The Steel Slip-On Welding Flange, commonly abbreviated as SO Flange, features a short tapered hub transition on the flange face. The pipe is inserted directly into the flange bore and secured by both internal and external fillet welds, providing a reliable and detachable sealing connection between pipes, valves, and pressure equipment.
Compared with the flat welding plate flange (PL Flange) without a hub, the short hub design significantly improves the overall rigidity, bending resistance, and deformation resistance of the flange. It helps distribute pipeline bending moments more effectively and provides a higher pressure-bearing capability. The hub height is relatively low, which distinguishes it from the long welding neck flange (WN Flange) featuring an extended tapered neck.
The flange bore diameter is slightly larger than the outside diameter of the connecting pipe, allowing the pipe to slide easily into the flange bore. After positioning, the pipe is fixed by double-sided fillet welding (inside and outside welding). This structure simplifies pipe alignment during installation, reduces assembly difficulty, and requires lower fabrication and cutting precision compared with butt-welding flange connections.
SO Flange (Steel Slip-On Welding Flange) is a widely used detachable pipe connection component designed for connecting pipes, valves, pumps, and other industrial equipment. With a short tapered hub structure, it offers enhanced mechanical strength compared with standard plate flanges while maintaining easy installation and cost-effective fabrication.
The slip-on design allows the pipe to be inserted into the flange bore before welding. The double fillet welding configuration ensures secure fixation and improved sealing performance. Due to its simple installation process, excellent alignment tolerance, and reliable performance, SO flanges are widely applied in petroleum, chemical, power generation, water treatment, and general industrial piping systems.
Note: The flange inner diameter (B) is determined by the wall thickness of the steel pipe and shall be specified by the user at the time of ordering.（Unit: mm）
| DN | NPS | Type | Part No | A | D | K | L | Th | n | C | B | N | H | P | E | F |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 15 | 1/2 | SO-150LB-RF | XLXSO150SO05 | 21.3 | 90 | 60.3 | 16 | M14 | 4 | 9.6 | 22.5 | 30 | 14 | |||
| 20 | 3/4 | SO-150LB-RF | XLXSO150SO075 | 26.9 | 100 | 69.9 | 16 | M14 | 4 | 11.2 | 27.5 | 38 | 14 | |||
| 25 | 1 | SO-150LB-RF | XLXSO150SO1 | 33.7 | 110 | 79.4 | 16 | M14 | 4 | 12.7 | 34.5 | 49 | 16 | 48 | 6 | 9 |
| 32 | 1 1/4 | SO-150LB-RF | XLXSO150SO125 | 42.4 | 115 | 88.9 | 16 | M14 | 4 | 14.3 | 43.5 | 59 | 19 | 57 | 6 | 9 |
| 40 | 1 1/2 | SO-150LB-RF | XLXSO150SO15 | 48.3 | 125 | 98.4 | 16 | M14 | 4 | 15.9 | 49.5 | 65 | 21 | 65 | 6 | 9 |
| 50 | 2 | SO-150LB-RF | XLXSO150SO2 | 60.3 | 150 | 120.7 | 18 | M16 | 4 | 17.5 | 61.5 | 78 | 24 | 83 | 6 | 9 |
| 65 | 2 1/2 | SO-150LB-RF | XLXSO150SO25 | 76.1 | 180 | 139.7 | 18 | M16 | 4 | 20.7 | 77.6 | 90 | 27 | 102 | 6 | 9 |
| 80 | 3 | SO-150LB-RF | XLXSO150SO3 | 88.9 | 190 | 152.4 | 18 | M16 | 4 | 22.3 | 90.5 | 108 | 29 | 114 | 6 | 9 |
| 100 | 4 | SO-150LB-RF | XLXSO150SO4 | 114.3 | 230 | 190.5 | 18 | M16 | 8 | 22.3 | 116.0 | 135 | 32 | 149 | 6 | 9 |
| 125 | 5 | SO-150LB-RF | XLXSO150SO5 | 139.7 | 255 | 215.9 | 22 | M20 | 8 | 22.3 | 143.5 | 164 | 35 | 171 | 6 | 9 |
| 150 | 6 | SO-150LB-RF | XLXSO150SO6 | 168.3 | 280 | 241.3 | 22 | M20 | 8 | 23.9 | 170.5 | 192 | 38 | 194 | 6 | 9 |
| 200 | 8 | SO-150LB-RF | XLXSO150SO8 | 219.1 | 345 | 298.5 | 22 | M20 | 8 | 27.0 | 221.5 | 246 | 43 | 248 | 6 | 9 |
| 250 | 10 | SO-150LB-RF | XLXSO150SO10 | 273 | 405 | 362.0 | 26 | M24 | 12 | 28.6 | 276.5 | 305 | 48 | 305 | 6 | 9 |
| 300 | 12 | SO-150LB-RF | XLXSO150SO12 | 323.9 | 485 | 431.8 | 26 | M24 | 12 | 30.2 | 328.0 | 365 | 54 | 381 | 6 | 9 |
| 350 | 14 | SO-150LB-RF | XLXSO150SO14 | 355.6 | 535 | 476.3 | 30 | M27 | 12 | 33.4 | 360.0 | 400 | 56 | 397 | 6 | 9 |
| 400 | 16 | SO-150LB-RF | XLXSO150SO16 | 406.4 | 595 | 539.8 | 30 | M27 | 16 | 35.0 | 411.0 | 457 | 62 | 454 | 6 | 9 |
| 450 | 18 | SO-150LB-RF | XLXSO150SO18 | 457 | 635 | 577.9 | 33 | M30 | 16 | 38.1 | 462.0 | 505 | 67 | 518 | 6 | 9 |
| 500 | 20 | SO-150LB-RF | XLXSO150SO20 | 508 | 700 | 635.0 | 33 | M30 | 20 | 41.3 | 513.5 | 559 | 71 | 559 | 6 | 9 |
| 600 | 24 | SO-150LB-RF | XLXSO150SO24 | 610 | 815 | 749.3 | 36 | M33 | 20 | 46.1 | 616.5 | 663 | 81 | 673 | 6 | 9 |
The connecting pipe is inserted into the flange bore with a recommended 2–3 mm welding gap between the pipe end and flange hub face. Internal and external welding is preferred to ensure maximum joint strength and sealing reliability. Single-side external welding is only permitted for low-pressure, non-critical piping applications.
Following welding operations, all weld residues and slag shall be removed carefully. The flange sealing surface must be kept clean and free from scratches, dents, or mechanical damage to maintain sealing performance.
For applications involving hazardous, corrosive, or leakage-sensitive media, PT (Penetrant Testing) inspection can be performed on the weld seam to verify welding quality and ensure system safety.
RF – Raised Face：The most commonly used flange facing type. Suitable for general industrial applications such as water, oil, and low-pressure steam services. Provides reliable sealing performance with various gasket materials.
MFM – Male and Female Face：The male and female raised sealing surfaces provide better gasket positioning and prevent gasket displacement. Suitable for applications where the medium has slight pressure fluctuations or vibration conditions.
TG – Tongue and Groove Face：Designed for high-integrity sealing applications. The tongue and groove structure provides excellent gasket retention and is widely used for highly corrosive media and high-cleanliness process systems.
FF – Flat Face：The entire flange face is flat and in the same plane. Mainly used with non-metallic soft gaskets in low-pressure piping systems to reduce the risk of flange deformation.
