| Flange Type | Connection |
|---|---|
| Threaded Boss Flange | Has an internal (female) thread cut into the boss |
| Plain (Unthreaded) Boss Flange | Has a smooth bore for welding |
Standards, Materials, Sizes and Variations
- Flange Standards:
AS2129 Table D, E, F, H, J used mostly for threaded flanges
AS4087 PN 16, PN21, PN35 used mostly for threaded flanges
EN1092 PN 6, PN 10, PN 16, PN 25, PN 40, PN 63, PN 100 refered to as hubbed flanges
ASME/ANSI
All these flanges have a boss. Special dimensions are required for tapped or threaded flanges
Plus many others - Pressure Ratings: Bossed flanges exist in most standards and pressure ratings
- Material Grades: Stainless Steel (304, 316, Duplex 2205) ;Forged Carbon Steel (ASTM A105) ;Mild Steel (Plate)
Balcombe Engineering can produce these in a huge range of materials to meet your application - Sealing Type: Flat Face / Raised Face / Ring Joint
- Sealing Finish: Spiral Finish 3.2 μm – 12.0 μm
- End Connection Type: Threaded (NPT / BSPT / BSPP) / Welded (Socket / Butt)
- Thread Type: Male / Female
- Fast Delivery - Stock items same day, custom orders typically 10-20 days
Boss Flange Types
Boss Flange Variations and Uses
| Type | Description | Use / Advantage |
|---|---|---|
| Slip-on bossed flange | Flange slips over pipe, then welded, and has a boss reinforcement disc | May be a more robust weld joint, better load distribution |
| Bossed blind flange | A blind flange (i.e. one that blocks flow) but with a boss (reinforcement) around its bore even though there's no through bore | Usually a specific customer specification |
| Forged boss (socket weld / threaded) | A standalone piece (often short and thick) with either threading or socket weld ends used to attach a branch outlet | Used for taps, instrumentation, as auxiliary connections |
Boss Flange Applications
Oil and Gas – Seals and isolates pipelines for testing or maintenance; withstands high pressures in upstream, refining, and offshore operations.
Chemical and Petrochemical – Handles corrosive fluids and high-temperature process lines in reactors and pressure vessels.
Power Generation – Provides leak-proof joints in steam lines, boilers, turbines, and cooling systems under fluctuating conditions.
Water and Wastewater Treatment – Secures high-pressure mains, pumping stations, and desalination plant connections.
Marine and Shipbuilding – Connects engine-room piping, ballast systems, and hydraulic circuits with compact, reinforced joints.
HVAC and Industrial Systems – Ensures durable sealing in large-diameter heating, cooling, and steam distribution networks.
Materials & Grades
Our flanges are available in multiple materials:
- Forged Carbon Steel (ASTM A105, A350 LF2) for general industrial applications
- Stainless Steel (304L, 316L) for corrosion-resistant requirements
- Duplex Steels (2205,2507, F51,F53) for high-strength, corrosive environments
- Specialized alloys available for extreme service conditions
| Material | Grades | Key Features | Applications |
|---|---|---|---|
| Carbon Steel | A105 | High strength, cost-effective | General industrial, oil & gas |
| Stainless Steel | 304L, 316L | Corrosion resistance | Chemical processing, marine |
| Duplex Steel | 2205 | Superior corrosion resistance | Offshore, high-chloride environments |
Applications & Industries
Threaded Boss Flanges
- Commonly used in low to medium-pressure systems requiring threaded connections
- Ideal for instrumentation ports, drains, vents, and small-bore piping
- Used in chemical, marine, and compressed air systems
- Enables easy assembly and disassembly without welding
Plain (Unthreaded) Boss Flanges
- Designed for welded connections where a strong, permanent joint is needed
- Suitable for process piping, pressure vessels, and water treatment systems
- Used in refinery, power generation, and mechanical engineering applications
- Provides a smooth bore for secure, leak-tight welding
Advantages
- Allows easy installation and removal without full pipe disassembly
- Suitable for both threaded and welded connections
- Provides a secure seal for low to medium-pressure systems
- Compact and versatile — fits confined or maintenance-heavy piping layouts
Limitations
- Not ideal for high-pressure or high-temperature service
- Requires accurate thread specification (BSP, NPT, etc.) to ensure compatibility
- Threaded versions may be prone to leakage under vibration or thermal cycling
- Less strength compared to weld neck or integral flanges
FAQ
A flange boss is a short, raised section on a component that provides a flat, reinforced surface for attaching a flange or threaded fitting. It ensures proper sealing, strength, and alignment during installation.
In piping, a boss is a reinforced pad or protrusion welded or cast onto a pipe or vessel to allow a threaded or welded connection—often used for instrumentation, drains, or vent points.
A bossed flange features a raised hub or collar (boss) around the bore to strengthen the connection and improve pressure handling. It’s commonly used where higher mechanical support or frequent assembly is required.
The main types are:
Weld Neck Flange
Slip-On Flange
Socket Weld Flange
Blind Flange
Threaded Flange
Lap Joint Flange
(Some standards also include Boss and Orifice flanges as specialized types.)
Fillet Weld: Common for slip-on and socket weld flanges.
Full penetration weld: Used in weld neck and long weld neck flanges for high-pressure systems.
Check the flange face (RF, FF, RTJ), hub shape, bore connection type, and markings (standard, size, and class) stamped on the rim.
The weld neck flange is the strongest due to its tapered hub and full-penetration weld, ideal for high-pressure and high-temperature applications.
No this does not work as leakage will occur through the bolt holes
Why Choose Balcombe Engineering
Quality Assurance
Flanges meeting international standards (ANSI, DIN, JIS, AS)
Extensive Range
Wide selection of flange types, sizes, and materials
Expert Support
Real people, knowledgeable and experienced. Ready to assist with your needs
Competitive Pricing
High-quality flanges at affordable prices
Nationwide Delivery
Prompt service across Australia
Custom Manufacturing
Able to manufacture flanges to your custom drawing specifications
