Flange welding can also produce perfect fish scale patterns

These precautions should be taken. If you want to obtain high-quality welds, you need to pay attention to the following issues during the assembly and welding of the flange.

First, the assembly and connection of the flange and the steel pipe.

The assembly quality of the flange and the steel pipe not only affects the strength and tightness of the pipeline connection, but also affects the eccentricity of the entire pipeline. Therefore, when assembling the flange on the steel pipe, the following basic requirements must be met.

(1) The center of the flange should be on the same straight line as the center of the steel pipe.

(2) The flange sealing surface should be perpendicular to the center of the steel pipe.

(3) The position of the flange screw hole on the steel pipe should correspond to the position of the flange screw hole on the matching equipment or steel pipe fittings. The screw hole positions of the flanges at both ends of the same steel pipe should correspond to each other.

1. Assembly of a flat welding flange and steel pipe.

When assembling the flat welding flange and the steel pipe, first put the flange into the steel pipe end. A certain distance should be left between the steel pipe end and the flange sealing surface (generally 1.5 times the thickness of the steel pipe wall). At this time, spot welding can be performed on either side of the steel pipe, and then the flange bend ruler or angle ruler can be used to align the opposite side of the spot welding point. After alignment, spot welding can be performed at the point where the bend ruler is aligned. Turn the steel pipe 90 degrees so that the spot welding position is placed at the top and bottom. At this time, the bend ruler can be used to align the left and right sides of the steel pipe. Spot welding can be performed on both sides. When PN is less than 1.6MPa, only the outer end can be welded; when PN is greater than or equal to 1.6MPa, internal and external welding can be performed.

2. Assembly of butt-welded flange and steel pipe.

The assembly of the butt-welded flange and steel pipe adopts butt welding. The welding method and requirements are the same as the welding connection method of steel pipe.

Second, the flange gasket

The gasket plays a sealing role in the flange connection. It is in contact with the sealed medium and is directly affected by the physical properties, temperature, and pressure of the medium.

(1) Generally speaking, when flanges of the same pressure grade are used on the same pipeline, the same type of gasket should be selected for interchangeability.

(2) For water pipes, medium-pressure asbestos rubber sheets are generally used. Since rubber has a long service life, rubber gaskets should be used for pipes that are not frequently disassembled and have a long service life.

(3) The correct selection of gaskets is based on the principle of using small-width gaskets to reduce excessive bolt tightening force while ensuring that the gaskets will not be damaged by pressure.

Third, flange connection

1. Check and process flanges, bolts, and gaskets before installation

(1) At first, check the flange dimensions, including outer diameter, inner diameter, groove, bolt hole center distance, flange height, etc., which should meet the design requirements.

(2) The flange sealing surface should be flat and smooth, without burrs or radial grooves.

(3) The threaded part of the threaded flange should be complete and undamaged; the convex and concave flanges should be able to fit naturally, and the convex height should not be lower than the depth of the groove.

(4) Hose gaskets such as rubber asbestos sheets, rubber sheets, and plastics should be flexible and free of aging, deterioration, and delamination; the surface should not have defects such as wrinkles. The material should be consistent with the design selection.

(5) The processing size, precision, surface roughness, and hardness of the metal gasket should meet the requirements, and the surface should be free of defects such as cracks, burrs, grooves, radial scratches, and rust spots.

(6) Metal wound gaskets should not have defects such as radial scratches, looseness, or warping.

(7) Before assembling the flange, rust, oil stains, and other debris on the surface and sealing surface must be removed until the metallic luster is exposed. The sealing line of the flange sealing surface must be clearly cut.

2. Flange assembly

(1) When installing a flat weld flange, the pipe end should be inserted into the flange 2/3. Since the flat weld flange will suddenly break due to mechanical stress and thermal stress, the flat weld flange should adopt a reinforced welding method on both the inside and outside when conditions permit. After welding, the slag should be removed cleanly, the inner hole should be smooth, and the flange surface should be free of spatter.

(2) When assembling the flange, the flange surface must be perpendicular to the center line of the steel pipe. For DN < 300mm, the allowable deviation is 1mm; for DN > 300mm, the allowable deviation is 2mm.

Flange connections should be coaxial, with the center deviation of the bolt holes generally not exceeding 5% of the hole diameter, and free insertion of the bolts must be ensured.

Flange connections should use bolts of the same specification and be installed in the same direction, meaning the nuts should be on the same side. Bolts and nuts connecting the valve should generally be placed on one side of the valve. Bolts should be tightened symmetrically and evenly, with appropriate tightness. The length of the bolt protruding from the nut after tightening should not exceed 5mm or 2-3 turns.

Bolt hole position on the flange: For horizontal pipes, the top two bolt holes should be horizontal; for vertical pipes, the two closest bolt holes should be parallel to the wall. Furthermore, the two connecting flanges should be parallel, with a parallelism deviation of no more than 2mm.

The clearance between the flange on the branch pipe and the outer wall of the riser should be at least 100mm, or sufficient for bolt insertion. To facilitate disassembly, the flange should be at least 200mm away from the edge of the support or building.

Flanges should not be buried directly underground. Inspection wells should be installed at flange joints on buried pipelines and pipelines in inaccessible trenches. If flanges must be buried underground, anti-corrosion measures should be implemented.

Flanges operate in high and low temperatures. Stainless steel and alloy steel bolts and nuts should be coated with graphite oil or graphite powder.