Flange connection is a commonly used detachable connection method between components such as pipelines, valves, and equipment. Its operation process must strictly follow the specifications to ensure sealing, safety, and service life. The following is a detailed flange connection operation process and key precautions:

1、 Preparation before operation

1. Material and tool inspection

Flange: Confirm that the material (such as carbon steel, stainless steel), pressure rating, nominal diameter match the pipeline, and that the surface is free of defects such as cracks, sand holes, burrs, etc.

Gasket: Select the appropriate material (such as metal wrap pad, rubber pad, PTFE pad) according to the medium, temperature, and pressure, and check the thickness uniformity and integrity.

Bolt/nut: Check the specifications (length, diameter), material and flange matching, ensure that the surface is free of rust and the threads are complete.

Tools: Prepare torque wrench, hydraulic wrench (high-pressure connection), pry bar, level, angle ruler, wire brush, sandpaper, lubricant (such as molybdenum disulfide), marking pen, etc.

2. Environmental and safety inspections

Ensure that the operating area is clean and unobstructed by debris.

Check whether the interior of the pipeline is clean and free of residual welding slag, rust, and other impurities.

Confirm that there is no stress at the flange connection (if the pipeline is not straightened, it may cause sealing failure).

Wear safety protective equipment (gloves, goggles, safety shoes).

2、 Flange installation steps

1. Align the flange with the pipeline

Pipeline straightening: Use a level or angle ruler to check whether the pipeline axis is consistent, and the deviation should meet the standard (such as when DN ≤ 300, the axial deviation should be ≤ 1mm; when DN>300, it should be ≤ 2mm).

Flange positioning: Insert the flange vertically into the end of the pipeline, ensuring that the flange sealing surface is perpendicular to the pipeline axis (deviation ≤ 1 °).

Spot welding fixation: Symmetrically spot weld 2-3 locations to prevent welding deformation, with a spot welding length of approximately 2-3 times the wall thickness.

2. Flange welding

Welding process: Select the welding method based on the material (such as manual arc welding, argon arc welding), and control the welding current, voltage, and speed.

Symmetrical welding: Starting from the free end of the flange, weld in sections in a symmetrical order (such as 12 o'clock → 6 o'clock → 3 o'clock → 9 o'clock) to reduce thermal deformation.

Weld seam inspection: After welding is completed, inspect the appearance of the weld seam (without pores, cracks, slag inclusions), and perform non-destructive testing (such as X-ray, penetrant testing) if necessary.

3. Gasket installation

Cleaning the sealing surface: Use a wire brush, sandpaper, or specialized cleaning agent to remove oil stains, rust, and burrs from the flange sealing surface.

Gasket positioning: Place the gasket in the center on the flange sealing surface to avoid deviation or folding.

Special gasket treatment:

Metal wrapped gasket: Ensure that the inner and outer rings are in good contact with the flange sealing surface without warping.

Rubber pad: Check the uniformity of thickness and avoid using aged or cracked gaskets.

4. Bolt installation and fastening

Bolt penetration: Insert the bolt vertically into the flange bolt hole, with the nut on the other side, ensuring that the thread is clean and undamaged.

Initial tightening: Tighten the nut symmetrically by hand or wrench to compress the gasket initially, but without excessive force.

Cross fastening:

Low voltage connection: Tighten gradually 2-3 times in diagonal order, increasing torque by 30% -50% each time.

High voltage connection: Use a hydraulic wrench to tighten in stages according to the manufacturer's recommended torque values (such as initial 50% → middle 75% → final 100%).

Final inspection: Confirm that all bolt tightening torques are consistent, and the parallelism deviation of the flange surface is ≤ 0.5mm.

3、 Check after connection

1. Appearance inspection: There is no leakage at the flange connection, the exposed length of the bolts is consistent, and the gasket is not extruded or damaged.

2. Sealing test:

Hydraulic test: Conduct a water pressure test at 1.5 times the design pressure and maintain the pressure for 30 minutes without any leakage.

Pressure test: Use inert gas (such as nitrogen) at a pressure of 1.15 times the design pressure and hold for 10 minutes without leakage.

Bubble method: Apply soapy water to the interface and observe whether bubbles are generated.

3. Recording and identification: Record information such as tightening torque, test pressure, and inspection personnel, and mark the connection date and inspection status on the flange.

4、 Key precautions

1. Material matching: The flange, gasket, and bolt materials should be compatible with the medium (such as stainless steel or Hastelloy for highly corrosive media).2. Temperature effect: Under high temperature conditions, thermal expansion should be considered, and gaps should be reserved or spring washers should be used.

3. Avoid excessive tightening: Excessive torque may cause gasket crushing or flange deformation, and strict adherence to standards is required.

4. Regular maintenance: Regularly check the loosening of bolts during operation, and shorten the inspection cycle under high temperature or vibration conditions.

5. Special working condition handling:

Vacuum system: Vacuum specific gaskets (such as graphite composite gaskets) are required.

Low temperature system: Avoid using rubber pads and choose polytetrafluoroethylene or metal gaskets.

5、 Common Problems and Prevention

Possible causes and preventive measures of the problem

Leakage gasket skewed, bolt not tightened. Reinstall gasket and tighten bolt symmetrically

Flange deformation welding thermal deformation, excessive tightening correction flange, control welding sequence and torque

If the material of the bolt fracture does not match and the torque is too high, qualified bolts should be selected, and the torque should be controlled according to the standard

Replace temperature/corrosion resistant gaskets due to aging, high temperature or chemical corrosion

By strictly following the above precautions, the reliability and safety of the flange connection can be ensured, and the service life of the equipment can be extended.