Large cylindrical tanks are often used to store liquids like milk and chemicals for distribution. These structures are considered thin shells because of their geometry, dimensions, and aspect ratios. In this paper, an actual failure of a large vertical tank is investigated. The tank contained milk and buckled as a result of an internal vacuum caused by human error. Inspection done after failure revealed that an internal vacuum could have been created by a large drop in the inside air temperature. Such tanks are usually washed using extremely hot water. If the water temperature exceeded the manufacturer’s recommendations and the tank is not vented during the cool-down cycle, the contracting air would have drawn a vacuum. Another possibility is that the tank may have been drained without venting the headspace above the milk. This would cause the fixed mass of air above the milk to expand and draw a vacuum. To investigate these scenarios we consider stability of a vertical cylindrical shell under external pressure. The critical pressure differential for buckling of a vertical cylindrical shell with closed ends subjected to uniform axial and radial pressure is known from basic theories of buckling. These equations are used to determine the critical pressure for lobar buckling. The results show that each of these scenarios could have caused failure of the tank. Recommendations to prevent future failures in such storage tanks and design considerations for tanks having arbitrary dimensions and aspect ratios are presented in the paper.

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