I. Basic Concept of Flanges

A flange is a disc-shaped component, commonly used for connections between pipe and pipe, pipe and equipment, and pipe and valve. It achieves a sealed connection by clamping two flanges together with bolts and placing a gasket between them.Flange connections feature easy disassembly, high strength, and good sealing performance, making them one of the most common connection methods in piping systems.

II. Main Functions of Flanges

• Pipe connection: Reliably connects two pipe sections for easy installation and disassembly.
• Equipment connection: Used for connecting pipelines to equipment ports such as pumps, valves, and heat exchangers.
• Convenient maintenance: Flange connections allow quick disassembly, facilitating system maintenance and cleaning.
• Excellent sealing performance: By compressing the gasket with bolts, reliable sealing can be maintained under high pressure, high temperature, and corrosive media.
• Strong load-bearing capacity: The flange structure can withstand relatively high pressure and external forces.

III. Basic Structure of Flanges

A complete flange assembly normally consists of the following parts:

• Flange disc: The main circular disc body with bolt holes.
• Sealing face: The surface on the flange disc that contacts the gasket, which is critical for sealing.
• Hub (for welded neck flanges): Used to increase flange strength and reduce stress concentration.
• Bolt holes: For inserting bolts to clamp the flanges.
• Bolts and nuts: Provide clamping force.
• Gasket: Placed between the two flange sealing faces to achieve sealing.

IV. Main Types of Flanges

Flanges are classified into various types according to their structure, suitable for different pressure ratings and service conditions.

• Plate Flat Welding Flange (PL): Simple structure and low cost, suitable for low-pressure pipelines.
• Slip-on Welding Flange (SO): Equipped with a short hub, stronger than PL flanges, used for medium and low-pressure conditions.
• Welded Neck Flange (WN): Long hub, butt-welded to the pipe, featuring high strength and excellent sealing, suitable for high-pressure and high-temperature pipelines.
• Integral Flange (IF): Cast or forged integrally with equipment or valve bodies, used for high-pressure equipment connections.
• Socket Welding Flange (SW): Pipe is inserted into the flange and then welded, suitable for small-bore, high-pressure pipelines.
• Threaded Flange (Th): Connected to the pipe by threads without welding, suitable for flammable, explosive areas or sites where hot work is prohibited.
• Lap Joint Flange (LJ): The flange can slide along the pipe, suitable for stainless steel, non-ferrous metal pipes and other materials difficult to weld.
• Blind Flange (BL): Without a central bore, used to close pipe ends or equipment openings.
• Lap Joint Flange with Butt Weld Ring (PJ/SE): Composed of a butt weld ring and a loose flange, suitable for corrosion-resistant pipelines.

V. Flange Face Types

The sealing face is the core of flange sealing, with different types adapted to various media and pressures.

• Raised Face (RF): The most widely used, with concentric serrations on the surface, suitable for most working conditions.
• Flat Face (FF): Smooth surface, used for low-pressure, large-diameter applications or where uniform bolt distribution is required.
• Male and Female Face (MFM): Matched male and female faces prevent the gasket from being extruded, suitable for medium and high pressure.
• Tongue and Groove Face (TG): Consists of a tongue and a groove; the gasket is fully embedded, suitable for flammable, explosive, and toxic media.
• Ring Joint Face (RJ): Used in high-pressure and high-temperature pipelines, sealing with metal ring gaskets.

VI. Flange Pressure Ratings

Flange pressure rating indicates the maximum working pressure it can withstand, with different notations in various standard systems.

• PN series (European & Chinese standards): e.g., PN6, PN10, PN16, PN25, PN40, PN63, PN100, etc.
• Class series (American standards): e.g., Class 150, 300, 600, 900, 1500, 2500.
• Japanese standard pressure ratings: e.g., 5K, 10K, 16K, 20K, 30K.

The pressure rating is not the actual working pressure and must be adjusted based on temperature, material, and other factors.

VII. Common Flange Materials

Flange material is generally consistent with piping material to ensure weldability and corrosion resistance.

• Carbon steel: e.g., 20#, Q235, A105, A234 WPB
• Stainless steel: e.g., 304, 316, 316L
• Alloy steel: e.g., 15CrMo, F11, F22
• Cast iron: e.g., gray cast iron, ductile iron
• Non-ferrous metals: e.g., copper, aluminum
• Plastics: e.g., PVC, PP, PE (mostly used for low-pressure pipelines)

VIII. Flange Connection Methods

• Welded connections: butt welding, flat welding, socket welding
• Threaded connections: suitable for small-bore, low-pressure systems
• Lap joint connections: for stainless steel or non-ferrous metal pipelines
• Clamp connections: used in special quick-connect systems

IX. Flange Selection Principles

• Select the appropriate pressure rating according to pipeline pressure.
• Choose suitable material based on operating temperature.
• Determine the sealing face type according to the conveyed medium.
• Select the flange type based on installation space and maintenance requirements.
• Ensure the flange standard matches that of pipes and valves.
• Gasket material must be compatible with the medium.