The flange is an indispensable key component in modern piping and equipment systems. As a standardized, detachable piping connection method, it is primarily used to achieve reliable interfaces between pipes, valves, pumps, and equipment, greatly facilitating system sealing, installation, routine maintenance, and inspection. Flange connections utilize bolt pre-load to tightly join components, making them one of the most widely used connection forms in fluid transport engineering.
What is a Flange
A flange is essentially a round or square connection piece with bolt holes, mounted on the end of a pipe or equipment. In industrial practice, a complete flange connection assembly consists of three core components:
- Flange Body: Typically two pieces, used to connect the pipe or equipment ports.
- Gasket: Placed between the two flange faces to provide a flexible seal.
- Bolting: The application of uniform pre-load via bolts and nuts.
Exploded diagram illustrating the main components of a flange connection—flange body, gasket, and bolting.
The core value of the flange connection lies in its ability to achieve rapid, reliable, and repetitive disassembly and maintenance while guaranteeing leak-free operation, which is a significant advantage over welded connections.
Beyond piping applications, the term “flange” can also refer more broadly to any projecting rim or edge used to strengthen or connect mechanical structures.
How a Flange Connection Works
The sealing principle of a flange connection is based on mechanical force. The process primarily follows these steps:
- Initial Placement: The gasket is precisely positioned between the sealing faces of the two flanges.
- Pre-load Application: Significant axial pre-load is applied to the gasket through the uniform tightening of bolts and nuts.
- Gasket Deformation: Under the pre-load, the gasket undergoes controlled deformation (elastic or plastic), fully filling and conforming to the minor irregularities, grooves, or defects on the two flange faces.
- Sealing Achievement: When the system is operational, the separating forces generated by the internal media pressure, temperature, and external loads are balanced by the bolt pre-load. As long as the bolt force exceeds the sum of all separating forces, the gasket maintains its minimum sealing stress, thereby achieving a reliable and long-lasting seal.
Common Flange Types
Flanges come in various types, each suited for different operating conditions, pressure ratings, and installation requirements.
Weld Neck Flange
The weld neck flange features a long tapered hub that connects to the pipe through butt welding. Its gradual transition from the flange thickness to the pipe wall minimizes stress concentration, providing excellent strength and sealing reliability. It is ideal for high-pressure, high-temperature, and fluctuating service conditions such as steam lines and heat exchangers.
Slip-On Flange
The slip-on flange slides directly over the pipe and is fillet welded on both the inner and outer sides. It offers simple alignment and requires less welding work. Due to its moderate strength, it is best suited for low- to medium-pressure applications at normal temperatures, but not recommended for severe pressure or temperature variations.
Blind Flange
The blind flange is a solid plate without a bore, used to close the end of a pipeline, valve, or pressure vessel. It allows easy access for inspection, maintenance, or hydrostatic testing, and provides a reliable means of isolating a section of the piping system.
Socket Weld Flange
The socket weld flange has a recessed bore where the pipe is inserted and joined with a single external fillet weld. This design provides high structural integrity and is commonly used for small-diameter, high-pressure systems such as instrument lines. However, it is less suitable for corrosive fluids or applications requiring strict weld inspection.
Threaded Flange
The threaded flange connects to pipes via internal threads, eliminating the need for welding. It allows quick installation and is especially useful in areas where welding is hazardous or not permitted. Threaded flanges are typically used in low-pressure, non-critical systems involving flammable or explosive media.
Lap Joint Flange
The lap joint flange consists of a stub end and a loose backing ring that can rotate freely around it. This allows easy alignment of bolt holes and convenient assembly or disassembly. It is ideal for piping systems requiring frequent maintenance or handling of corrosive fluids, where the stub end can be made of corrosion-resistant material while the backing flange uses cost-effective carbon steel.
Flange Facing Types
The sealing face is the part of the flange that directly contacts the gasket, and its geometry determines sealing performance and suitable gasket materials.
Comparison of three flange facing types: Ring Type Joint (RTJ), Raised Face (RF), and Flat Face (FF).
- Ring Type Joint (RTJ):The sealing face includes a precisely machined groove that holds a metal ring gasket. This configuration offers extremely high sealing integrity and is widely used in high-pressure, high-temperature, or hazardous media conditions.
