Cifras y categorías: Análisis de los resultados electorales regionales

The construction of large-scale cylindrical storage tanks requires automatic weld-ing processes (Figure 8-68) such as submerged arc weldweld-ing (SAW) and electrogas arc welding (EGW). In addition, manual shielded metal arc welding (SMAW) and semi-automatic gas metal arc welding (GMAW) may also be used according to the accessibility and applicability of the welding position and the targeted efficiency for individual welding joints, [34].

Table 8-3 below shows the typical welding joints for a crude oil storage tank, ap-plicable welding positions and processes and well typical types of steel plates used for fabrication.

The welding Process (Notes: (1) HT steel: 550-610 MPa X80-90 HT steel (2) Dis-similar: Mild steel and 550-610 MPa, X80-90 HT steel), [34].

Figure 8-67. Large storage tanks (Kharge Island, Persian Gulf)

Figure 8-68. Automatic welding of large crude oil tanks (photo source: KHK, Safety &

Tomorrow, Mar. 2000, [34])

Some joint designs are indicated in Figure 8-69 below.

The following details the welding procedures for particular welding joints as re-ferred to in the previous Table 8-3.

8.7.4.2.1 Shell Plate Horizontal butt Joints: account for 90% of the total weld-ing length of the shell, and the plate thickness of the joints is as high as 12 to 40 mm.

Therefore, welding efficiency has a significant effect on the total construction cost.

To achieve high welding efficiency, the SAW process is usually carried out with special equipment for horizontal welding is generally used. Figure 8-70 outlines this process, in which a welding wire is fed at a certain angle into a granular flux that is sustained by conveyor tracking along the lower part of a double bevel groove. Figure 8-71 shows an application of this process at a construction site, in which the SAW equipment tracks along the shell plate.

This specific horizontal SAW process uses a particular flux and a thin solid wire (typically of about 3.2 mm Ø).

While SAW is the main welding process for shell plate horizontal welding in large-capacity cylindrical tanks, SMAW is equally indispensable. Because tack welding uses low hydrogen electrodes even for mild steel base metal, cold cracks are prevented by decreasing the diffusible hydrogen in the weld metal.

8.7.4.2.2 Shell Plate Vertical butt: are typically welded by EGW, using portable equipment suitable for welding short lengths of large-capacity storage tanks. For this application, the SEGARC process (Figure 8-72) which features easy-to-handle equip-ment can be used.

TAblE 8-3. main joints for cylindrical storage tank and suitable welding procedures (reproduced from [34])

As shown in Figure 8-72, the welding progresses while the weld pool is shielded with CO₂ gas and is dammed up by a water-cooled copper shoe on the front side and refractory backing (KL-4) or water-cooled copper backing on the backside of the welding joint. The welding head tracks during welding on a guide rail attached by a magnet on the surface of the base metal as shown in Figure 8-73.

The SEGARC process is widely used in the construction of storage tanks due to

·

the following outstanding characteristics of efficiency and operability:

High deposition rates (e.g. 180 g/min at 380A) ensure high welding

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efficiency.

Light-weight, compact equipment makes for easy set up.

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There is constant control of the wire extension in varied welding conditions.

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The welding line can be located either on the left side (Standard) or, by

reas-·

sembling, the right side of the tracking rail.

Figure 8-69. Typical vertical and horizontal weld joint design (all dimensions in mm) (Repro-duced from [34])

With the oscillator (Optional), one-pass completion welding can be conducted

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for steel plates with a thickness of 32 mm max.

8.7.4.2.3 Shell to Annular Plate Tee Joints: can be subjected repetitively to severe bending stresses over the lifetime of the storage tank because of frequent loading and unloading of the liquid and uneven settling of the foundation under the tank. In addi-tion, the welding of this joint is likely to be affected by sand, rust, dirt, oil, rain and dew because this joint is located close to the foundation at the construction site. Weld-ing durWeld-ing fabrication must be conducted carefully in order to prevent weldWeld-ing defects and ensure the durability of the tank. In particular, root pass welds can easily contain porosity and cracks. To prevent these defects, the SMAW process is recommended for the root passes on the backing side and final side because it resists the difficulties in such a critical welding environment.

