Not every weld neck flange looks identical. The tapered hub and butt-weld end are constant, but the face type, hub length, and bore can all change to fit a specific joint or industry requirement. This guide covers the main types of weld neck flanges and the industries where they are the preferred choice.
Weld Neck Flange Types
Standard Weld Neck
This is the most common type, with a tapered hub of moderate length that butt-welds directly to the matching pipe. This type is available across the full range of ANSI/ASME B16.5 and B16.47 pressure classes and sizes.
- Manufactured in forged ASTM A105 carbon steel or ASTM A182 stainless steel.
- Rated across the full range of ANSI/ASME B16.5 pressure classes, 150 through 2500.
- The tapered hub transitions gradually into the pipe wall thickness, distributing stress along the joint.
- This is the main choice for general inline process and pipeline connections.
Long Weld Neck
A long weld neck flange has an extended hub longer than the standard configuration and is often used as a nozzle on pressure vessels rather than an inline pipe connection. The extra hub length provides additional reinforcement where the flange penetrates the vessel wall.
- Hub length is typically specified to match the vessel wall thickness plus the projection needed for internal components.
- Common in pressure vessel, heat exchanger, and boiler nozzle connections.
- Often referred to as a “nozzle flange” on vessel fabrication drawings.
- This type reduces the number of separate weld joints needed at a vessel penetration.
Reducing Weld Neck
A reducing weld neck flange has a hub with a reducing bore. This type of flange allows a larger flange to connect directly to a smaller-diameter pipe without a separate reducer fitting. This is common where a line changes size at a valve, pump, or equipment connection.
- A reducing weld neck flange combines the flange and reducer into a single forged component, eliminating one weld joint from the system.
- This type is commonly specified where a line steps down in size at a pump, valve, or piece of equipment.
- Reduces total part count and installation time compared to a separate reducer plus standard flange.
- Available in the same pressure classes as standard weld neck flanges.
Raised-Face Weld Neck
Raised face weld neck flanges have a small raised sealing area right above the bolt circle, which improves gasket compression. This is the standard facing for high-pressure, high-temperature systems and the most commonly specified face type for weld neck flanges in process piping.
- Raised face height is typically 1/16 inch (1.6 mm) above the bolt circle face for Class 150 and Class 300, and 1/4 inch (6.4mm) for Class 400 and higher per ANSI/ASME B16.5.
- This type is compatible with standard ring-type gaskets, including spiral-wound and flat ring gaskets.
- Available across all ANSI/ASME B16.5 pressure classes.
- This is the default face type unless a project specification calls for flat face or ring-type joint.
Flat-Face Weld Neck
Flat face (FF) weld neck flanges have a flush sealing surface with no raised area. Because forcing a raised face flange against brittle material can cause severe cracking under bolt load, the flat face design is used in lower-pressure lines or when mating directly to cast iron, bronze, or fiberglass equipment.
- The sealing surface of this flange type sits flush with the bolt circle face, with no raised area.
- Typically paired with a full-face gasket rather than a ring gasket.
- FF weld necks are usually specified per ANSI/ASME B16.5 when mating to cast iron, fiberglass, or other equipment that cannot handle the concentrated bolt load of a raised face.
- They are common in low-pressure water, wastewater, and utility piping systems.
Applications of Weld Neck Flanges
Chemical and Process Piping
Specified throughout critical process lines in chemical plants and refineries, weld neck flanges provide the long-term reliability and leak-resistant integrity essential for handling corrosive media and high-pressure spikes.
Steam and Thermal Systems
Boilers and steam lines rely on the butt-weld connection to handle thermal expansion without warping or leaking. The full penetration weld tolerates cyclic thermal stress better than a fillet welded joint.
Offshore and Subsea Platforms
Marine environments favor weld neck flanges for their robust weld design. They can withstand vibration, shifting loads, and corrosive conditions better than lighter flange types.
Large-Diameter Water Mains
Municipal and industrial water systems use weld neck flanges where large pipe diameters require secure, well-aligned connections. They also help minimize disruption to flow.
Conclusion
There are five common weld neck flange types, which include: standard, long, reducing, raised-face, and flat-face configurations. Each type solves a specific joint requirement, ranging from pressure vessel nozzles and pipe size transitions to compatibility with cast iron equipment. Matching the flange configuration to the application ensures the connection remains structurally sound and properly sealed.
API International supplies weld neck flanges machined from A105 forged carbon steel and A182 and 304/316 forged stainless steel, meeting ANSI/ASME B16.5 and ANSI/ASME B16.47 standards. Explore the full range in our online product catalog, or contact us for custom machining on nonstandard sizes and configurations. Get connected with a dedicated sales representative today, or call us at 503.692.3800.
Frequently Asked Questions
What is the difference between a standard and a long weld neck flange?
A standard weld neck flange has a moderate-length tapered hub for general pipe connections. A long weld neck flange has an extended hub and is typically used as a nozzle on pressure vessels rather than an inline pipe joint.
What is a reducing weld neck flange used for?
A reducing weld neck flange has a reducing bore in the hub, allowing it to connect directly to a smaller-diameter pipe without needing a separate reducer fitting.
When is a flat-face weld neck flange used instead of raised-face?
Flat-face weld neck flanges are used in lower-pressure lines or when connecting to cast iron equipment, since a raised face can crack a brittle cast iron mating flange under bolt load.
Which industries rely most on weld neck flanges?
Power generation and steam systems, offshore and subsea platforms, large-diameter water infrastructure, and chemical and process plants all specify weld neck flanges for critical, high-pressure connections.
Why are weld neck flanges preferred for high-pressure pipelines?
The tapered hub distributes stress evenly along the joint, and the full-penetration butt weld provides stronger, more fatigue-resistant performance than fillet-welded flange types, which matters directly under pressure spikes and cyclic loading.