Overview

Flange connection is a common detachable connection form in industrial piping systems. It connects flanges, gaskets and pipes together through different processing technologies, facilitating pipeline assembly, maintenance and inspection. According to the connection mode between flanges and pipes, pipe flange connections can be divided into the following mainstream types.

Main Pipe Flange Connection Methods

1. Butt Weld Connection

Applicable flange: Weld Neck Flange (WN)

The tapered hub of the weld neck flange is butt-welded with the pipe end by full penetration welding.

Features: Uniform stress transition, high structural strength, excellent sealing performance.

Advantages: Suitable for high temperature, high pressure, impact and cyclic load working conditions.

Application: Oil & gas transmission, petrochemical, power station critical pipelines.

2. Fillet Weld Connection (Slip-on Weld)

Applicable flange: Slip On Flange (SO)

The pipe penetrates through the flange inner hole, then fillet welding is carried out on both inner and outer sides of the flange.

Features: Simple positioning and easy alignment during installation.

Limitation: Weld cannot achieve full penetration; lower load capacity than butt weld.

Application: Medium and low pressure, normal temperature conventional pipelines.

3. Socket Weld Connection

Applicable flange: Socket Weld Flange (SW)

The pipe is inserted into the flange socket cavity, and fillet weld is made at the pipe mouth.

Features: Small assembly space, good concentricity.

Application: Small diameter pipelines (generally DN ≤ 50), medium-high pressure equipment pipelines.

4. Threaded Connection

Applicable flange: Threaded Flange

Internal threads are machined on the flange bore, matched with externally threaded steel pipes without welding.

Features: Fast installation, no thermal welding operation required.

Limitation: Not suitable for severe vibration and high temperature conditions, easy leakage under temperature change.

Application: Fire protection pipeline, compressed air system, occasions where welding is forbidden.

5. Loose Sleeve Connection (Lap Joint Connection)

Applicable flange: Lap Joint Flange (LJ)

A stub end is butt-welded on the pipe end. The lap joint flange is sleeved outside the stub end and can rotate freely. Sealing relies on the stub end flange face.

Features: Flexible rotation, easy bolt hole alignment; low cost for expensive stainless steel and alloy piping.

Application: Stainless steel pipelines, frequent disassembly engineering projects.

Auxiliary Sealing Connection Principle of Flange Joints

No matter which connection method is adopted between flange and pipe, the sealing of the flange joint surface follows the same principle:

Place a gasket between two opposite flanges, tighten bolts to compress the gasket, fill tiny gaps on the sealing surface, and prevent medium leakage.

Common sealing face types: RF, FF, RTJ, MFM, T&G.

Key Selection Tips

Confirm working pressure, temperature and medium corrosivity first to choose matching flange connection form.

Vibration, thermal expansion and cyclic load pipelines prefer butt weld connection.

Explosion-proof and fire hazard sites can select threaded flanges if pressure and temperature allow.

Stainless steel pipelines often adopt lap joint structure to save material cost.