Storage tank welding project is more common, especially
Large Storage Tank welding requirements are more stringent, here is a brief introduction to the operation process and welding skills of tank welding project with large
Vertical Storage Tank as an example.
Vertical cylindrical storage tank is a vertical tank composed of a circular tank wall with the central axis perpendicular to the ground, a flat disc-shaped tank bottom and different forms of tank roof, a vertical tank, accessories (such as ladders, platforms, etc.) and accessories (such as liquid level measurement, fire fighting facilities, etc.). General nominal volume greater than 100m?
Storage Tanks called large storage tanks, the standard used: GB50128-2005.
Second, the storage tank foundation
Tank foundation is the place where the tank is placed, requiring strong, solid; it not only affects the aesthetics, and will lead to uneven stresses on the tank. Tank foundation is a civil construction category, must be in accordance with the foundation construction drawings and technical standards requirements. Tank installation must be checked before the foundation of the tank, qualified before installation.
Three, prefabricated assembly
For large vertical storage tank manufacturing, general division, segmentation and combination of prefabrication, and then assembly, to improve production efficiency is very helpful, the first should be the construction of storage tanks selected materials and accessories for review, prefabricated assembly includes: bottom plate, wall plate, floating roof and internal floating roof, fixed roof top plate, wind ring, strengthening ring, wrapped edge angle steel and other components, parts.
Fourth, the welding of storage tanks
1, welding process assessment
For the first use of steel, according to the steel number, plate thickness, welding methods and welding materials, etc., according to the current national standards for process evaluation to determine the appropriate welding process. Welding process assessment is carried out using butt weld specimens and T-shaped fillet weld specimens, of which butt weld specimens are subjected to tensile and transverse bending tests.
2, welder requirements
For welding electrode arc welding, submerged arc welding and gas-electric standing welding of ordinary welders, usually should be assessed according to GB50236-1998; for "special equipment welding operator assessment rules" examination and obtain the quality and technical supervision department issued by the steel category, group and test piece classification code certificate of welders, can Engaged in the welding of storage tank parts, do not need to test again.
3. Preparation before welding
In order to ensure the quality of welding, welding equipment should meet the requirements of tank welding construction, for tensile strength σb ≥ 430MPa, plate thickness ≥ 13mm tank wall butt welding seam should be applied using low-hydrogen welding rod. Welding consumables should be dried and used in accordance with the product instructions or the provisions of Figure 1 before welding, and the dried low-hydrogen welding rod should be stored in a holding cylinder at 100°C to 150°C and taken with use. It is worth mentioning that sealed flux-cored wires and flux-cored wires in sealed boxes are in principle no longer dried and the flux-cored wires should be cooled to room temperature after drying before use.
Drying and use of welding consumables
4, Welding construction
The qualified steel plate will be cleaned up, bevels will be made according to the requirements of the diagram, and positioning welding will be carried out with fixtures that must be fixed securely. The length of each positioning weld, ordinary carbon steel and low-alloy steel should not be less than 50mm; yield strength greater than 390MPa low-alloy steel should not be less than 80mm. should check the quality of assembly before welding, remove the bevel surface and bevel within 20mm on both sides of the mud, rust, water, and oil, scale, etc., and should be re-dried.
In welding to ensure the quality of the beginning and end of the welding channel, the beginning should be in the bevel using the backward arc method, if necessary, can be used to lead the shield; terminal arc off should be filled with the arc pit; multi-layer welding layer between the head should be staggered. Note: Both the arc initiation and arc extinguishing should be within the bevel or on the weld path. Double-sided welded butt joints on the back side should be cleared before welding; when carbon arc gas planing is used, a grinding wheel should be used to trim the roots after clearing; when σs ≥ 390 MPa, penetration flaw detection should be carried out after clearing. Lapped corner welds with a plate thickness greater than 6 mm should be welded at least twice.
Special weather shall not be welded, such as: rain, snow; wind speed ≥ 8m/s for manual welding; wind speed ≥ 2m/s for gas-electric stand-up welding or gas shielded welding; ordinary carbon steel < -20°C; low-alloy steel < -10°C; σs ≥ 390MPax low-alloy steel < 0°C; relative atmospheric humidity ≥ 90%. Preheating temperature should be based on the material of the steel plate, thickness, joint constraint, welding materials and climatic conditions and other factors, the weldability test and welding process evaluation to determine, preheating should be uniform heating, preheating range shall not be less than 3 times the plate thickness on each side of the centerline of the weld, and not less than 100mm. preheating temperature should be measured by temperature measuring pen, surface thermometer, infrared thermometer. For preheated welds before welding, the interlayer temperature of the weld must not be lower than the preheating temperature. For common steel preheating temperatures see Figure 2.
Preheating of steel
Weld seams that require post-heating for hydrogen elimination should be treated immediately after welding with a heating temperature of 200 to 350°C and a holding time of not less than 0.5h (reference standard: JB/T4709-2000). For welding of different strengths of steel, it is appropriate to use the lower strength of steel to match the welding material, the choice and the higher strength of steel corresponding welding process.
