Is 95% of allowable thermal expansion stress range cause for concern?


Is 95% of allowable thermal expansion stress range cause for concern?

February 11, 2021

The stress that is generated by thermal expansion is self-limiting in nature. These self-limiting stresses behave quite differently from the sustained stress caused by weight and pressure. In a sustained loading condition the stress generated has to be in static balance with the applied load. On the other hand, when a pipe is subjected to thermal expansion or other displacement type of loads such as the displacement of a nozzle on a large storage tank that settles upon first filling, the mechanism of balance shifts to strain rather than load. Once the displacement reaches its final position the whole thing stops because the displacement is limited by either final temperature (i.e. the design temperature) or by movement of a nozzle on a storage tank. The stress due to a displacement type load is self-limiting and will never be greater than its first application. The thermal expansion stress range relaxes over time. A self-limiting stress has the following characteristics: 1) It does not break ductile pipe (A-106 Grade B) in one application of the displacement. 2) The mode of failure is thermal fatigue requiring many cycles of application. The piping codes recognize this by introducing a factor f, the stress range reduction factor. F equals 1.0 at 7000 cycles and 1.2 at 3100 cycles. The stress can reach yield without causing failure. 7000 cycles is approximately one full cycle per day for 20 years. One full cycle is a cold start up to operating temperature.

If one were to consider 3100 cycles, then the allowable thermal expansion stress range is 20% higher so the report of stress for 3100 cycles in the pipe stress analysis program would be 75% of allowable rather than 95% of allowable. So is 95% of allowable thermal expansion stress range cause for concern?  You decide for your own specific application. Generally, you want the sustained stresses and expansion (displacement stress range stresses) to be about 80% or lower as a goal. However, when they are above that, you might consider the criticality of the system. For instance, a system with Category M content and located in an area with humans would normally be a highly critical system.

Leave a Comment