Flange Selection Guide by Pressure Class

Selecting the right flange for a piping system depends on the pressure rating (Class) and the operating conditions. Here’s a breakdown of flange types, materials, and applications based on pressure levels:

1. Low-Pressure Systems (Class 150 & 300)

Typical Applications: Water, air, low-pressure steam, HVAC, light chemicals.

Recommended Flange Types

Slip-On (SO) – Easy to install, cost-effective, but weaker than WN.

Socket Weld (SW) – Better for small-diameter, high-vibration lines.

Threaded (TH) – Used where welding isn’t possible (e.g., hazardous areas).

Lap Joint (LJ) – Good for systems requiring frequent disassembly.

Common Materials

Carbon Steel (A105) – General-purpose use.

Stainless Steel (A182 F304/F316) – For corrosive fluids.

PVC/FRP – Non-metallic option for water/chemical handling.

Face Type

Raised Face (RF) – Most common.

Flat Face (FF) – Used with cast iron or brittle materials.

2. Medium-Pressure Systems (Class 600 & 900)

Typical Applications: Process piping, refineries, power plants, high-pressure steam.

Recommended Flange Types

Weld Neck (WN) – Best for high-stress applications (prevents leakage).

Socket Weld (SW) – For small-bore piping (≤2").

Threaded (TH) – Only for non-critical, low-vibration systems.

Common Materials

Carbon Steel (A105, A350 LF2) – For non-corrosive fluids.

Stainless Steel (A182 F316/F321) – High corrosion resistance.

Alloy Steel (A182 F11/F22) – High-temperature steam/oil service.

Face Type

Raised Face (RF) – Standard for ASME B16.5.

Ring-Type Joint (RTJ) – If high sealing integrity is needed.

3. High-Pressure Systems (Class 1500, 2500 & Above)

Typical Applications: Oil & gas wellheads, high-pressure steam, chemical reactors.

Recommended Flange Types

Weld Neck (WN) – Mandatory for high-pressure integrity.

Ring-Type Joint (RTJ) – Preferred for critical sealing (metal gaskets).

Blind (BL) – Used to terminate high-pressure lines.

Common Materials

Alloy Steel (A182 F91/F92) – Extreme temperature/pressure.

Duplex/Super Duplex (A182 F51/F53/F55) – Corrosion + high strength.

Forged Steel (A694 F60/F65/F70) – For sour service (H₂S environments).

Face Type

Ring-Type Joint (RTJ) – Standard for Class 1500+.

Raised Face (RF) – Only if gasket material can handle pressure.

4. Special Considerations

Temperature Effects

Low Temp (Below -29°C) → Use A350 LF2/LF3 (impact-resistant).

High Temp (Above 400°C) → Use A182 F11/F22/F91 (creep-resistant).

Corrosive Environments

Acids/Caustics → Stainless (316L), Hastelloy, PTFE-lined flanges.

Seawater → Super Duplex (F53/F55) or Titanium.

Vibration/Pulsation

Avoid Threaded Flanges (risk of loosening).

Use Weld Neck or Socket Weld for stability.

5. Standards & Compliance

ASME B16.5 – Covers Classes 150 to 2500.

ASME B16.47 – For larger diameters (Series A/B).

API 6A – Wellhead flanges (up to 20,000 psi).