Welding storage tanks

Tank Shell. Another example of where thickness is set by minimums for fabricabihty but not for strength is in small-diameter tanks. For example, a water storage tank built using a steel of an allowable stress of 20,000 psi (138 mPa), 9 ft (3 m) in diameter by 21-ft (7-m) high, requires a shell thickness to resist hoop stress of only 0.023-in. (0.58-mm) thick. However, if built to API Standard 650, the shell would be fabricated at least 0.1875-in. (4.76-mm) thick. The code requires this thickness so that when fabrication, welding, and tolerances are considered, a tank of acceptable quaUty and appearance meeting the requirements of most services in most locations is provided. 

Storage tanks should be designed in accordance with the ASME code for unfited pressure vessels. AH-welded constmction is recommended. Ethylene oxide storage tanks should be electrically grounded, isolated from potential fire hazards, and equipped with pressure rehef devices. New equipment should be cleaned of iron oxide and immediately purged with inert gas. 

Low-Pressure Tanks (below 15 psig) Low-pressure storage tanks for highly hazardous toxic materials should meet, as a minimum, the American Petroleum Institute (API) 620 Standard, Recommended Rules for the Design and Construc tion of Large Welded, Low-Pressure Storage Tanks (API Standards). This standard covers... 

API Std 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks, Ninth ed., February 1996. 

Many steel structures - like bridges, storage tanks, and ships - are held together by welds. And when incidents arise from fast fractures or fatigue failures they can often be traced to weaknesses in the welds. The sinking of the Alexander Keilland oil platform in 1980 is an example. 

Weak roof-to-shell attachment for fixed roof tanks, as per API-Std-650 for Welded Steel Tanks for Oil Storage. The joint fails and excess pressure can be relieved (above the normal design provided). Such tanks do not require additional emergency vent equipment however, it can be provided in order to prevent the roof seam failure with its attendant replacement/ maintenance requirements. This type can only be used outside of a building, not confined. 

The correct form to use for a given joint will depend on the material, the method of welding (machine or hand), the plate thickness, and the service conditions. Double-sided V- or U-sections are used for thick plates, and single V- or U-profiles for thin plates. A backing strip is used where it is not possible to weld from both sides. Lap joints are seldom used for pressure vessels construction, but are used for atmospheric pressure storage tanks. 

API 620 (2002) The design and construction of large, welded, low pressure storage tanks, 10th edn (American Petroleum Institute). 

BS 2654 (1989) Specification for the manufacture of welded non-refrigerated storage tanks for the petroleum industry. 

Source Welded Steel Tanks for Oil Storage, American Petroleum Institute, Washington, D.C., 1973. ... 

These requirements do not apply to large field-erected storage tanks covered by the Standard for Welded Steel Tanks for Oil Storage, ANSI/API 650, or the Specification for Field-Welded Tanks for Storage of Production Liquids, API 12D or to the Specification for Shop-Welded Tanks for Storage of Production Liquids, API 12F. 

UL 80, Steel Tanks for Oil-Burner Fuel, covers the design and construction of welded steel tanks of the atmospheric type with a maximum capacity of 0.23 to 2.5 m (60 to 660 gal) intended for unenclosed installation inside of buildings or for outside aboveground applications as permitted by the Standard for Installation of Oil-Bum-ing Equipment, NFPA 31, primarily for the storage and supply of fuel oil for oil burners. 

API 12F, Shop Welded Tanks for Storage of Production Liquids RP 12R1, Setting, Maintenance, Inspection, Operation, and Repair of Tanks in Production Service RP 575, Inspection of Atmospheric and Low Pressure Storage Tanks... 

RP 579, Fitness-For-Service API 650, Welded Steel Tanks for Oil Storage API 652, Lining of Aboveground Storage Tank Bottoms API 653, Tank Inspection, Repair, Alteration, and Reconstruction... 

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