We performed a API 579-1/ASME FFS-1 2007 Fitness-for-Service evaluation on a tank for a brewery.
The tank is comprised of an 8-foot diameter cylindrical shell 13’-10” tall, with two dished heads on the ends – of welded (carbon steel) construction. It contains multiple penetrations and is supported by a total of six vertical posts attached to the bottom head. The penetrations are stainless steel and welded into the tank. There were indications that the penetrations were replaced sometime after the initial installation. While the exact number of cycles the tank had been subjected to over its 70+ year service life is unknown, reasonable estimates of the minimum number of cycles (910 cycles) is greater than the number of available cycles (less than 100).
FEA analysis determined that the stresses in the bottom head in the area of the supports are significantly greater than the allowable stress limit. Considering the corrosion of the tank was less than 0.0625 inches yet still had stresses greater than the allowable limit, it is suspected that the original design calculations did not consider the effects of the supports on the tank. This is evident in that the maximum allowable working pressure (MAWP) would be greater than the operating pressure if the areas of the supports were ignored.
Other analyses were performed – and it was concluded that the tank in its current condition no longer met the current industry accepted safety margins and should be removed from service.
We perform Fitness for Service Evaluations for clients in industries including food service.
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– (PDF) “Weld Defects and Failures: Quantifying Fitness For Service – How Finite Element Mathematical Simulations can Mitigate Risk, Cut Costs and Improve Overall Safety”
– Publications – Heat Exchangers, Pressure Vessels, Welds and Other Applications, Fatigue, Elevated Temperature