Introduction
Flanges are the mechanical connectors that join pipes, valves, pumps, and other equipment into a complete piping system. Unlike butt weld fittings — which are permanent welded joints — flanges allow for periodic disassembly, making them essential wherever inspection, cleaning, or maintenance access is needed.
Among the dozen or so flange types defined by ASME B16.5, two dominate industrial procurement: Weld Neck (WN) and Slip-On (SO). Understanding when to use each can save you both money and engineering headaches.
1. Weld Neck Flanges (WNRF / WNRF)
Design Features
Weld neck flanges have a long, tapered hub that is butt-welded to the pipe. This hub gradually transfers stress from the flange ring to the pipe wall, making WN flanges the strongest and most fatigue-resistant flange type.
- Butt weld joint = full-penetration weld, X-ray quality
- Tapered hub reduces stress concentration at the weld
- Available in Raised Face (RF) and Ring Type Joint (RTJ) face types
When to Specify Weld Neck
- High-pressure systems (Class 300 and above)
- High-temperature services where thermal cycling is expected
- Critical applications requiring radiographic inspection of the weld
- Systems subject to vibration, pulsation, or repeated bending loads
- LNG, refinery, petrochemical, and power generation piping
2. Slip-On Flanges (SORF)
Design Features
Slip-on flanges slide over the pipe and are fillet-welded on both the inside and outside of the flange. The pipe "slips on" — hence the name.
- Two fillet welds (inside + outside) instead of one butt weld
- Lower fabrication cost — no bevel preparation, easier alignment
- Available in Raised Face (RF) and Flat Face (FF)
When to Specify Slip-On
- Low-pressure, non-critical services (Class 150 and Class 300)
- Where cost efficiency is a priority and the service is non-hazardous
- Cooling water, fire water, utility piping, HVAC systems
- Temporary piping or systems requiring frequent modification
3. Head-to-Head Comparison
| Parameter | Weld Neck | Slip-On |
|---|---|---|
| Joint type | Butt weld (full penetration) | Double fillet weld |
| Pressure rating | Class 150–2500 | Class 150–300 (typically) |
| Fatigue resistance | Excellent | Moderate |
| Welding cost | Higher (bevel prep + radiography) | Lower (simple fillet welds) |
| Alignment | Requires precise fit-up | Easy — pipe slips on |
| Material cost | ~15–20% higher than SO | Lower unit cost |
| NDT requirement | RT or UT mandatory for Class 600+ | MT/PT typically sufficient |
4. Pressure Class Reference (ASME B16.5)
Flange pressure classes define the maximum allowable working pressure at a given temperature. For carbon steel (ASTM A105) at 38°C:
| Class | Max Pressure (bar) | Max Pressure (psi) | Typical Application |
|---|---|---|---|
| 150 | 19.6 | 285 | Utility water, low-pressure air |
| 300 | 51.1 | 740 | Process piping, steam condensate |
| 600 | 102.1 | 1480 | Hydrocarbon processing |
| 900 | 153.2 | 2220 | High-pressure reactor feeds |
| 1500 | 255.3 | 3705 | Superheated steam, HP gas |
Values decrease with rising temperature. Always verify with ASME B16.5 tables for your specific design temperature.
5. Material Grades for Flanges
- Carbon Steel: ASTM A105 (forgings, NPS ≤ 24), ASTM A105N (normalized for improved toughness).
- Low-Temp Carbon Steel: ASTM A350 LF2 — service down to −46°C, common in cryogenic and Arctic installations.
- Alloy Steel: ASTM A182 F5, F9, F11, F22, F91 — for high-temperature refinery and power plant service.
- Stainless Steel: ASTM A182 F304/L, F316/L, F321, F347 — for corrosive and hygienic applications.
- Duplex: ASTM A182 F51 (UNS S31803), F53 (UNS S32750) — for offshore, desalination, and chloride environments.
6. Common Procurement Mistakes
- Specifying SO flanges for high-pressure service — Fillet welds do not provide full penetration and are not suitable for NDT radiography. If the code requires RT, you need weld neck.
- Forgetting face finish requirements — RTJ flanges require a specific serrated finish per ASME B16.5. Raised Face flanges typically use a 125–250 μin Ra finish. Specifying the wrong finish leads to gasket leaks.
- Not ordering bolts and gaskets together — Flange assemblies require matching stud bolts (ASTM A193 B7/B8) and gaskets (spiral wound per ASME B16.20). Sourcing them separately often leads to mismatched materials.
- Confusing A105 with A105N — Non-normalized A105 may lack sufficient toughness for low-temperature or impact-test-required service. Always confirm whether normalization is required by your project specification.
Conclusion
For critical, high-pressure, or high-temperature piping, weld neck flanges are the correct choice despite their higher cost. For non-critical utility services, slip-on flanges offer a cost-effective and installation-friendly alternative.
Need a competitive flange quotation? Contact our team — we supply WN and SO flanges from Class 150 to Class 2500, in carbon steel, stainless, and alloy grades, all with EN 10204 3.1 MTCs.