The 7 key flange types explained: what are weld-neck, long weld-neck, slip-on, socket weld, lap joint, threaded and blind flanges and what are their key characteristics.
The different types of flanges are used depending on the strength of the connection to be made: welding neck, slip-on, and socket weld flanges are used for mission-critical connections; threaded flanges are used for low-pressure joints; lap joint flanges are used in connection with stub ends in specific circumstances (example to lower the cost of flanged joints in high-grade pipelines).
Let’s now dive into each the flange types more in detail:
1- WELD NECK FLANGE
The tapered hub allows a smooth distribution of the mechanical stress between the pipe and the weld neck flange and facilitates the execution of radiographic inspections to detect possible leakages and welding defects.
The dimension of the flange (NPS and schedule) shall match the dimension of the connecting pipe. Therefore, when ordering a welding neck flange, the buyer shall inform the supplier about the nominal size and the wall thickness (schedule) of the connecting pipes.
A welding neck flange is connected to a pipe by a single full penetration V-shaped butt weld.
2 – LONG WELD NECK FLANGE
3 – SLIP ON FLANGE
The bore size of a slip-on flange is larger than the outside diameter of the connecting pipe, as the pipe has to slide inside the flange to be connected by the execution of a fillet weld.
SLIP ON FLANGE VS WELD NECK
Field experience shows that slip-on flanges tend to have a way shorter service life than welding neck flanges, under the same mechanical stress conditions, due to the absence of a tapered hub (which distributes the stress between the pipe and the flange evenly) and the presence of two different welding areas, instead of one. Furthermore, while welding neck flanges can be connected to pipes and fittings, slip on flanges can be only connected to pipes.
Slip-on flanges are also defined “Hubbed flanges” and they are easy to recognize by their slim shape.
4 – THREADED FLANGE
5 – SOCKET WELD FLANGE
Socket weld flanges are connected to pipes using a single fillet weld executed on the outer side of the flange.
According to ASME B31.1, to execute a flanged connection using a socket weld flange, the pipe shall be at first inserted in the socket of the flange until it reaches the bottom of the flange, then it should be lifted by 1.6 mm and finally welded. This gap shall be left to allow a proper positioning of the pipe inside the flange socket after the solidification of the weld.
Socket Weld Flanges are used for small-size and high-pressure piping that do not transfer highly corrosive fluids. This due to the fact that these flange types are subject to corrosion in the gap area between the end of the pipe and the shoulder of the socket.
Their static strength of socket weld flanges is similar to slip-on flanges’, but their fatigue strength is higher due to the presence of a single, instead of double, fillet weld.
6 – LAP JOINT FLANGE
The combination of a nickel alloy stub end with a stainless steel lap joint flange.
7 – BLIND FLANGE