Failure Investigation of a 500 Gallon Liquid Nitrogen Storage Tank

Abstract

A 500-gallon liquid nitrogen (LN2) storage tank failed while being filled by a pump truck. The failure of the tank was the first of its kind and quite unusual. The cryogenic storage tank was a typical double-wall construction. The inner and outer vessels were made of 5083-O aluminum and low-carbon ferritic steel, respectively. The inner liquid container was an ASME Section VIII, Div. 1 Code stamped vessel. The outer vessel was not Code stamped since it was designed for vacuum service. The outer vessel was made to support the inner vessel and insulation material and maintain this vacuum for thermal insulation purposes. The heads on both the outer carbon steel and the inner aluminum vessel were fractured at a girth weld resulting in a rocketing of the vessel. A detailed investigation was conducted to find the root cause. This investigation showed that a failure of a nozzle in the annular area between the two tanks caused LN2 to pour into that area. The warmer carbon steel outer shell caused the LN2 to vaporize and rapidly pressurize the annular area. This pressurization of the annular region caused the inner aluminum tank to buckle and resulted in the head separating from the main (inner) cylinder during the buckling process. The liquid LN2 from the inner tank flowed into the outer tank (along with possible flow from the truck) and the pressure continued to increase as the LN2 level increased. The pressure in the annular space reached critical level to cause the failure of the weld in the carbon steel tank. This paper describes the analyses that were carried out for this investigation which involved determining the crack-driving force from the combined weld residual stresses, thermal stresses from the LN2 liquid level, and pressure.