- Raised Face (RF):The sealing surface is slightly raised above the bolt circle face. It is the most commonly used flange facing, providing broad adaptability to different gaskets, especially non-metallic sheet gaskets and spiral wound gaskets.
- Flat Face (FF):The sealing surface is flush with the entire flange face. It is typically used for low-pressure systems and equipment made of brittle materials, such as cast iron or fiberglass, to avoid bending stress during bolt tightening.
Dimensions
Flange dimensions define the geometric parameters necessary to ensure proper alignment, sealing, and mechanical integrity within a piping system.
They determine not only the flange’s compatibility with the connected pipe but also its ability to accommodate gaskets and bolting correctly under pressure.
Key Dimensional Parameters:
- Nominal Diameter (DN / NPS): Represents the nominal pipe size corresponding to the inner diameter of the connected pipe — DN for metric systems and NPS for imperial systems.
- Outside Diameter and Bolt Circle: Specify the overall flange size and the exact positioning of bolt holes to guarantee alignment and interchangeability.
- Design Basis: All dimensional parameters — including bolt number, size, and spacing — must strictly conform to the applicable pressure rating standard (e.g., ASME B16.5 or EN 1092) to ensure that the flange, gasket, and bolting assembly perform safely under the intended design pressure.
Pressure Ratings
Flange pressure ratings define the maximum internal pressure a flange can safely withstand at a given temperature. They are the core indicator of a flange’s strength and pressure-bearing capacity, and serve as a key parameter in system design and material selection.
Classification Systems:
Different standards express flange pressure ratings in distinct systems:
- American System (ASME Class): Uses designations such as Class 150, Class 300, Class 600, etc. A higher class number indicates greater pressure resistance.
- European System (EN/DIN PN): Uses designations such as PN10, PN16, PN40, etc., where PN stands for Nominal Pressure (typically measured in bar).
It is important to note that the allowable working pressure of a flange decreases as operating temperature rises. Design engineers must always consult the applicable Pressure–Temperature Rating Tables for the specific material and standard to ensure the selected flange meets actual service conditions.
Flange Standards and Marking
Flange standards define the dimensional, material, and testing requirements that enable global interchangeability and consistent performance across piping systems.
They are categorized by regional and industrial systems, each with its own pressure rating series and dimensional conventions.
Mandatory markings on every flange ensure full traceability and identification throughout manufacturing and service life.
Common Standard Systems:
- ASME B16.5 and ASME B16.47 – Predominantly used in the Americas.
B16.5 covers flanges up to NPS 24, while B16.47 applies to large diameters from NPS 26 to NPS 60. - EN 1092 / DIN Standards – Common in Europe, using the PN pressure series to specify nominal pressure ratings.
- HG/T 20592–20635 – Chinese chemical industry standards, harmonized with international systems and supporting both PN and Class series.
All compliant flanges are permanently marked with critical data such as standard designation, material grade, nominal size, pressure class, and heat number to ensure quality control, regulatory compliance, and reliable traceability.
Flange Applications in Industry
Due to their reliability and maintainability, flange connections are widely applied in critical infrastructure across various industries:
- Oil and Gas Pipeline Systems: Used to connect long-distance transmission lines, pump stations, compressor stations, and oil and gas processing equipment.
- Chemical Process Piping: Used to transport corrosive, high-temperature, or high-pressure chemical media, facilitating regular inspection and equipment replacement.
- Marine and Offshore Engineering Systems: Used to connect ship piping, desalination equipment, and various systems on offshore platforms.
- HVAC and Water Treatment Systems: Used to connect boilers, heat exchangers, pumps, and large municipal water supply and drainage networks.
Conclusion
The flange, as a standardized, highly maintainable, and broadly applicable pipe connection method, plays a crucial role in industrial safety and efficiency. When designing and sourcing, engineers must carefully consider factors like medium properties, operating temperature, and pressure to correctly select the flange type, facing, pressure rating, and standard. Only through proper selection and installation can the long-term safety and reliable operation of the entire fluid transport system be guaranteed.
To ensure your next critical piping project is compliant and optimized, contact our engineering team today for expert consultation on material compatibility, pressure tolerance, and custom flange solutions.
