Tank blanketing, also known as tank padding, is the injection of a gas into the open space of a liquid storage tank. It is to maintain a layer of gas above the liquid to prevent atmospheric gasses from entering the tank. Air in the tank contains oxygen, moisture, along with whatever else may be in the air. The type of gas used for blanketing is dependant on process, environmental constraints and available gas. Smaller tanks may be blanketed with nitrogen. Tanks used in the oil industry are blanketed with natural gas, when available.
Blanketing is used to remove Oxygen from the vapor space which will cause corrosion of the tank. Corrosion could lead to contamination of the process or failure of the tank. Additionally, by keeping an inert gas or natural gas blanket on the process conditions will not be right for combusion should there be a spark.
Tank padding works by setting an inlet valve to open when the tank vapor space drops below a preset pressure. This indicates that the tank level is dropping and a vacuum is forming. The valve will remain open until the set pressure is met at which point the valve will close. To capture gas that is displaced by rising fluid, a vapor recovery unit must be installed.