Flanges and Bolts
Flange connections are the most common detachable joint for pipes, pressure vessels and valves. A full flange assembly consis
Flange connections are the most common detachable joint for pipes, pressure vessels and valves. A full flange assembly consists of flange, gasket, bolts and nuts.
Bolts apply uniform clamping force to compress the gasket between two flange faces, creating a sealed joint to prevent medium leakage.
Working Principle
When tightening the bolts, axial preload is generated. This force squeezes the gasket to form a continuous sealing layer between flange sealing surfaces.
• Sufficient bolt preload: keeps gasket compressed and maintains sealing under working pressure & temperature.
• Insufficient preload: leakage may occur.
• Over-tightening: gasket crush or flange deformation.
Bolt Selection for Flange
1. Type: Double end studs (stud bolts) are the preferred choice for high-pressure / high-temperature flanges. Hex head bolts are used for low-pressure general flanges.
2. Material: Match bolt material with flange & service environment.
◦ Carbon steel flange: B7 alloy steel studs
◦ Stainless steel flange: A2 / A4 stainless bolts
◦ Corrosive offshore / petrochemical service: Duplex or Super Duplex stainless steel studs (2205, 2507)
3. Strength grade: Higher pressure class flange requires higher strength fasteners.
Key Points for Assembly
1. Tighten bolts in a cross / star sequence to evenly distribute pressure across the flange face, avoid flange warping.
2. Tighten in multiple passes (3~4 stages) step by step, not one-time full torque.
3. For high-temperature service, hot bolting may be required after equipment warming up.
Common Application
Pipeline flanges, pressure vessel nozzles, valve flange ends, heat exchanger connections.
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