A typical tank battery contains two or more tanks and usually has a capacity equal to four days production.
Inlet connections, preferably, should be located in the deck of the tank and should have a valve located near the inlet and capable of closing off against pressure.
Drain connections should be located immediately above the tank bottom in the side of the tank or in the tank bottom immediately adjacent to the side. They should be equipped with a valve and sealing device located next to the tank. Drains from all the tanks in a battery should be connected together and piped well away from the tanks.
Tank batteries are operated by flowing into a single tank that is "equalized" to another. The "equalizer line" allows flow from the primary tank to overflow to a secondary tank when the primary tank is full. The operator then equalizes the second tank to another empty tank so that there is a new primary and new secondary tank.
The original primary tank is then ready to be run to sales. Before the tank is accepted by the crude purchaser, the water should be drained from the tank if necessary and the water valve sealed closed. All other valves should be sealed, except the vent or vapor-recovery line. The pipeline valve is then unsealed and opened for delivery to the purchaser. When a closing gauge is taken, and before the tank is filled again, the pipeline valve should be sealed, the drain valve checked to ensure that it is closed, and the seal removed. The seal from the equalizer-line valve can then be removed, and the tank is ready to be put in service as an equalizing tank.
Equalizer connections should be installed below the deck in the tank shell. A valve and sealing device should be installed immediately adjacent to the tank if more than two tanks are in the battery and should be connected in such a manner that any two tanks can be equalized together.
Vent connections should be installed in the center of the tank deck and all tanks connected to a common line. The line should have a pressure-vacuum valve installed in the line or on the end of it. The line should be sloped to prevent accumulation of liquids in it or in the valve. The use of gas to roll stored products is usually considered poor practice and should be restricted to temporary or emergency use. If a roller line is used, it should enter the tank through the deck and be equipped with a valve next to the tank.
Tank battery for hydrogen sulfide crude storage
Constant attention should be given to the hazardous condition created by iron sulfide deposits. These occur most frequently within the vapor space and particularly on the underneath exposed side of the deck. These iron sulfide deposits generate severe corrosion that can go unnoticed when deck conditions are observed from the topside only. When sour crude is stored, all openings on the tanks should be kept closed because hydrogen sulfide is poisonous. This can be accomplished by equipping the tanks with some type of ground-level gauging and by locating thermometers in the tank shell. Gauges and temperatures then can be read from the ground without the tank being opened. These gauging devices usually require approval by the crude purchaser. Grounding-level sampling also can be accomplished by installing pipes that extend into the tank at any desired level and to any desired distance. Valves are located at a convenient level to permit sampling on the ground without the tanks being opened. If available, a small amount of sweet gas should be fed into the top of the tank continuously to establish a "gas sweep." This ensures positive pressure within the tank at all times and prohibits air from entering the tank, thereby greatly reducing corrosion. It is advisable to extend the tank vent line well beyond the tank battery and to use a back pressure valve and flame arrester in the vent line. The vapors should be flared and not vented.
Storage tanks that are properly designed, constructed, and maintained can provide 30 to 50 years of service.
Steel tanks should be kept clean and free from spilled oil or other material. They should be kept painted and all water or accumulated dirt should be removed from around the bottom edge of the tanks. Thief hatches and vent-line valves should be kept closed and inspected periodically for proper operation and gasket condition. Should any leaks occur, they may be repaired temporarily with lead sealing plugs or toggle bolts. These leaks should be repaired permanently as soon as possible.