Thermal fatigue is attributable to cyclic stresses similar to mechanical fatigue but differs in that the stresses are developed by temperature gradients (i.e., thermal stresses). Temperature gradients that exceed 50°F through the thickness of a pressure vessel’s shell, or that span a distance less than 2.5(Rt)1/2 (where R = radius of vessel component and t = thickness of shell) along the surface, are considered to be sufficient to cause thermal fatigue.
Thermal fatigue cracks tend to occur at locations where the thermal gradient is greatest, and, therefore, they may not develop at locations of high stress concentration, where mechanical fatigue cracks are most likely to occur. Thermal fatigue cracks almost always originate at the surface, and closely spaced cracks can develop. Because nozzle reinforcement pads and partial penetration welds can magnify temperature gradients, they may be especially prone to thermal fatigue.
The locations most susceptible to thermal fatigue are surfaces alternately wetted by cooler liquids and dried by hotter vapors, and nozzles where fluids are introduced at fluctuating rates at either higher or lower temperatures than the bulk contents.
Thermal fatigue cracks in pressure vessels occasionally can be detected by visual examination (VT), but the greater sensitivity of magnetic particle examination (MT) or dye-penetrant examination (PT) is superior. Ultrasonic examination (UT) using a shear wave technique can be used to determine the depth of thermal fatigue cracks, or to detect and size those that might originate at a surface that is not accessible for MT or PT examination.
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