SS Flange Types Explained — WNRF vs SORF vs Blind vs Socket Weld
Flanges are the bolted connection points of every piping system — and selecting the wrong type for your service conditions leads to leaks, gasket blowouts, or expensive re-work. This guide covers the five main stainless steel flange types, when to use each, how to select pressure class and face type, and ASME B16.5 rating considerations. Creative Metal Industries Vadodara has supplied SS flanges in all types and pressure classes to EPC projects and maintenance teams across India since 2012, with full MTC documentation.
Five Main Flange Types — When and Why
1. Weld Neck (WN) Flange
The strongest and most reliable flange type. Its long tapered hub provides a smooth stress transition from pipe to flange, resisting bending moments and cyclic loads. Butt-welded to the pipe with a single full-penetration V-weld that can be 100% radiographed.
- Use when: High-pressure (ASME 600# and above), high-temperature, cyclic service, critical process piping, IBR-regulated steam systems
- Advantages: Highest structural integrity, full NDE capability, excellent fatigue resistance, smooth bore flow
- Bore: Matches pipe ID — no turbulence or crevice at the joint
2. Slip-On (SO) Flange
Slides over the pipe and is fillet-welded both inside and outside. Easier to align and cheaper than weld neck, but the fillet welds cannot be radiographed and provide less fatigue strength (~2/3 of WN).
- Use when: Low-to-moderate pressure (up to ASME 300#), non-critical utility services, where cost savings matter and NDE is not required
- Avoid when: ASME 600# and above, cyclic/vibration service, services requiring radiography inspection
- Cost: Typically 20-30% cheaper than equivalent WN flange
3. Blind Flange
A solid disc with no bore — bolted to close off pipe ends, vessel nozzles, or valve outlets. Experiences the highest bending stress of all flange types because it resists full line pressure as a flat plate with no pipe support.
- Use when: Dead-end closures, future connections, maintenance isolation (spading), pigging launcher/receiver doors
- Design note: Must be rated for full design pressure AND hydrotest pressure — no reduction allowed
- IBR note: Blind flanges on IBR steam systems require Form III-C certification like all other pressure components
4. Socket Weld (SW) Flange
Has a recessed bore (socket) into which the pipe is inserted, then fillet-welded on the outside. Used for small-bore (up to 2") high-pressure piping where buttweld alignment is difficult.
- Use when: Small-bore (1/2" to 2") high-pressure process connections, instrument piping, sampling points
- Advantage: Easier pipe-to-flange alignment than WN in small sizes, stronger than threaded
- Limitation: Socket gap can trap corrosive media — avoid in severe crevice corrosion environments
5. Lap Joint Flange
Used with a stub end (lap) that is butt-welded to the pipe. The flange slides over the pipe and sits against the stub end face. The flange never contacts the process fluid — only the stub end does.
- Use when: Exotic/expensive pipe materials (Inconel, Hastelloy, Titanium) where only the stub end needs to be corrosion-resistant — the backing flange can be cheaper carbon steel
- Advantage: Easy bolt-hole alignment (flange rotates freely), significant cost savings on expensive alloy systems
- Common in: Chemical plants with Inconel/Monel piping, offshore desalination systems, titanium heat exchangers
Pressure Class Selection — ASME B16.5 Ratings
ASME B16.5 defines seven pressure classes for pipe flanges. The pressure rating decreases with increasing temperature — always check the pressure-temperature table for your material group:
| Class | Rating at 38°C (SS 304/316) | Rating at 260°C | Typical Application |
|---|---|---|---|
| 150# | 19.6 bar (285 psi) | 16.2 bar | Utility water, low-P process |
| 300# | 51.1 bar (740 psi) | 42.5 bar | Medium-pressure process |
| 600# | 102.1 bar (1480 psi) | 85.0 bar | High-pressure hydrocarbon |
| 900# | 153.2 bar (2220 psi) | 127.5 bar | HP separators, compressors |
| 1500# | 255.3 bar (3705 psi) | 212.5 bar | Wellhead, HP/HT service |
| 2500# | 425.5 bar (6170 psi) | 354.2 bar | Ultra-HP, research reactors |
Note: Ratings shown are for Material Group 2.2 (SS 304, 316). Carbon steel (Group 1.1) has different values. Always refer to ASME B16.5 Table 2 for the exact P-T rating of your material group.
