If you want to get high-quality welds, you need to pay attention to the following issues during the assembly and welding of flanges.
First, the assembly and connection of flanges and steel pipes
The assembly quality of flanges and steel pipes not only affects the strength and tightness of the pipeline connection but also affects the eccentricity of the entire pipeline. Therefore, when assembling flanges on steel pipes, the following basic requirements must be met.
(1) The center of the flange should be in 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 pipe fittings, and the screw hole positions of the flanges at both ends of the same steel pipe should correspond to each other.
1. Assembly of flat welding flanges and steel pipes
When assembling flat welding flanges and steel pipes, first put the flange into the pipe end, and leave a certain distance between the pipe mouth and the flange sealing surface (generally 1.5 times the pipe wall thickness). At this time, spot welding can be performed on either side of the 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 bend ruler alignment point. Turn the steel pipe 90° 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 mouth is 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 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, 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 level 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 are suitable for pipes that are not often disassembled and have a long service life.
(3) Correct selection of gaskets. Under the premise of ensuring that the gaskets will not be damaged by pressure, it is a principle to use small-width gaskets to reduce excessive bolt tightening force.
Third, flange connection
1. Check and handle flanges, bolts, and gaskets before installation
(1) 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 and 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 stratification; the surface should not have defects such as defects and wrinkles. The material should be consistent with the design selection.
(5) The processing size, accuracy, 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 radial scratches, looseness, warping, and other defects.
(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 welding flange, the pipe end should be inserted into the flange 2/3. Since the flat welding flange is subjected to mechanical stress and thermal stress, the entire connection will suddenly break when it breaks. Therefore, if conditions permit, the flat welding flange should adopt the reinforcement welding method on both the inside and outside. After welding, the slag should be removed, 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. When DN is less than 300mm, the allowable deflection is 1mm, and when DN is greater than 300mm, it is 2mm.
The flange connection should be coaxial, the center deviation of the bolt hole should generally not exceed 5% of the hole diameter, and the bolt should be able to penetrate freely.
The flange connection should use bolts of the same specification and the installation direction should be consistent, that is, the nuts should be on the same side. The bolts and nuts connecting the valve should generally be placed on one side of the valve. When tightening the bolts, they should be symmetrical and uniform, with appropriate tightness. The length of the bolt exposed outside the nut after tightening should not exceed 5mm or 2~3 buckles.
Bolt hole position on the flange: The top two bolt holes of the horizontal pipeline should be kept horizontal; the two holes closest to the wall of the bolt holes on the vertical pipeline should be parallel to the wall. At the same time, the two connecting flanges should be parallel and natural, and the parallelism deviation should not exceed 2mm.
The net distance between the flange on the branch pipe and the outer wall of the riser is more than 100mm, or it can be penetrated by bolts. For easy disassembly, the distance between the flange and the edge of the bracket or the building should be more than 200mm.
The flange should not be buried directly underground. Inspection wells should be set up at the flange joints of buried pipelines and pipelines in inaccessible trenches. If the flange must be buried underground, anti-corrosion measures should be taken.
Flanges work at high and low temperatures, and bolts and nuts of stainless steel and alloy steel should be coated with graphite oil or graphite powder.
Post time: Apr-09-2025