I. Five core categories and characteristics of flanges
1. Classification by connection method
- Slip-on flange (SO flange): Requires only single-sided welding, low cost, but weak pressure bearing capacity (common pressure ≤ 2.5 MPa), mostly used in low-pressure piping systems.
- Weld-on flange (WN flange): Has a neck structure and requires double-sided welding. High strength (can withstand high pressures up to 42 MPa), suitable for critical applications such as petrochemicals and nuclear power.
- Socket weld flange (SW flange): The pipe is inserted into the flange inner ring and welded. It provides good sealing but is difficult to disassemble. It is commonly used in small-diameter, high-pressure pipes (DN ≤ 80 mm).
- Threaded flange (Th flange): Has an internal thread connection, requires no welding, and is easy to assemble and disassemble, but is prone to leakage. It is mostly used for low-pressure, non-corrosive media (such as fire protection pipes).
- Lap joint flange: Consists of a flange ring and a short pipe, which can be rotated for alignment. It is suitable for pipelines that are frequently disassembled, but the cost is higher.
2. Classification by Sealing Surface Type
- Raised Face (RF): Flat surface with concentric grooves (easy to replace the gasket), accounting for over 60% of the market (according to ASME B16.5 standard), highly versatile.
- Male-Female-Meat Face (MFM): Used in pairs of one male and one female, it offers better sealing than the RF surface, but requires precise alignment during installation. It is commonly used in medium- and high-pressure systems.
- Ring Joint (RTJ): A metal ring gasket embedded in a trapezoidal groove, with a pressure resistance of up to 100 MPa, designed for extreme operating conditions (such as deep-sea oil and gas pipelines).
- Full Face (FF): The entire end face is flat, used with non-metallic gaskets, and is suitable for low-pressure corrosive media (such as acid and alkali transportation).
II. Other Key Classification Dimensions
3. Classification by Pressure Rating
- The American Standard (ASME B16.5) divides flanges into seven grades: 150#, 300#, 600#, 900#, 1500#, 2500#, and 4500#. The number represents the poundage (e.g., 150# ≈ 2.0 MPa).
- The European Standard (EN 1092-1) uses PN: PN6 to PN100 (e.g., PN16 ≈ 1.6 MPa). Be aware of these differences when selecting flanges.
4. Classification by Material
- Carbon Steel Flanges (Q235, A105): Highly cost-effective, accounting for over 70% of industrial use, but with poor corrosion resistance.
- Stainless Steel Flanges (304/316L): Highly corrosion-resistant, essential for the food and pharmaceutical industries, but cost 3-5 times more than carbon steel.
- Alloy Steel Flanges (16Mn, CrMo): Resistant to high temperatures and high pressures, used in power plant boilers (operating temperatures up to 600°C).
- Plastic Flanges (PP/PVC): Lightweight and insulated, suitable for chemically corrosive environments (pressure bearing capacity ≤ 1.6 MPa).
5. Special-Purpose Flanges
- Blind Flange (BL): Closes the end of a pipe, used for maintenance. Thickness must be ≥ 1.5 times that of a flange of the same specification (GB/T 9124).
- Reducing Flange: Connects pipes of different diameters, reducing the need for reducers, but requires custom processing.
- Insulating Flange: Includes insulating gaskets to prevent electrochemical corrosion. A set should be installed every 20 km on long-distance pipelines (SY/T 0516 standard).
III. Selection Recommendations and Industry Trends
- Preferred Standard: Domestic projects often use GB/T 9119, while export projects must comply with ASME or EN standards.
- Emerging Technologies: 3D-printed flanges are beginning to be used for complex custom structures, but the cost remains 8-10 times that of traditional methods (2023 market research data).
- Environmental Requirements: Asbestos-free gasket flanges are becoming mainstream, as the EU has completely ban