Welding stainless steel tanks, before welding should be beveled on both sides of the 20mm range of water, oil, dirt clean, in the 100mm range coated with chalk powder or anti-spatter coating; austenitic stainless steel to ensure weld through and fusion good conditions, should be selected with a small line energy, short arc and multi-layer multi-channel welding process, the interlayer temperature should not be high; high corrosion resistance requirements of double-sided welds or surface and media contact welds should be the last Welding; stainless steel tanks in pickling, passivation treatment, pay attention to the protection of the environment.
When welding stainless steel composite storage tanks, should be strictly prevented from the grassroots and transition layer welding rod melting on the cladding; welding transition layer to reduce the dilution of alloying elements, it is advisable to choose a small current, narrow channel, short arc welding; welding cladding should fall on the surface of the cladding bevel carefully cleaned spatter, and finally cladding welding.
5, the tank welding sequence
In the tank manufacturing process, in order to reduce welding deformation, to ensure the size of the tank and reduce the welding stress, in addition to the necessary fixtures, the welding sequence is very important.
Medium width plate, welding should be welded first short weld after welding long weld; welding long weld, welders should be evenly and symmetrically arranged, from the centre outward welding, the first layer of weld using segment back welding or jump welding.
For the rim plate, first weld the 300mm butt weld on the outside of the rim plate below the tank wall; then weld the fillet weld connecting the tank wall to the bottom of the tank, after the fillet weld is completed, weld the remaining butt weld on the rim plate, the welder should be symmetrically arranged and weld from the outside in; then weld the lap weld between the bottom plate, rim plate and the centre panel, this ring weld has more lap to compensate for shrinkage deformation and is often called a shrinkage seam This ring weld has more lap to compensate for shrinkage deformation and is often referred to as a shrinkage joint. When welding shrinkage joints, welders should be symmetrically arranged and welded in the same direction, with the first layer being back welded in sections or jump welded.
The bottom of the tank and the tank wall should be arranged symmetrically by several pairs of welders respectively in the tank and outside the tank, the welders inside the tank about 500mm in front of the welders outside the tank, and then welded in sections along the same direction, the first layer of the welding channel using section back welding or jump welding.
Tank wall, should first weld longitudinal welds, and then weld circumferential welds bottom circle longitudinal welds after welding and then weld the bottom circle of the tank wall and the bottom of the corner weld; other adjacent two circles of wall plate longitudinal seams after welding, and then weld the circumferential welds between them, welding welders symmetrically arranged, along the same direction welding;.
Fixed top top plate, first weld the interrupted weld on the inside of the top plate, then weld the long weld on the outside, continuous weld should first weld the short weld in the circumferential direction, then weld the long weld in the radial direction, from the centre to the outer section back welding.
Cladding angle, it and the wall plate butt should first weld the angle connection butt weld after welding the angle and tank wall butt weld; cladding angle and wall plate lap, after welding the angle butt weld, then weld the angle and wall plate lap weld.
Floating roof, for the cabin inside and outside edge plate, should be welded first longitudinal weld, after welding corner weld; single plate, cabin bottom plate, cabin top plate welding order and the welding order of the middle width plate is the same; for cabin and single plate connection should wait for cabin and single plate all welded seam welding after welding, welding welders evenly symmetrical distribution, segment back welding; floating roof such as directly in the tank bottom laying assembly, its lower surface of all welds should be waiting for the floating roof to rise and fall onto the pillar before welding.
For
Stainless Steel Storage Tank bottom and tank wall connection of the corner weld, in order to prevent overheating, should not be inside the tank, outside the tank at the same time welding, should be welded first inside the tank corner weld, and then welded outside the tank corner weld.
6, the tank rework
In the manufacturing, transportation and construction process inevitably produce a variety of surface defects (such as scratches, arc abrasions, weld scars, etc.), only grinding can be trimmed, but the thickness of the trimmed steel plate should be greater than or equal to the nominal thickness of steel plate minus the allowable negative deviation value. When the grinding depth exceeds 1mm, welding should be carried out.
Before the weld is reworked, the location and depth of the defect should be detected to determine the extent of defect removal, the depth of removal should not exceed 2/3 of the plate thickness, and defects greater than 2/3 of the plate thickness should be removed from both sides. After reworking the weld should be flaw detection according to the prescribed method and should reach the qualified standard. The length of the weld channel shall not be less than 50 mm when reworking the welding. the number of reworking of the same part shall not exceed 2 times, otherwise the technical chief of the construction unit shall agree.
For σs ≥ 390MPa of low-alloy steel, defects should be removed after penetration flaw detection to confirm that the defects are cleared before rework welding, welding should be used to temper the weld channel, after welding should be polished and repaired, so that its surface is smooth and penetration flaw detection or magnetic particle inspection; when the depth of welding defects exceeds 3mm, the reworked parts should be subject to radiation flaw detection.
Five, the acceptance of the tank project
After the completion of the tank, the construction unit should be in accordance with the design documents and standards for a comprehensive inspection and acceptance of the quality of the project, the construction unit should be submitted to the completion of the information are: tank delivery acceptance certificate; as-built drawings or construction drawings (with design modification documents and row material chart); materials and accessories factory quality certificate or inspection report; tank foundation inspection records; tank body geometry inspection records; concealed works inspection records. Weld ray inspection report; weld ultrasonic inspection report; weld magnetic particle inspection report; weld penetration inspection report; weld rework report; strength and tightness test report; foundation settlement observation records.
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