Figure 8-70. Outline of horizontal submerged arc welding (Reproduced from [34])

Figure 8-71. Application of horizontal submerged arc welding (photo source: KHK, Safety &

Tomorrow, Mar. 2000 [34])

For the filling passes and the capping passes, SAW provides the highest efficiency.

Due to the severe welding environment, the best flux-wire combination for this joint should emphasize porosity resistance.

With these flux-wire combinations, DCEP polarity will produce better bead ap-pearance in single SAW than AC polarity. Figure 8-74 shows a typical weld pass se-quence for this joint, combining SMAW for the root pass and single SAW for the filling and capping passes.

Figure 8-72. The SEGARC (submerged electro-gas arc welding) process for one-run vertical butt welding (KHK, Safety & Tomorrow, Mar. 2000 [34])

Figure 8-73. SEGARC (submerged electro-gas arc welding) process: a portable EGW process for vertical welding (Courtesy Kobe Steel [34])

8.7.4.2.4 bottom Plate Joints (Figure 8-75): are typically made of mild steel lap joints in small-capacity storage tanks and butt joints with steel backing in 10,000 kl (10,000 BBLS) or larger storage tanks. As with the shell to annular plate joints, a similarly severe welding environment can lead to the inclusion of rust and dirt in the welding groove because the bottom plate rests directly upon the foundation of the tank.

This is why SMAW is generally suggested for the root pass welds to minimize the occurrence of porosity. Because of its high efficiency, SAW is more beneficial for the filling and capping passes.

Bottoms are usually to either of the following two construction methods:

Lap Welded Plates

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Butt welded Plates

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The Lap welded plates must be reasonably rectangular and square edged. Three-plate laps are required not be closer than 300 mm from each other and also from the tank shell.

Plates are welded on top side only with a continuous fillet weld on all seams. Joints are lapped to 5 times the thickness of the thinner plate, but generally are not to exceed 25 mm (see Figure 8-75, Section BB).

Portion of the sketch plates shown in Figure 8-76A, coming under the bottom shell ring shall have the outer ends of the joints fitted and lap welded to form a smooth bear-ing for the shell plates, as shown in the same figure.

Bottom plate attachment with the shell plate may be made by an annular ring of segmental plates as shown in Figure 8-76B. Such annular rings (if used) must have their radial seams butt welded with a backing strip as shown in the same figure. Bottom sketch and rectangular plates can be lapped over the annular ring of segmental plates with the lap not less than five times the nominal thickness the thinner plates joined.

For bottoms butt welded construction, plates shall have the parallel edges prepared for butt welding with either square or V-grooves. If square grooves are employed, the root opening are generally required to be equal or over 6 mm. The butt welds may be

Figure 8-74. A typical pass sequence for shell plate to annular plate tee joint welded by using a combination of SMAW and single SAW processes [34]

made by applying a backing strip 3 mm thick or heavier by tack welding to the un-derside of the plate (see Figure 8-76B, Section XX). A metal spacer may be used, if necessary, to maintain the root opening between the adjacent plates.

In bottom plate butt welding three-plate joints are generally not be closer than 300 mm from each other and also from the tank shell.

Tank erectors may however propose other methods of butt welding the tank bottom.

8.7.4.2.5 Nozzle Peripheral and Manhole Joint (Figure 8-77): Some tanks are fitted with manholes. Such manholes are welded in the factory in advance. They can be welded by SAW using special automatic welding equipment that can track along the three-dimensional saddle-shaped welding line.

However, where such automatic welding equipment is not available, semi- automatic GMAW and SMAW are the second and third best processes respectively in terms of welding efficiency. These welds are postweld heat treated to remove the residual stresses;

therefore, the filler metal should be selected taking into account the weld metal properties after the post-weld heat treatment at the specified temperature and soaking time.