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DKL Engineering, Inc.
Welded Bottom Joints
A minimum thickness of 12.7 mm (1/2”) including corrosion allowance is recommended for the bottom of the tank.
Three types of welded joints are commonly used to assemble the bottom of a vertical cylindrical storage tank:
- Double Welded Butt Joint
- Single Welded Butt Joint with Backing Strip
- Single Welded Lap Joint
The double welded butt joint is the preferred method of constructing the bottom of the tank. This requires that the bottom be fabricated in an elevated position since access is required to both sides of the plate. The advantage of this method of construction is that the welds can be radiographically examined.
The next preferred method is the single welded butt joint with backing strip. Backing strips are applied to the plates before being placed into position on the foundation. The adjoining plate is butted into position and welding is done from the topside of the plate only. A backing strip is required to prevent excessive weld drop-through.
The least preferred method of construction is the single welded lap joint which is the easiest and quickest method of construction. The plates are overlapped and a fillet weld is applied at the edge of the plate. The disadvantage of this method is that as the tank corrodes, metal loss in a fillet weld occurs more rapidly to the point where very little weld metal may remain even though the plate thickness may still be ample.
The height of a storage tank should be specified in multiples of standard plate widths to achieve an economical design and minimize material wastage and work. Standard plate widths are 72” and 96”.
The plate in the first course in the tank wall will be the thickest since it must be design to contain the pressure resulting from the head of acid when the tank is full. The wall thickness should be no less than 5 mm (3/16”) excluding the corrosion allowance. As well, the shell thickness should not exceed 38 mm (1½”) since the material cannot be easily stress relieved when erected in the field.
The maximum shell height will be determined by the allowable soil bearing pressure. A 9.754 m (32 ft) high tank filled with acid (s.g. = 1.84) will exert a pressure of 3200 lb/ft² on the ground. The ground and foundation design must accommodate this pressure otherwise an reduction in the height of the tank is required.
A minimum corrosion allowance of 6 mm (1/4”) should be specified for the shell of the tank.
Vertical joints in plates of adjoining courses are to be a minimum of 600 mm apart.
Tank roofs are generally umbrella type or domed and are self-supporting. Conical roofs or roofs equipped with internal supports are not recommended. Conical roofs tend to be uneconomical and internal supports are subject to corrosion.
A corrosion allowance of 3 mm (1/8”) is generally used in the design of a roof.
Snow loads must be considered in colder climates but a minimum load of 25 psf is recommended regardless to account for live loads that must be accommodated during construction and during operation.
For domed roofs a spherical radius of 1.0 x shell diameter is typical. The spherical radius should not be less than 0.8 and not more than 1.2 times the shell diameter.
External stiffeners may be considered to reduce the required plate thickness of the roof. The plate thickness should not be less than 9.5 mm (3/8”) thick.
When a roof is a partial sphere, the membrane stresses in the roof pull inward on the periphery of the side walls. This pull results in circumferential compressive forces at the juncture, which may be resisted either by a knuckle curvature n the roof or by a limited zone at the juncture of the intersecting roof and sidewall plates, supplemented in some cases by an angle, rectangular bar, or a horizontally disposed ring girder. An excerpt from API Standard 620 - Design and Construction of Large, Welded, Low-Pressure Storage Tanks indicates permissible compression ring details.