Differences between a flange with a neck for seamless welding and a flange with a neck for butt welding

1. Material: A flange with a neck for seamless welding is fabricated from ordinary steel plates whose thickness meets specified requirements, whereas a flange with a neck for butt welding is typically fabricated from forged steel components followed by machining; 

2. Weld type: For a flange with a neck for seamless welding, the weld between the pipe and the flange is a fillet weld; for a flange with a neck for butt welding, the weld between the pipe and the flange is a circumferential weld;

 3. Nominal pressure: The nominal pressure range for a flange with a neck for seamless welding is 0.6–4.0 MPa, whereas for a flange with a neck for butt welding, the nominal pressure range is 1–25 MPa;

 4. Additionally, the weld between a flange with a neck for butt welding and a pipe joint is classified as Class B, whereas the weld between a flange with a neck for seamless welding and a pipe joint is classified as Class C; consequently, their post-weld non-destructive testing (NDT) requirements differ. 

5. A flange with a neck for seamless welding and a flange with a neck for butt welding cannot be used interchangeably. In terms of manufacturing design, a flange with a neck for seamless welding (SO stands for "Slip-on") has a larger inner diameter, which results in reduced weight, lower weight, and lower manufacturing costs; furthermore, flanges with a neck for butt welding (WN stands for "Weld-neck") with a nominal diameter exceeding 250 mm require inspection, whereas SO flanges do not require such inspection, thereby further reducing their overall cost.