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TanksTypes of TanksAbove ground storage tanks are available in a wide variety of designs. The most common type of tank is the atmospheric tank used for storing petroleum products. Atmospheric tanks are designed to contain pressures of 0 to 0.5 psig. There are five common types of atmospheric storage tanks used for petroleum products:
Floating roof tanks minimize vapor loss because the roof rests on the liquid, which greatly reduces the vapor space between the top of the tank and the top of the liquid. Lifter roof or vapor dome tanks reduce the amount of vapor loss by moving up or down with vapor volume changes. Ordinary fixed cone roof tanks and vertical roof tanks tend to have higher vapor loss than the other three types. Aboveground tanks often are painted with aluminum or white paint to reflect heat, which decreases the temperature rise of the liquid contents and slows evaporation. The thickness of the metal used for tank construction is based on the strength required to hold the mass of the liquid. There is also an added allowance for corrosion encountered during the service life of the tank. Storage tanks should be constructed of materials compatible with the contents contained in the tank. Steel tanks resist heat from exposure to fires and maintain structural integrity for a longer period of time than any other material currently used for tank construction. Low pressure (0-15 psig) storage tanks are commonly used for storing petroleum products with low vapor pressures. Low pressure tanks are designed to contain liquids with vapor pressures slightly higher than atmospheric pressure, but not exceeding 15 psig. Spheroid and horizontal tanks are the two common types used for petroleum products; however, spheroid tanks are normally used for storing very "light" hydrocarbons, such as pentane, butane, and propane, which do not present a spill hazard if released. Horizontal tanks are used for storing products such as light crude oil, gasoline additives, gasoline blending stocks, and naphtha solvents which constitute a spill hazard if released. Most horizontal tanks are constructed of welded steel, but some older riveted steel tanks may still be in service. Horizontal storage tanks should be frequently examined for cracks, corrosion, buckling, or other damage. Tank Inspections and TestingTanks, pipes, joints, gaskets and seams should be inspected routinely for for visible oil leaks. The inspection should be recorded on an inspection form and kept for three years. Typically tanks are inspected once a week. All visible leaks should be reported to the person in charge of spill prevention as noted in the SPCC Plan. Tank corrosion and bottom deterioration is also a concern. Tanks bottoms can corrode and leak with out any signs of a visible leak. Corrosion protection can be provided by dielectric coatings and cathodic protection. In some cases double bottom tanks have been used. The type f material and foundation must be considered and should be evaluated by an engineer. Corrosion of a tank's surface may also result in tank failure. Corrosion that is concentrated in small areas of a tank's surface or "pitting" creates a high potential for tank failure. If tanks are rusty, holes may form causing the tank to leak. Tank supports and foundations should also be inspected for cracks, crumbling, deterioration, and seepage. ASTs should be subjected to periodic integrity testing. Some of the accepted methods for testing are the following:
Level Gauges and AlarmsLevel gauges and alarms are most commonly used to ensure that tanks are not overfilled. There are two basic objectives for using alarms and gauges:
Larger tanks may be designed with gauges, high-level alarms, and high-high level alarms to satisfy this requirement. Smaller tanks may be gauged by "stick". Generally it is not adequate to only "stick" a tank during filling operations. A second overfill protection measure should be used as a backup. Some trucks have automatic shutoff systems, that will shut off the pump once the meter reaches a pre-selected volume of product such as 90% of the tanks capacity. In the simplest case, the gauge is a small-diameter glass or plastic tube vertically attached to two openings in the tank shell. Liquid level in the tank is shown by the level in the tube.Another common sight level gauge is a float gauge. A float rides on top of the liquid in the tank and moves a marker attached to a cable or chain on the outside of the tank. The marker moves up or down with the product level in the tank. High liquid level alarms are usually tied into a float gauge or level gauging system. The alarms produce an audible or visual signal when the liquid level in the tank reaches a predetermined height. This consists of a fill-level alarm connected to a pump control that automatically shuts down the pump when a preset liquid level is reached. This system eliminates the possibility of human failure and is effective at stopping overfilling of tanks. This system consists of communication between the tank gauger and pumping station and relies on human perception of liquid levels in the tanks and pumping rates to avoid overfilling tanks. Human error could cause a spill if the tank gauger or pumping station misreads an audible or code signal to start or stop pumping. Underground and Partially Buried TanksUnderground tanks have both advantages and disadvantages for storing petroleum products. The advantages are reduced vapor loss, increased safety, efficient land use and greater security. The disadvantages are undetected leaks and higher corrosion factors for metal tanks. Fiberglass-reinforced plastic tanks are commonly used for storing petroleum products underground. They have a distinct advantage over metal tanks in being corrosion-free. Steel tanks should be protected from corrosion by coatings and cathodic protection.. Underground corrosion of metal surfaces is a direct result of an electric current generated by the reaction between the metal surfaces and chemical ions present in the soil and water. The flow of current from one portion of the tank to another causes metal ions to leave the surface of the metal, creating pits. The rate of destruction of the metal is directly related to soil moisture and chemical makeup of the soil. Underground tanks should also be leak test and adequate
records kept of such tests. These records must be made part of the SPCC plan and
kept for at least three years. The Federal UST regulations found in 40 CFR 280 have technical requirements consistent with the underlying regulatory purposes of the SPCC program and are equally protective for purposes of preventing discharges of oil into waters of the United States. These regulations contain provisions for corrosion protection, leak detection, tank overfill and spill prevention equipment, and tank tightness testing. Facilities should refer to the full text of 40 CFR 280 or seek the advice of a consultant when making determinations of compliance. Portable Tanks, Temporary Tanks and Tank TrucksMobile or portable oil storage tanks including tank trucks and temporary tanks should be positioned or located so as to prevent spilled oil from reaching navigable waters. A secondary means of containment, such as dikes, basins, or spill pallets, must be provided for temporary tanks and parked tank trucks whether full or not. The containment area must hold the contents of the largest container stored in the area. Temporary tanks are generally contained in an earthen berm with a plastic liner. Overnight tank truk parking may vary from a bermed area to a fully contained garage. These temporary storage areas and truck parking area must be located where they will not be subject to periodic flooding or washout. Containment for drums and other small containers does not have to be expensive. If there are a small number of drums, a facility may purchase plastic spill pallets or portable containment devices designed for drum containment.
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