Stainless Steel Flanges: Types, Pressure Ratings & Selection Guide (2026)
Stainless steel flanges are the most common way to connect pipes, valves, and equipment in industrial piping. But there are so many types: WN, SO, SW, threaded, lap joint, blind… and pressure ratings from PN6 to Class 2500. As a stainless steel flange manufacturer with 20+ years of experience, we’ll break down the different flange types, how to specify them correctly, and which one to use for your application.
1. What Is a Pipe Flange?
A flange is a ring (usually made of metal) that’s welded or threaded to the end of a pipe. Two flanges are bolted together with a gasket between them to create a leak-proof joint. Flanges make it easy to disassemble piping systems for maintenance, inspection, or modification.
Flanges come in different materials (carbon steel, stainless 304/316, alloy steel), pressure ratings (PN6 to Class 2500), and face types (RF, FF, RTJ, tongue & groove). Choosing the wrong flange type can lead to leaks, vibration, or premature failure.
2. Main Flange Types
| Type | Design | Pressure Range | Best For |
|---|---|---|---|
| WN (Weld Neck) | Long tapered neck, butt-welded to pipe | PN6 to Class 2500 (highest) | High pressure, high temp, critical service, oil & gas |
| SO (Slip-On) | Pipe slips inside, fillet welded both sides | PN6 to Class 600 (medium) | General industry, water, HVAC, low cost |
| SW (Socket-Weld) | Pipe slides into socket, fillet welded | Class 300 to Class 2500 (high) | Small sizes, high pressure, oil & gas piping |
| Threaded (NPT/BSPT) | Threads onto threaded pipe | PN10 to Class 300 (low-medium) | Small sizes, low pressure, easy assembly |
| Lap Joint (LJ) | Loose flange, used with a stub end | PN6 to Class 300 (medium) | Stainless systems (cheaper), frequent disassembly |
| Blind (BL) | Solid disc, no hole — closes the end | All pressure ratings | Closing pipe ends, future expansion, isolation |
Most popular choice: SO (Slip-On) flanges for general industry (70% of all flanges used). WN for high pressure/oil & gas. Lap joint for stainless systems where you want to save money. Blind to close pipe ends.
3. Pressure Ratings: PN vs Class
There are two common pressure rating systems: PN (European) and Class (American). They’re not exactly equivalent, but here’s a rough conversion:
| PN Rating (Europe) | Equivalent Class (USA) | Max Pressure (room temp) | Common Use |
|---|---|---|---|
| PN6 | Class 150 (approx) | 6 bar (87 psi) | Low pressure, water, HVAC |
| PN10 | Class 150 (approx) | 10 bar (145 psi) | Most common, water, wastewater, general |
| PN16 | Class 150 / Class 300 (approx) | 16 bar (232 psi) | General industry, oil, gas |
| PN25 | Class 300 (approx) | 25 bar (362 psi) | Medium-high pressure, process industry |
| PN40 | Class 300 (approx) | 40 bar (580 psi) | High pressure, oil & gas, steam |
| PN63 | Class 600 (approx) | 63 bar (913 psi) | Very high pressure, refinery |
Important note: PN and Class ratings are NOT directly interchangeable — the bolt hole pattern is different. A PN16 flange will NOT bolt onto a Class 150 flange. Always match the rating system to your piping standard.
4. Face Types
| Face Type | Description | Best For |
|---|---|---|
| RF (Raised Face) | Small raised ring (1.5mm or 6mm) in the center | Most common, general industry, oil, gas, water |
| FF (Flat Face) | Flat surface, no raised ring | Cast iron flanges, low pressure, water treatment |
| RTJ (Ring Type Joint) | Groove for metal ring gasket | Very high pressure/temp, Class 600+, oil & gas |
| Tongue & Groove | Tongue on one, groove on other | Precise alignment, corrosive service |
Most common: Raised Face (RF) — 90% of all flanges sold. Use RTJ only for high pressure Class 600+. Use FF only when mating with cast iron flanges.
5. Material Options
| Material | Grade | Price | Best For |
|---|---|---|---|
| Stainless Steel 304/304L | CF8 | Medium | General corrosion resistance, food, chemical |
| Stainless Steel 316/316L | CF8M | Expensive | Coastal, saltwater, corrosive chemicals, pharma |
| Carbon Steel | A105 / A105N | Cheapest | Oil, gas, steam, general industry (non-corrosive) |
| Alloy Steel | F11 / F22 | Very expensive | High temperature, refinery, power plants |
6. Key Standards
| Standard | Region | What It Covers |
|---|---|---|
| ASME B16.5 | USA / International | Pipe flanges NPS 1/2″ to 24″, Class 150 to Class 2500 |
| EN 1092-1 | Europe | European flanges PN6 to PN100 |
| JIS B2220 | Japan | Japanese flanges (5K, 10K, 20K, etc.) |
| DIN 2527 / 2573 / 2576 | Germany / Europe | German standard flanges |
Important: ASME B16.5 and EN 1092-1 flanges are NOT interchangeable — bolt hole circles and pressure ratings are different. Always specify which standard you need.
7. Buying Checklist
When you send an RFQ for stainless steel flanges, include:
• Flange type (WN / SO / SW / threaded / lap joint / blind)
• Size (NPS / DN, inches or mm)
• Pressure rating (PN / Class)
• Material (304 / 304L / 316 / 316L)
• Face type (RF / FF / RTJ)
• Standard (ASME B16.5 / EN 1092-1 / JIS / DIN)
• Quantity
• Required certification (MTC / EN 10204 3.1)
Need stainless steel flanges? Tell us the type, size, and pressure rating — we’ll send you a factory-direct price with full material test certificates within 24 hours.