1. Confirm Correct Installation Direction
The flange face must face inwards: The toothed or smooth flange face should be tightly against the surface of the connected parts to achieve anti-loosening and pressure distribution functions.
The threaded end should face outwards: This facilitates tightening with tools and subsequent maintenance. Incorrect direction will lead to anti-loosening failure, insufficient contact area, and even crushing of the connected parts.
2. Check Tightening Quality
Diagonal Tightening Sequence: Confirm that tightening is done in a diagonal or star-shaped sequence (e.g., 50% → 80% → 100% torque) to avoid flange misalignment.
Torque Retest: Use a torque wrench to randomly check key bolts to ensure that the design preload is reached and the value is stable.
Compliant Tool Use: For critical equipment involving high temperature, high pressure, flammable, or explosive materials, a torque wrench should be used for precise control, rather than relying solely on feel.
3. Verify Sealing Performance
Pressure Test: Conduct a pneumatic or hydrostatic test on the pipeline or pressure system (pneumatic pressure 1.15 times the working pressure, hydrostatic pressure 1.5 times) and hold the pressure for 30 minutes.
Soapy Leak Detection: Apply soapy water to the flange connection, bolt roots, and gasket edges, and observe for continuous bubbling. Brief bubbling is a false alarm; continuous bubbling indicates a leak.
Additional High-Temperature Inspection: Perform hot tightening after the equipment has heated up to compensate for preload loss due to thermal expansion and contraction.
4. Check Exposed Thread Length
After tightening, the bolt should have at least two complete threads protruding beyond the nut to ensure full engagement.
According to industry standards, a protruding length of 0-3 thread pitch is recommended, with 2 thread pitch being optimal, ensuring connection strength while avoiding interference risks associated with excessive length.
5. Appearance and Structural Integrity
The flange face should fit completely without warping or gaps.
Toothed flanges should show slight indentations, indicating effective engagement.
Inspect nuts and bolts for deformation, cracks, or stripped threads.
