Flanges Explained: Types, Face Finishes & Pressure Ratings

Flanges are critical connectors in every industrial piping system. They allow engineers to attach:

Pipes
Valves
Pumps
Heat exchangers
Instruments

— creating a strong, leak-proof, and serviceable connection.

But most buyers struggle with questions like:

What is the difference between Weld Neck and Slip-On flanges?
What does RF, FF, RTJ face mean?
What is Class 150, 300, 600… 2500?
Which flange is best for high pressure?

This beginner-friendly guide answers everything in simple English.

What Are Flanges?

A flange is a mechanical plate used to connect two pipes or equipment using bolts and gaskets.

Why flanges are essential

Easy assembly & disassembly
Perfect sealing using gaskets
Suitable for high pressure and temperature
Helpful for maintenance
Enable modification in pipelines

Types of Flanges (Most Used in Industry)

1) Weld Neck Flange (WNRF)

Has a long tapered neck for smooth stress distribution.

Best for:

High pressure
High temperature
Critical lines (steam, oil & gas, boilers)

2) Slip-On Flange (SORF)

Pipe slips inside the flange before welding.

Best for:

Low/medium pressure
Easy installation
Low-temperature services

3) Blind Flange (BLRF)

Used to close the end of a pipeline or vessel.

Best for:

Maintenance
Testing
Isolation

4) Socket Weld Flange (SWRF)

Pipe fits into a socket and is fillet welded.

Best for:

Small-bore high-pressure lines
Hydraulic lines
Chemical injection lines

5) Threaded Flange (THRF)

Screwed connection → no welding required.

Best for:

Flammable areas where welding is unsafe
Temporary lines

6) Lap Joint Flange (LJRF)

Used with stub ends for easy rotation and alignment.

Best for:

Frequent disassembly
Corrosive systems (stub end handles the media)

Flange Face Finishes (Critical for Leak-Proof Sealing)

Face finish determines which gasket you must use.

1) Raised Face (RF)

Most common → provides better gasket compression.

Used in: Class 150, 300, 600, 900 flanges.

2) Flat Face (FF)

Flat sealing surface.

Used in:

Cast iron fittings
Low pressure applications

3) Ring Type Joint (RTJ)

Metal-to-metal sealing groove → strongest sealing type.

Best for:

Extremely high pressure
High temperature oil & gas lines
Offshore & subsea systems

Flange Pressure Ratings (Class 150 to 2500)

Pressure class = how much pressure the flange can safely handle.

Pressure Class Chart (ASME B16.5)

Class 150 → Low pressure (water, HVAC)
Class 300 → Medium pressure (chemical plants)
Class 600 → High pressure (steam, refinery)
Class 900 → Higher pressure (oil & gas)
Class 1500 → Very high pressure
Class 2500 → Extreme pressure (critical plants)

Higher class → thicker flange → stronger sealing.

Flange Materials (Choose Based on Media & Temperature)

Stainless Steel: 304 / 304L, 316 / 316L, 321, 347
Carbon Steel: ASTM A105
Alloy Steel: F5, F9, F11, F22
Duplex / Super Duplex: 2205, 2507
Nickel Alloys: Inconel, Monel, Hastelloy

Choosing the correct material prevents corrosion, cracking, and leakage.

How to Select the Right Flange (Simple Guide)

✔ High pressure → WNRF or RTJ
✔ Low/medium pressure → SORF
✔ Maintenance needed → Blind flange
✔ No welding allowed → Threaded flange
✔ Need easy alignment → Lap joint flange
✔ Small-bore high-pressure lines → SWRF

Why Moksh Tubes Is a Reliable Flange Supplier

Weld Neck, Slip-On, Blind, Lap Joint, Threaded, Socket Weld
Class 150 to 2500 available
Stainless, Carbon, Alloy, Duplex, Super Duplex, Nickel Alloys
CNC-machined flanges for perfect finish
MTC 3.1 / 3.2 provided
PMI, UT, Hydro-test done
Export to 45+ countries
Custom flanges as per drawing

Conclusion

Flanges may look simple, but choosing the wrong type can cause:

❌ Leaks
❌ Pressure loss
❌ Gasket failure
❌ Safety hazards
❌ Costly breakdowns

When selected correctly, flanges ensure:

✔ Leak-proof operation
✔ Long-lasting performance
✔ High reliability
✔ Easy maintenance

Moksh Tubes supplies high-quality, certified flanges suitable for all industrial applications.

FAQ's

Which flange is best for high pressure?

Weld Neck Flange + RTJ face.

Which flange is cheapest?

Slip-On Flange.

What is a Blind Flange used for?

To close pipe ends or for testing.

Do you supply custom flanges?

Yes — as per drawing.