Thermal Fatigue Stresses In Reinforcement Pads And Shell Wall Below, In An Un-Insulated Shut Down Vessel Operating At High Temperature

International Journal of Mechanical Engineering

© 2019 by SSRG - IJME Journal

Volume 6 Issue 8

Year of Publication : 2019

Authors : Ajay Mehta

10.14445/23488360/IJME-V6I8P103

How to Cite?

Ajay Mehta, "Thermal Fatigue Stresses In Reinforcement Pads And Shell Wall Below, In An Un-Insulated Shut Down Vessel Operating At High Temperature," SSRG International Journal of Mechanical Engineering, vol. 6,  no. 8, pp. 16-25, 2019. Crossref, https://doi.org/10.14445/23488360/IJME-V6I8P103

Abstract:

Un-insulated pressure vessels designed for higher temperature process fitted with reinforcement plates (re-pads) around the openings, operating under 'shutdown' conditions, develop very high induced stresses (approaching their yield limits and beyond) to differential thermal expansion, causing thermal fatigue. The reinforcement pads and corresponding patch of shell wall develop many fatigue stress cracks along the weld joints where the reinforcement pads are constrained to shell by fillet welds. The cracks initiate the blind side of the shell-reinforcement assembly (at the common interface), which cannot be easily detected by visual examination. An advanced non-destructive testing technique called Phased Array Ultrasonic Testing (PAUT) or Time of Flight Diffraction (ToFD) is required to be applied to detect the presence and extent of crack propagation. Australian pressure vessel design code identifies one of the design loads as 'Forces due to temperature conditions, including the effects of differential expansion of parts or attached piping.' However, there is no specific mandating guideline for steps to be taken when designing an un-insulated vessel operating at high temperatures and frequently shutting down. The author emphasizes to mandate insulation of shutdown vessels operating above ambient temperature or use a single heavy wall shell insert in place of reinforcement plate at the openings. There are chances of the fatigue cracks propagating un-detected, potentially causing catastrophic failure of the vessel.

Keywords:

Shutdown Vessel – The pressure vessel that operates with batch processes and shuts down and starts up at batch frequency,
Reinforcement Plate (Re-pad) – The doubling plate to compensate for the loss of shell wall when an opening is cut out Or compensation plate to take extra load due to external attachment, e.g., support legs, platforms above vessel, etc.,

References:

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