1、 Preparation work before installation
Confirmation and inspection:
Verification: Confirm that the specifications, materials, pressure rating, and sealing surface form of the flange meet the requirements.
Inspection: Carefully inspect the flange sealing surface, gasket, and bolts/nuts, ensuring proper cleaning and no damage. There shall be no scratches, bumps, rust spots, oil stains or residual old gaskets on the sealing surface.
Alignment: Confirm that the two flanges are parallel and concentric with each other, and that the misalignment is within the allowable range. The pipeline should be deeply supported to avoid using flanges as tools for correcting pipeline misalignment.
Gasket installation:
Select the right gasket: Confirm that the gasket type, material, and size match the flange sealing surface.
Clean placement: Carefully align the gasket and place it on the sealing surface to confirm that it is not skewed. For wrapping pads, metal ring pads, etc., special attention should be paid to placing them in place.
2、 The Golden Rule of Bolt Tightening
Core principle: By applying uniform bolt loads, the gasket is evenly stressed, thereby forming a stable seal.
Bolts and lubrication:
Use bolts that meet the standard and have an appropriate length (2-3 pitches should be exposed after tightening the nut). The bolt should be able to penetrate freely without any forced impact.
Key step: Apply one-on-one high-temperature anti seize agent or lubricant on the thread and nut bearing surface. This can confirm the accurate torque value and prevent bolt biting at high temperatures.
Manual pre tightening:
Firstly, tighten all nuts by hand to confirm that the flange clearance is roughly uniform.
Formal fastening - Cross symmetry method:
Sequence: Imagine a clock face, starting from the 12 o'clock position, then going to 6 o'clock, then to 3 o'clock, and finally to 9 o'clock. This is the foundation of a cycle.
Step: Tighten in at least three cycles:
Good sales cycle (pre tightening): Tighten all nuts in a cross symmetric order to approximately 30% of the final torque. This step aims to eliminate most of the gaps and preliminarily fix the gasket.
Cycle (tightening): Similarly, increase the torque to approximately 60% -70% of the final torque in a cross symmetric order.
Third cycle (final tightening): Continue to increase the torque slowly and steadily to the specified final torque value in a cross symmetrical order.
Inspection: After all cycles are completed, all bolts should be checked again in a clockwise direction to confirm that there are no omissions and the torque is uniform.
3、 Key control points and precautions
Torque control: A calibrated torque wrench is required. Do not use impact wrenches or uncontrolled extension rods to avoid overloading, stretching, or even breaking bolts, or causing flange deformation.
Uniformity principle: During the fastening process, the gap between the two flanges should be monitored at all times to confirm that it maintains uniform convergence, which is a visual indicator of whether the force is uniform.
Hot tightening: For high-temperature conditions (especially when first put into operation), after the system is heated to operating temperature and stabilized for a period of time (hot tightening) or cooled down (cold tightening), it may be necessary to re tighten according to the same specifications to compensate for the stress relaxation of bolts caused by differences in material thermal expansion. It is strictly prohibited to perform fastening operations under pressure!
Recording and tracing: For critical pipelines and important equipment, the initial pre tightening torque, final torque, and hot tightening data of bolts should be recorded for maintenance and fault analysis.
4、 Systematic thinking
Flange installation is not an isolated action, it is closely related to the entire pipeline system:
Stress considerations: The stress generated by the thermal expansion and contraction, vibration, or external loads of the pipeline itself should not be borne by flange connections. The support and flexibility of pipelines are prerequisites.
Sequential installation: In complex pipe sections composed of multiple flanges (such as valve groups), an overall installation sequence should be established to avoid rework due to the inability of a certain bolt to penetrate later.