Exchanger's Tubesheet Thermal Fatigue Cracking
Thermal fatigue is a type of fatigue failure with macroscopic cracks resulting from cyclic thermal stresses and strains due to temperature changes, spatial temperature gradients, and high temperatures under constrained thermal deformation. Thermal fatigue is the result of cyclic stresses caused by variations in temperature. Damage is in the form of cracking that may occur anywhere in a metallic component where relative movement or differential expansion is constrained during repeated thermal cycling.
Thermal fatigue differs from thermal shock in that the rate of temperature changes experienced is much greater and the magnitude of the temperature gradient is much less.
Every time a processing unit is started up or shut down, thermal stresses set up in equipment. Repeated application of thermal stresses can lead to progressive cracking,
Thermal fatigue cracks usually initiate on the surface of the component, on either the ID or OD surface. They are generally wide and often filled with oxides due to elevated temperature exposure and cracks may occur as single or multiple cracks. Thermal fatigue is best prevented through design and operation to minimize fluctuating thermal stresses and thermal cycling.
The following 2 preventive methods are usually applied:
Designs that incorporate reduction of stress concentrators, blend grinding of weld profiles, and smooth transitions should be used.
Controlled rates of heating and cooling during operation, start-up and shutdown of equipment can lower stresses.
Temperature monitoring can be performed by installing thermocouples, where practical, on components with thick sections or that are otherwise susceptible to thermal fatigue cracking.
Below picture shows example thermal fatigue cracking of tubesheet
In the Thermal Energy System, the tubes constructed of 316L stainless steel worked in a medium including water inside the tubes and process gas on the shell side. In the heat exchanger, the process gas was utilized at 360°C, while steam was produced at 306°C and 15 bar pressure. Thermal fatigue was discovered to be the underlying cause of tube failure, which was caused by temperature fluctuation due to inadequate water circulation.
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