A The term “nitrogen blanket” refers to a padding (or a layer) of nitrogen gas put into a tank car to: (1) protect the interior from developing rust; (2) prevent chemical reactions of the products shipped; and/or (3) reduce the level of dangerous gases in the expansion space. Because regulations require expansion space in shipments of hazardous materials to accommodate changes in temperature during transportation, oxygen or product gases may be present.
Nitrogen, a non-flammable gas, takes the space of oxygen or other gases present in the vapor space of a tank and, since it is an inert gas, it reduces the risk that may be present. Nitrogen is used for both empty, cleaned tank cars, and for tank cars containing hazardous materials. When cars are shipped from a tank car shop after repair or qualification, the facility will often, at the request of the owner or leasing party, inject nitrogen gas into the tank.
Likewise, shippers often inject nitrogen into the remaining space of a tank car after loading. However, only enough nitrogen is put into the car to inhibit rust or chemical reactions. Shippers and facilities must take care to not put too much gas into the tank, as doing so could result in the nitrogen becoming a regulated hazardous material. By definition, nitrogen, a non-flammable and nonpoisonous compressed gas, is regulated if it exerts a gauge pressure of 29.0 pounds per square inch or greater at 68 degrees Fahrenheit, in its package. You may also hear the term “nitrogen padding,” which has the same meaning. – Bill Schoonover, hazmat staff director, Federal Railroad Administration
