Correct Flange Connection Procedure - Flange Fastener Inspection

50. Gasket Installation: Before installation, verify that the gasket is new, clean, and dry, and inspect it for any defects or damage. It is essential not to reuse old gaskets. Ensure that the size and grade of the gasket match the requirements and confirm that the flange specifications are identical before proceeding with installation.
50. Flange Inspection: Prior to installation, inspect the flange surface for any signs of damage, including scratches, nicks, mud, corrosion, burrs, dents, and scratches deeper than 0.2mm radially across the water line of the flange sealing surface. If the damage extends over half the width of the gasket sealing surface or if the flange surface is significantly flawed, replacement or reprocessing and repair of the sealing surface is necessary. Ensure that the nut-supporting surface on the back of the flange is parallel and smooth. Verify that the flange is properly centered according to the inspection method outlined in SH3501-2011, Section 6.2, which details pipeline installation requirements.

50. Bolts and Nuts: Ensure that bolts and nuts are utilized by equipment and piping design specifications. Threads and contact surfaces must be clean and free from dirt, rust, scale, nicks, burrs, chips, or any other contaminants that may affect torque during tightening. Welding or machining for bolt repair is strictly prohibited. Following flange installation and tightening, at least two threads should be exposed outside the nut. Bolts and nuts must be lubricated before use to reduce friction during tightening and enhance their resistance to slipping and corrosion. Prior to lubrication, degrease and dry stud threads, nut threads, and contact surfaces. Apply a uniform layer of lubricant to bolt threads, nut threads, nut bearing surfaces, washers, and flange surfaces. Use high-temperature anti-seize compound as necessary.

Flange connection.

50. The alignment of flange connections must preserve the same axis, with the center deviation of bolt holes not exceeding 5% of the hole diameter, allowing bolts to penetrate freely. Flange connecting bolts should match the exact specifications, be installed in the same direction, and be tightened symmetrically and uniformly.
50. To address flange non-parallelism, avoid using slanted washers of varying thicknesses or double gaskets for compensation. When joining large-diameter gaskets, refrain from using flat butt joints or labyrinth configurations for splicing.
50. To simplify the installation and removal of flanges and ensure proper bolt tightening, maintain a minimum distance of 200mm between the flange plane, the bracket, and the wall.
50. When tightening the bolts, follow a symmetrical and diagonal sequence to ensure even stress distribution across the gasket.
50. In the instances outlined below, it is advisable to coat bolts and nuts with molybdenum disulfide, graphite oil, or graphite powder to ease future disassembly:

1) Stainless steel or alloy steel bolts and nuts.

2) Pipeline designed for temperatures exceeding 100 ℃ or below 0 ℃;

3) Outdoor installations.

4) Exposed to atmospheric corrosion or corrosive media.

50. Utilize copper, aluminum, mild steel, or other metal gaskets, ensuring they undergo annealing before installation.

50. During the trial operation of high-temperature or low-temperature pipelines, the flange connection bolts should typically undergo hot-tightening or cold-tightening, adhering to the following guidelines:

1) Hot-tightening or cold-tightening should occur after maintaining the working temperature for 24 hours.

2) When fastening the pipeline with bolts, determine the maximum internal pressure of the pipeline based on the design pressure. If the design pressure is less than 6MPa, the maximum internal pressure for thermal tightening should be 0.3MPa; if the design pressure exceeds 6MPa, the maximum internal pressure for thermal tightening should be 0.5MPa. Cold tightening should generally be carried out with pressure relief. Both hot and cold tight tightening should be conducted moderately, and safety precautions should be implemented.

50. Flange connections must not be directly buried. Inspection wells should be installed at the flange connections of buried pipelines. If burial is unavoidable, appropriate anti-corrosion measures must be implemented.​​

Conclusion

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