11 is not well understood in the industry, and has caused confusion. One typical comment is — Why have an Eq. 10 limit if it can be exceeded? The history and development of the thermal expansion stress limits are presented. The thermal expansion stress limits from B31.1 are based on relaxation considerations and prevention of yield or creep strain even though the failure mode of concern is fatigue cycling. Hence, the thermal expansion stress limit is an implied and approximate limit on fatigue. Recommendations for changes to the Section III Class 2/3 rules are provided. A direct fatigue based stress limit is proposed.

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