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:



Weld neck flange ASME B16.5 and B16.47A weld neck flange (“WN”), one of the most common types of flanges, features a long tapered hub that can be welded with a pipe.
This flange type is used, normally, in high-pressure and high/low temperatures applications that require an unrestricted flow of the fluid conveyed by the pipeline (the bore of the flange matches with the bore of the pipe). The absence of pressure drops prevents negative effects as turbulence and erosion/corrosion of the metals in the proximity of the flanged joints.

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.

Weld Neck Flange welding

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.




Long weld neck flangeLong weld neck flanges (“LWN”) are similar to weld neck flanges, with the exception that the neck (tapered hub) is extended and acts like a boring extension. Long weld neck flanges are generally used on vessels, columns or barrels. These flange types are available also in the heavy barrel (HB) and equal barrel (E) types.




Slip on flange ASME B16.5A slip-on flange is connected to the pipe or the fittings by two fillet welds, one executed inside and one outside the cavity of the 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.



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 Flange, fillet welds

Slip-on flanges are also defined “Hubbed flanges” and they are easy to recognize by their slim shape.




Threaded flange ASME B16.5Threaded flanges are connected to pipes by screwing the flange onto the pipe, without any welding (in certain cases, though, small welds are applied to increase the strength of the connection).
Threaded flanges are available in sizes up to 4 inches and multiple pressure ratings, however, they are predominantly used for pipes below 2 inches with wall thicknesses above schedule 80.
Threaded flanges are also a mandatory requirement in explosive areas, such as gas stations and plants, as the execution of welded connections in such environments would be dangerous.
Threaded flange
1: flange; 2 threading; 3 pipe



Socket weld flange ASME B16.5Socket 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.

Socket Weld Flange, 1 fillet weld




Lap joint flange ASME B16.5Lap joint flanges feature a flat face and are always used in conjunction with a stub end. Lap joint flanges resemble, in shape, slip-on flanges except for the radius at the crossing of the flange face and the bore to accommodate the flanged portion of the stub end.
A lap joint flange slips over the pipe and seats on the back of the stub end and the two are kept together by the pressure of the bolts. The use of lap joint flanges in combination with stub ends is a cost-effective solution for stainless steel or nickel alloy pipelines, as the material of the lap joint flange can be of a lower grade (generally carbon steel) than the material of the stub end (which has to match the pipe grade, as in contact with the conveyed fluid).
This arrangement, therefore, has these two advantages:
a. reduces the overall cost of the pipeline’s flanged joints, as the use of higher grade materials is minimized;
b. bolting operations are simplified, as the lap joint flange can be rotated around the pipe to help with bolts alignment.

Stub ends and lap joint flange

The combination of a nickel alloy stub end with a stainless steel lap joint flange.




Blind flangeContrary to all the flange types seen above, blind flanges do not have a center hole, and are used to blind or seal a pipeline, a valve/pressure vessel and block the flow of the fluid.
Blind flanges have to withstand remarkable mechanical stress due to the system pressure and the required bolting forces.
Blind flanges allow easy access to the pipeline, as they can be easily unbolted to let the operator execute activities inside the terminal end of the pipe.
It is maybe interesting to observe that, while this flanges type is easier to manufacture, they are sold at a premium average cost per kilogram compared to the other flange types.

Example of installed blind flange