Flange Manufacturing Process

The flange manufacturing process primarily involves forging and casting methods. Each method has distinct advantages and disadvantages.

Forged and cast flanges offer benefits such as accurate shape and dimension, low production cost, and generally requiring minimal machining. However, forged flanges may contain air pockets, cracks, or other manufacturing defects, and their internal structure may not be as uniform as that of cast flanges.

Forged flanges provide the following advantages:

• Flexibility in forging various shapes
• Lower production cost
• Higher resistance to shear and tensile forces

Forged flanges generally have lower carbon content compared to cast flanges, making them more resistant to corrosion. They also have a more uniform internal structure and superior mechanical performance. While cast flanges are free from air pockets and cracks, improper forging may lead to coarse and irregular grains or solidification cracks, potentially resulting in higher costs compared to casting.

Manufacturing Process of Cast and Forged Flanges

Cast and forged flanges undergo different manufacturing processes. For example, a centrifugal flange is a type of cast flange. Centrifugal casting reduces internal defects such as porosity, sand holes, and loose texture, providing a more uniform internal structure than sand-cast flanges.

The general production steps include:

1. Cutting a circular plate from the central plate, defining inner diameter, outer diameter, and thickness.

2. Machining bolt holes and water lines for functional features.

3. Welding and flattening the circular shape.

4. Machining serrated spiral surfaces and bolt holes.

This process improves flange durability and assembly fit. For large-diameter flanges, the cutting and welding method enhances production efficiency and ensures precise alignment during installation.

Flange Materials

Flanges can be manufactured from various materials depending on the application and operating conditions. Common materials include:

• Forged carbon steel: Most commonly used for its machinability.
• Stainless steel: Provides high corrosion resistance and durability.
• Cast iron: Suitable for low-pressure and low-temperature applications.
• Aluminum, brass, bronze: Lightweight and used for special purposes.
• Plastic: For low-pressure systems requiring chemical resistance.

In certain applications, flanges may include layers of materials different from the flange itself to provide additional strength or corrosion resistance. Flange materials are typically selected to be compatible with the pipe material to prevent mechanical or chemical incompatibility.

All flanges discussed here, unless otherwise specified, comply with ASME and ASTM standards. ASME B16.5 defines flange dimensions and tolerances, while ASTM defines mechanical and chemical properties of materials. Correct material selection directly impacts pipeline life and safety; improper material choice may lead to pressure loss, corrosion, and mechanical failure.

Flange Sizes

Each flange has specific dimensions according to ASME B16.5, ensuring global compatibility in piping systems. For instance, if a welder in Japan, a pipe fitter in Canada, or an installer in Australia refers to a “NPS 6, Class 150, Schedule 40 ASME B16.5” weld neck flange, it indicates production according to the standard.

Suppliers usually require material grades when ordering flanges, such as:

• ASTM A105: Forged carbon steel flange
• ASTM A182: Forged stainless steel flange

Flange dimensions are selected according to pipe diameter, pressure class, and operating conditions. Incorrect sizing may result in leakage, pressure loss, or mechanical failure.

Dimensions and Tolerances

ASME B16.5 specifies:

• Inner and outer diameters
• Thickness
• Number and size of bolt holes
• Weight according to pressure class

This ensures flanges from different manufacturers can be assembled seamlessly. Proper sizing is crucial not only for the flange itself but also for the integrity and safety of the entire piping system.

Standard Flange Types

The choice of flange type depends on the required mechanical strength and ease of maintenance. Flanged connections allow easy disassembly, reducing maintenance and repair times.

Weld Neck Flange (WN)

• Contains a long tapered hub welded to the pipe.
• Suitable for high-pressure and high/low temperature applications.
• Allows unrestricted fluid flow, minimizing pressure drop.
• Tapered hub evenly distributes mechanical stress between pipe and flange.
• Suitable for radiographic inspection.

Long Weld Neck Flange (LWN)

• Similar to WN but with an extended neck.
• Commonly used in ships, columns, and tanks.
• Available in heavy barrel (HB) and equal barrel (E) types.

Slip-On Flange

• Connected to the pipe using two fillet welds, one inside and one outside.
• Pipe slides into the flange for welding.
• Compact design allows economic and simple installation.

Threaded Flange

• Mounted on the pipe without welding (typically NPT standard).
• Available up to 4 inches in multiple pressure classes.
• Suitable for low-pressure and small-diameter pipes.
• Used where welding is not possible, such as gas stations or explosive areas.

Socket Weld Flange

• Welded to the pipe using a single socket weld on the flange outside.
• Suitable for small-diameter, high-pressure pipes.
• Provides similar static strength to slip-on flanges.
• Fatigue resistance is higher due to a single weld instead of double fillet welds.

Lap Joint Flange

• Flat-faced flange used with a stub end.
• Can slide over the pipe and fit behind the stub end.
• Reduces material cost and optimizes use of different material grades in pipelines.

Blind Flange

• Has no center hole; seals pipes or pressure vessels.
• Must withstand high mechanical stress.
• Easy to remove for maintenance access.
• Can act as a manhole for pressure vessels.

Special Flange Types

Special flanges are designed for unique applications or piping requirements, including Nipoflange, Weldoflange, Rotating Flange, Expander Flange, and Reducing Flange.

Nipoflange

• Used for 90° branch pipe lines.
• Combines a weld neck flange with a forged nipolet.
• Single-piece forged steel; not assembled from separate parts.
• Nipolet is welded to the run pipe and bolted to the branch flange.
• Materials: ASTM A105, A350, A182 (duplex/super duplex stainless steel), Inconel, Incoloy, Hastelloy.
• Reinforced versions provide additional mechanical strength.

Weldoflange

• Conceptually similar to Nipoflange.
• Combines a weld neck flange with a Weldolet branch connection.
• Produced from a single-piece forged steel; no welded parts.

Rotating Flange

• Used in large-diameter pipes, submarine and offshore pipelines, petrochemical, and water management applications.
• Facilitates alignment of bolt holes between two mating flanges.
• Materials: Carbon steel (ASTM A105), alloy steel (ASTM A182 F1, F5, F9, F91), stainless steel (ASTM A182 F304, F304L, F316, F316L).
• Sizes: 3/8” to 60”, pressure classes: 150 to 2500.

Expander Flange

• Increases pipe diameter from one point to another or connects pipes to devices with different inlet sizes.
• Maximum 2–4 inch diameter increase.
• Cheaper and lighter compared to butt-weld reducers with standard flanges.
• Materials: A105, A350, ASTM A182.
• Pressure ratings and dimensions comply with ANSI/ASME B16.5 and available in RF or FF faces.

Reducing Flange

• Reduces pipe diameter safely.
• Transition can be reduced by only 1 or 2 sizes.
• Available in most sizes and material grades, generally not stocked.
• Same standards as expander flanges for features, sizes, and materials.