Tank collapse incident

A fixed atmospheric storage tank was taken out of service for repairs. During re commissioning, the tank was filled up with water to check for leaks. After the leak check, the contractor handling the testing process was instructed to drain the tank of water using the 4” drain valve located at the bottom of the tank. As the contractor was pressed for time, he decided to open the 24” tank bottom manway also, to speed up the process of draining. After some time, a loud bang was heard and the tank collapsed due to vacuum. The venting arrangement in the tank was not sufficient to prevent vacuum in the tank when the manway was opened.