Tank blanketing, also referred to as tank padding, is the process of applying a gas to the empty space in a storage container. The term storage container here refers to any container that is used to store products, regardless of its size. Though tank blanketing is used for a variety of reasons, it typically involves using a buffer gas to protect products inside the storage container. A few of the benefits of blanketing include a longer life of the product in the container, reduced hazards, and longer equipment life cycles.
At present, flammable vapors in fuel tanks are rendered inert by replacing the air in the tank with an inert gas, such as nitrogen, nitrogen enriched air, steam or carbon dioxide. This reduces the oxygen concentration of the ullage to below the combustion threshold. Alternate methods based on reducing the ullage fuel-air ratio to below the LFL or increasing the fuel-air ratio to above the UFL have also been proposed.
Three elements are required to initiate and sustain combustion: an ignition source (heat), fuel and oxygen. Combustion may be prevented by reducing any one of these three elements. If the presence of an ignition source can not be prevented within a fuel tank, then the tank may be made inert by:
1.reducing the oxygen concentration of the ullage—the space above a liquid fuel—to below that capable of combustion (the combustion threshold);
2.reducing the fuel concentration of the ullage to below the "lower explosive limit" (LEL), the minimum concentration capable combustion; or
3.increasing the fuel concentration to above the "upper explosive limit" (UEL), the maximum concentration capable of combustion.
In 1970, Appalachian Controls Environmental (ACE) was the world’s first company to introduce a tank blanketing valve. There are now many ready-made systems available for purchase from a variety of process equipment companies. It is also possible to piece together your own system using a variety of different equipment. Regardless of which method is used, the basic requirements are the same. There must be a way of allowing the blanketing gas into the system, and a way to vent the gas should the pressure get too high.
The most common gas used in blanketing is nitrogen. Nitrogen is widely used due to its inert properties, as well as its availability and relatively low cost. Tank blanketing is used for a variety of products including cooking oils, volatile combustible products, and purified water. These applications also cover a wide variety of storage containers, ranging from as large as a tank containing thousands of m³ of vegetable oil down to a quart-size container or smaller. Nitrogen is appropriate for use at any of these scales.
When considering the application for combustible products, the greatest benefit is process safety. Since fuels require oxygen to combust, reduced oxygen content in the vapor space lowers the risk of unwanted combustion.
Blanketing valves open/close manually to pressurize the storage tank. Oxygen is not monitored and this can increase explosion risk. 3BS.Solutions developed the Tank Blanketing Controller to control, monitor and measure the m³ Nitrogen and LEL/UEL to Oxygen in the storage Tank or container.