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The U.S. Department of Transportation (DOT) regulates the design and construction of shipping containers to help assure the safe transportation of hazardous materials. Specifications for fixed-facility containers, however, are quite different.
There is no mandatory regulation of fixed-facility containers. Recommendations for fixed tank specifications are issued by codes and standards organizations. I have seen a railroad tank car being used as an underground storage tank for a gasoline service station and highway transportation containers being used for fixed storage after they were no longer certified for highway use by the DOT.
Fixed-facility containers can be almost any size and shape. Some of them have names based on their designs and functions, such as the open floating roof tank. Others, however, do not have any specific name or designation, unlike their highway counterparts.
Several National Fire Protection Association (NFPA) standards address the storage of hazardous materials in fixed containers. These include NFPA 30 Flammable & Combustible Liquid Code and NFPA 58 Storage and Handling of Liquefied Petroleum Gases. Both of these codes address the design and safety requirements for specific fixed tanks such as propane and flammable liquid storage. The American Petroleum Institute, the Institute of Petroleum, the American National Standards Institute and others also publish standards - but these are "consensus standards" and do not become law or mandatory until a jurisdiction adopts them. Some jurisdictions have passed local codes and ordinances that regulate certain hazardous materials in fixed containers, but these vary from location to location and do not have the consistence that the DOT regulations provide for transportation of hazardous materials in containers.
Bulk Petroleum Storage
The first tanks to be discussed are those used for bulk petroleum storage. These include tanks with cone roofs, floating roofs, open floating roofs and retrofitted floating roofs. Many of these tanks are associated with tank storage facilities or "farms" where there are multiple tanks at one facility. Tank farms are often connected with pipelines as well as highway, rail and waterway transportation.
Photo by Robert Burke
A cone-roof bulk petroleum storage tank. Cone-roof tanks may be used to store gasoline, fuel oil, diesel fuel and corrosive liquids.
Fixed storage tanks have pressures that range from atmospheric (0-5 psi) and low (5-100 psi) to high (100-3,000 psi) and ultra high (above 3,000 psi). Bulk petroleum tanks are generally considered atmospheric pressure tanks.
Cone-roof tanks get their name from the inverted-cone-shaped construction of their roofs. They are atmospheric and low-pressure tanks with cylindrical outer walls supporting the cone roof. American Petroleum Institute Specification 650 calls for the roof-to-shell seam for this tank to be designed to fail as the result of a fire or explosion, reducing the possibility of a pressure buildup.
Cone-roof tanks may be used to store gasoline, fuel oil, diesel fuel and corrosive liquids. Some of these chemicals, such as gasoline, are very volatile and easily produce vapor at normal atmospheric temperatures. Because a cone-roof tank is open inside, the surface of the liquid is exposed to air and product will be lost to vaporization. Therefore, a cone-roof tank is primarily used for non-volatile materials. Contents of cone-roof tanks change frequently: daily, even hourly under some circumstances. These tanks are recognized by the cone-shaped roof and the lack of wind girders or external vents around the top of the sidewall.
An open floating-roof tank has a roof that literally floats on the liquid product in the tank. The outer walls of these tanks are vertical and cylindrical, with the floating roofs eliminating the vapor space in the tank. Drains in place are designed to remove accumulations of moisture on the surface of the tank.