During the cleaning operation of a very large storage tank, the roof suddenly and unexpectedly collapsed; Baker & O'Brien was retained to determine why. We evaluated the cleaning procedures, relevant valve positions, cleaning fluid temperatures, and then produced a computer simulation with these inputs to predict the forces that resulted in the tank damage. Baker & O'Brien produced an expert report of the incident cause for the related claims for property damage and business interruption.

Baker & O'Brien was engaged to investigate the cause and origin in a case where the roof of a very large cylindrical storage tank was crushed during a cleaning operation. The procedure for cleaning the tank required that a cleaning fluid be charged to the tank and pumped from a bottom outlet, through filters and a heater, and returned to the top of the tank. The procedure allowed for pumping to be periodically interrupted to clean the filters. The procedure specified that a tank vent remain open to ensure that internal and external pressures would be equalized throughout the cleaning process.

Large storage tanks are remarkable structures, which can be designed to safely contain hundreds of thousands of barrels of crude oil, gasoline, diesel fuel, chemicals, or drinking water. Tank shells are constructed of steel plate and, having large surface areas, are not built to resist internal or external pressures, i.e., an internal vacuum. Temperature decreases or condensation of vapors inside a sealed tank can result in pressure differentials that cause catastrophic damage. Storage tanks are designed with vents to safely equalize internal and external pressures, and operators are trained to recognize and prevent situations where unequal pressures may develop.

Baker & O'Brien determined that the vent, which was to remain open during cleaning to equalize pressures, had been covered with a plastic tarpaulin for housekeeping purposes. Hot liquid inside the tank cooled off during an interruption to clean the filters. Since the pressures could not equalize, a vacuum developed inside the tank, and the roof was crushed by atmospheric pressure. Baker & O'Brien prepared a computer simulation that predicted the forces that resulted in damage to the tank and evaluated claims for property damage and business interruption. Our findings were presented in a written report.