Weld Neck Flanges

High-Integrity Butt-Weld Connection for High-Pressure and High-Temperature Service

What Is a Weld Neck Flange?

A weld neck flange is a forged flange with a long tapered hub that transitions smoothly to the pipe wall thickness. It is connected to the pipe via a single full-penetration butt weld, providing the highest structural integrity of any flange type.

Weld neck flanges are designed per ASME B16.5 for sizes NPS 1/2 through 24, and per ASME B16.47 for larger sizes NPS 26 through 60. The tapered hub provides reinforcement at the flange base and distributes mechanical and thermal stress evenly into the pipe wall, reducing high stress concentrations at the joint.

Key Design Features

  • Long tapered hub for gradual stress distribution from the flange face to the pipe
  • Butt-weld connection using a single V-groove weld for full penetration
  • Bore matches the pipe inside diameter for smooth, unobstructed flow
  • Available in raised face (RF), flat face (FF), and ring type joint (RTJ) facings
  • Highest structural integrity of all flange types, making it the preferred choice for critical service

Common Applications

  • High-pressure piping systems
  • High-temperature service
  • Cyclic loading and thermal expansion environments
  • Critical process piping in refineries and chemical plants
  • Offshore and subsea pipelines

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Available Sizes and Pressure Classes

Weld neck flanges are available across a wide range of sizes and pressure classes:

  • ASME B16.5: NPS 1/2 through 24, Classes 150, 300, 600, 900, 1500, and 2500
  • ASME B16.47 Series A & B: NPS 26 through 60, Classes 75, 150, 300, 400, 600, and 900

The bore of a weld neck flange is matched to the pipe schedule to ensure a flush interior surface and eliminate turbulence at the joint.

Common Materials

  • Carbon Steel (ASTM A105) — the most common material for general high-temperature and high-pressure service
  • Stainless Steel (ASTM A182 F304/F316) — for corrosion resistance in chemical and food processing
  • Alloy Steel (ASTM A182 F11/F22) — for elevated temperature service in power generation and refining
  • Low-Temperature (ASTM A350 LF2) — for cryogenic and low-temperature applications

Weld Neck vs Other Flange Types

Weld Neck vs Slip-On: Weld neck flanges are significantly stronger and more fatigue-resistant than slip-on flanges, but they cost more and require more precise alignment during installation. Slip-on flanges are adequate for low to moderate pressure service where cost savings are important.

Weld Neck vs Blind: A blind flange is used to close the end of a piping system and has no bore. A weld neck flange has a full bore and is used for inline connections.

Weld Neck vs Socket Weld: Socket weld flanges are typically used for small-bore piping (NPS 2 and under), while weld neck flanges are preferred for larger sizes and higher pressure ratings.

Related References