Analysis of Connection Methods, Processes and Application Scenarios of Various Flanges

Flanges are core detachable connecting accessories in pipeline engineering, mainly used for the sealed connection between steel pipes, and between steel pipes and equipment (valves, pumps, vessels). They feature convenient disassembly and assembly, reliable sealing and high pressure resistance. The connection process between flanges and steel pipes directly determines the tightness, stability and service life of the pipeline system. According to flange types, welding structures and assembly methods, the mainstream connection modes include flat welding, butt welding, socket welding, threaded connection and loose sleeve connection. Each connection method varies significantly in construction technology, mechanical properties, applicable pressure, temperature and pipeline working conditions, which are analyzed in detail below.

I. Flat Welded Flange Connection (Slip-On Flange, SO)

The flat welded flange is the most widely used basic flange type in industrial pipelines. It adopts a plate-type non-neck structure. The steel pipe penetrates directly into the inner hole of the flange and is fixed by internal and external fillet welds, featuring simple construction and low cost.

Application: Medium-low pressure, normal temperature, non-vibration (PN10–PN25, DN15–DN200). Water supply, HVAC, atmospheric lines. Not for high/low temperature or flammable media.

II. Butt Weld Flange Connection (High-Neck & Short-Neck WN Flange)

Weld neck flanges (WN) are high-reliability integral types, using full penetration groove butt welding. They form an integrated structure with superior strength.

Application: High-neck: petrochemical, power, high-pressure gas (PN40–PN160+). Short-neck: general chemical, heating, steam lines.

III. Integral Flange Connection (Integral Forged Flange, IF)

Integrally forged with equipment ports (valves, pumps, vessels). No welding, extremely high rigidity.

Characteristics: No welding stress, excellent impact/alternating load resistance. Not for independent pipe joint; used for equipment ports.

IV. Socket Weld Flange Connection (SW)

Stepped socket, pipe inserted against step, outer fillet weld. For small-diameter high-pressure lines.

V. Threaded Flange Connection (TH)

Non-welding, threaded engagement. Fast, detachable.

Application: Low pressure, normal temp, small diameter, fire‑prohibited sites (DN15–DN80). Fire-fighting, civil gas. Not for high pressure, high temp, severe vibration.

VI. Loose Sleeve Flange Connection (Lapped Flange, LJ)

Flange rotates freely, used with stub ends. Saves costly alloys.

Main use: Anti-corrosion pipelines (stainless steel, titanium, lined plastic). Medium-low pressure, normal temp.

VII. Blind Flange Connection (Flange Cover, BL)

Solid non-through flange for blocking pipe ends, isolation, pressure testing.

Characteristics: High tightness, complete isolation, easy assembly. Used for reserved ports, maintenance isolation, test blocking.

VIII. Selection Summary and General Construction Specifications

General Construction Specifications

• Inspect sealing faces for scratches, deformation.
• Select gaskets matching medium, pressure, temperature; install centrally.
• Tighten bolts diagonally, evenly.
• Visual inspection for welded flanges; NDT mandatory for high pressure.
• Avoid over-standard working conditions; match pressure grades strictly.

IX. Conclusion

The selection of flange and steel pipe connection modes is mainly based on pipeline pressure, temperature, pipe diameter, medium characteristics and construction environment. Flat welding and threaded connection are preferred for ordinary civil and low-pressure industrial pipelines to balance cost and practicability; high-neck butt welding connection is mandatory for core process pipelines with high temperature, high pressure and flammable & explosive media to ensure system safety and stability; integral flanges are prioritized for equipment port connection, socket welding for small-diameter high-pressure pipelines, loose sleeve connection for anti-corrosion pipelines, and blind flanges for pipeline blocking and isolation. Strict matching of connection processes and standardized construction are the key to reliable sealing and long-term stable operation of pipeline systems.

All content is for reference. Always follow applicable codes and engineering standards.