Face Types — RF vs FF vs RTJ
RF (Raised Face)
Standard for steel-to-steel bolting. 1.6mm raised area concentrates gasket load. Uses spiral wound or sheet gaskets. Default face for ASME 150# to 2500# in process piping.
FF (Flat Face)
No raised area — full gasket covers entire face. MANDATORY when bolting to cast iron, FRP, or HDPE flanges (to prevent uneven bending). Also used in low-pressure HVAC.
RTJ (Ring Type Joint)
Machined groove holds a metal ring gasket (oval or octagonal). For high-pressure (900# and above), high-temperature, or zero-leak services. Self-energising — pressure increases sealing force.
SS Flange Material Grades
- ASTM A182 F304/F304L: General-purpose austenitic SS — most common grade for chemical, pharma, food processing flanges
- ASTM A182 F316/F316L: Molybdenum-enhanced — superior pitting resistance for chloride environments, marine, and offshore
- ASTM A182 F321: Titanium-stabilised — for high-temperature welded service above 425°C (see our SS 304 vs 321 guide)
- ASTM A182 F51 (Duplex 2205): High strength + corrosion resistance — seawater, chloride SCC, weight-critical offshore
- ASTM A182 F55 (Super Duplex 2507): Maximum pitting resistance — subsea, hot chloride environments
- ASTM A182 F44 (6Mo): 6% molybdenum superaustenitic — seawater systems, flue gas desulphurisation
SS Flanges from Creative Metal Industries
- All types in stock: WN, SO, Blind, SW, Lap Joint, Threaded — ASME 150# to 2500# in SS 304/316/321/Duplex
- All face types: RF, FF, RTJ grooved — machined to ASME B16.5 dimensional tolerances
- Full MTC: Chemical composition, mechanical properties, heat number traceability per EN 10204 3.1
- IBR flanges: Form III-C certified flanges for steam system and boiler piping applications
- Supplying since 2012: EPC contractors, refineries, chemical plants, pharma — Vadodara warehouse with pan-India delivery
- Non-standard sizes: We source custom bore, non-standard OD, and large-diameter flanges (above 24") on project basis
Frequently Asked Questions — SS Flange Types
What is the difference between WNRF and SORF flanges?
WNRF (Weld Neck Raised Face) is butt-welded with a tapered hub — strongest joint, 100% radiography capable, specified for ASME 600# and above and critical services. SORF (Slip-On Raised Face) slides over pipe and is fillet-welded — cheaper and easier to align but limited to ASME 300# and non-critical services. WNRF fatigue life is approximately 1.5x that of SORF.
When should I use a blind flange?
Blind flanges close off dead-end piping, provide future connection points, allow pigging access, and isolate equipment for maintenance. They experience the highest bending stress of all flange types (full pressure on unsupported flat plate) — must be rated for full design AND hydrotest pressure without reduction.
What is the difference between RF, FF, and RTJ flange faces?
RF (Raised Face) is standard for steel piping — 1.6mm raised area with spiral wound gaskets. FF (Flat Face) is mandatory when bolting to cast iron or FRP to prevent cracking. RTJ (Ring Type Joint) uses a metal ring gasket in a machined groove — for high-pressure (900#+), high-temperature, or zero-leak applications where elastomeric gaskets cannot perform.
Need SS Flanges — Any Type, Any Pressure Class?
WN, SO, Blind, SW flanges in SS 304/316/321/Duplex. ASME 150# to 2500#. Full MTC, IBR available. Supplying from Vadodara since 2012.