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An articulating-arm-type device on the top of the tank functions as a ladder for examination of the interior. As the product level and floating roof go down, so does the end of the arm, which rests on top of the roof. The location of this arm, when viewed from the outside, can be an indicator of the approximate amount of product in the tank. If the arm is completely visible the tank is near full. If the arm is completely inside the tank, it is near empty. A wind girder is located near the top and on the outside of the tank. Primarily a retaining band, the wind girder provides necessary rigidity to the container wall when liquid levels are lowered. An external staircase is also located on the tank wall to provide access to the articulating arm (walkway) on top of the floating roof.
Photo by Robert Burke
An open floating-roof tank. Notice the stairs on the outside and the reinforcing ring around the top.
Another version of the floating-roof tank is the covered or internal floating roof. These tanks may resemble cone-roof tanks, but are distinguishable by the vents around the tank near the roof to sidewall seam. The cone roof provides protection from rain and snow, while the internal floating roof eliminates the vapor space above the liquid and helps prevent the loss of vapor into the air.
This style of tank is sometimes used for polar solvent materials, which are miscible with water. If water or snow entered the tank, it would dilute the product. Materials stored in the internal floating roof tank are generally flammable and combustible liquids because of the vapor loss protection of the floating roof on top of the liquid.
A floating-roof tank may also be covered by a geodesic dome that keeps outside weather from affecting the floating roof in the tank. This form of internal floating roof tank also is used for flammable and combustible liquids.
Horizontal tanks are another kind of tank used for chemical storage in many parts of the country, particularly in rural areas. These tanks range in size from hundreds of gallons to thousands of gallons with low or atmospheric pressure. The ends of the tanks are flat, which usually indicates a limited amount of pressure.
Many horizontal tanks are supported on stands constructed of metal. Unprotected metal can be dangerous during fire conditions, as the metal will be stressed by the heat and fail very quickly.
Photo by Robert Burke
Horizontal high-pressure tanks containing liquid propane. Notice the extensions on the relief valve.
In the 1950s, six Kansas City, KS, firefighters were killed when the steel supports of a horizontal tank at a gasoline service station failed, sending burning gasoline into their location on the street. NFPA 30, Flammable and Combustible Liquids Code, requires the steel supports of horizontal tanks be protected from flame impingement by encasement in concrete or other non-flammable material. Horizontal tanks may be used to store flammable and combustible liquids, corrosive liquids, and many other types of hazardous materials. A direct result of the Kansas City fire was the requirement for gasoline tanks at service stations frequented by the public to have underground storage tanks.
According to the NFPA, there has never been a fire or explosion involving an underground fuel-storage tank. During the 1970s and '80s, it was discovered that underground storage tanks, which were usually constructed of steel, eventually corrode and leak fuel into the ground. Gasoline and other flammable fuels have a specific gravity less than water, so they will float on water. Moisture that accumulates inside underground storage tanks settles to the bottom of the tank and causes the metal to corrode. Because of the leakage, the U.S. Environmental Protection Agency (EPA) requires that all underground tanks be replaced and safeguards put in place to prevent and detect leaks.
An alternative to the placement of tanks underground was allowed in the form of an aboveground vault. The vault is constructed of concrete and the tank(s) are placed within the vault, partially underground with the vault top above the ground. There is access to the vault from aboveground. Vaults for flammable and combustible liquids must have safety precautions such as monitors installed and must be able to hold the contents of the largest tank if it should leak.
Propane and anhydrous ammonia are stored in high-pressure tanks that contain many safety features that are built-in through codes and standards. Industry associations also are heavily involved in safe storage and handling requirements for their members and emergency responders. The National Propane Gas Association (NPGA), in conjunction with the Propane Education & Research Council, has developed a training program for emergency responders with a 220-page text, Facilitator's Guide, CD-ROM and 50-minute video.