SSM is a manufacturer and supplier of flanges. If you are unfamiliar with the basics of flanges, we will provide you with a simple introduction to help you understand them better.
Flanges are critical connecting components in piping systems, used to join pipes, valves, pumps, and other equipment, facilitating cleaning, inspection, and modifications.
This guide will systematically explain the fundamentals of flanges, covering types, standards, materials, and selection criteria.
What is a Flange?
A flange is a protruding rim or collar (usually circular) with bolt holes, designed to create a high-strength, leak-proof seal between pipes or between a pipe and equipment using bolts and gaskets. It serves as an alternative to welding while allowing for easy disassembly.
Core Functions of Flanges:
✔ Quick connection of piping systems without welding
✔ Easy disassembly for maintenance
✔ Withstands high-pressure and high-temperature conditions
✔ Ensures precise alignment due to CNC-machined surfaces, minimizing leakage and safety risks
Common Types of Flanges
The market primarily offers six types of flanges, which cover most industrial applications. Custom or special flanges can also be manufactured upon request. Below are the most common types:
1. Weld Neck Flange (WN)
Structural Features:
- Tapered neck with a V-groove for butt welding
- The neck transition disperses stress, reducing fatigue cracking risks
- Common sealing faces: RF (Raised Face) or RTJ (Ring-Type Joint)
Selection Note:
- For thick-walled pipes, internal beveling is recommended to match pipe thickness
Technical Parameters:
- Pressure Rating: Class 150~2500 (ASME) / PN10~PN420 (EN)
- Temperature Range: -196°C ~ +800°C (material-dependent)
- Standard Sizes: DN15~DN600 (½”~24″)
2. Slip-On Flange (SO)
Structural Features:
- Slightly larger inner diameter than the pipe, fixed by fillet welding inside and outside
- Common sealing faces: FF (Flat Face) or RF (Raised Face)
Technical Parameters:
- Max Pressure: Class 300 (PN40)
- Economical Size Range: DN50~DN400
3. Socket Weld Flange (SW)
Structural Features:
- Pipe inserts into the flange socket and is welded at the end
- Includes a 1.6mm gap for thermal expansion compensation
Technical Parameters:
- Pressure Rating: Up to Class 3000 (PN100)
- Optimal Size Range: DN8~DN50
4. Blind Flange (BL)
Structural Features:
- Solid flange without a center hole
- Matches the sealing face of the paired flange
5. Lap Joint Flange (LJ)
Structural Features:
- Consists of a flange body + Stub End (backing flange)
- Rotatable 360° for bolt hole alignment
Limitations:
- Not suitable for toxic media (potential leakage risk)
- Max Pressure: Class 600
6. Threaded Flange (THD)
Structural Features:
- Internal threads per ASME B1.20.1 standard
- Requires external threads (NPT or BSPT) on pipes
Application Limits:
- Max Pressure: Class 300 (limited to Class 150 for steam systems)
- Prohibited for highly corrosive/toxic media or pulsating loads
Special Flanges
Beyond standard flanges, specialized designs address unique industrial needs:
Orifice Flanges integrate pressure taps for flow measurement, while Expander/Reducing Flanges manage pipe diameter transitions without the need for separate fittings. Long Neck Welding Flanges (LWN) reinforce high-stress offshore systems, and Spectacle Blinds enable visual flow isolation for maintenance safety. Swivel Ring Flanges simplify bolt alignment in tight spaces, Anchor Flanges absorb pipeline thrust forces, and Grooved/Van Stone Flanges offer quick-connect flexibility. Companion Flanges bridge equipment with non-standard connections.
Flange Face Types
The sealing surface design directly impacts leak prevention:
Flat Face (FF): Full-contact gasket seating, used for low-pressure cast iron flanges
Raised Face (RF): Most common (ASME B16.5), raised height is 1.6mm (150#) or 6.4mm (300#+)
Ring-Type Joint (RTJ): For ultra-high-pressure (oil & gas), requires metal ring gaskets
Tongue & Groove (T&G): Prevents gasket blowout, typical in hydraulic systems
Flange Standards & Specifications
Most flanges are standardized. Below is a comparison of global standards:
| Standard | Region | Key Features |
| ASME B16.5 | USA | General use (up to 24″) |
| ASME B16.47 | USA | Large-diameter (26″-60″) |
| EN 1092-1 | Europe | PN pressure ratings (PN6-PN100) |
| JIS B2220 | Japan | 5K/10K/16K pressure classes |
| DIN | Germany | Metric sizing |
Pressure Rating Comparison:
- ASME Classes: 150#, 300#, 600#, 900#, 1500#
- PN Ratings: PN6, PN10, PN16, PN25, PN40
Flange Material Selection
Choosing the right material ensures cost efficiency, safety, and longevity:
| Material | Best For | Temperature Range |
| Carbon Steel (A105) | General applications | -29°C ~ +425°C |
| Stainless Steel (304/316) | Corrosive media | Up to 800°C |
| Alloy Steel (F11/F22) | High temp/pressure | Up to 550°C |
| Duplex Steel (S31803) | Offshore/chemical | Highly corrosion-resistant |
Conclusion
Flanges are essential for safe and efficient piping systems. Proper selection requires evaluating media properties, pressure/temperature conditions, and installation requirements.
Need flanges? Consult our engineers for professional selection advice!
Request a flange selection guide or dimension charts? Contact us anytime!
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