Weld Neck (WN) vs. Slip-On (SO) Flange: A Comprehensive Technical Reference

1. Introduction In the complex landscape of PVF (Pipes, Valves, Flanges) procurement, selecting the correct flange type is critical for both operational safety and cost efficiency. The Weld Neck (WN) and Slip-On (SO) flanges are the two most prevalent designs in global infrastructure. This technical reference clarifies their structural differences, welding requirements, and standard applications according to ASME B16.5 and EN 1092-1.

2. Anatomical and Structural Comparison

• Weld Neck Flange (WN): Distinguished by a long, tapered hub that transitions gradually into the pipe. The bore of the flange is machined to match the inside diameter (ID) of the mating pipe, ensuring a smooth flow with minimal turbulence.
• Slip-On Flange (SO): A ring-like design where the inside diameter is slightly larger than the pipe’s outside diameter (OD). The pipe is inserted into the flange and secured with internal and external welds.

3. Welding and NDT (Non-Destructive Testing)

• WN Welding: Requires a single V-butt weld at the hub-to-pipe junction. This type of joint allows for 100% Radiographic Testing (RT), providing full volumetric verification of the weld integrity.
• SO Welding: Requires two fillet welds—one external at the back of the flange and one internal near the face. Due to the overlapping geometry, fillet welds can typically only be inspected via Surface NDT (Dye Penetrant or Magnetic Particle), which does not reveal internal weld defects.

4. Pressure and Temperature Ratings While both types are available in Class 150 to Class 2500, their performance under stress varies:

• WN: Superior at handling high-pressure surges and extreme temperature fluctuations (Cryogenic to High-Temp).
• SO: Generally limited to Class 150 and Class 300 applications in non-critical utility lines.