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Complete Guide to the Most Common Flanges in Pipe Welding

The industry-accepted flanges in pipe welding according to the ASME B16.5: Slip-On, Blind, Threaded, Lap Joint, Socket Weld, Welding Neck flange. Below you will find a short definition and description of each type.

Welding Neck Flange

It is an easily recognizable long tapered hub of a welding neck flange that provides a crucial reinforcement of the joint. It is useful for applications that involve sub-zero and elevated temperatures where the joint will be under a particular amount of stress.

This type of flange also offers a smooth relocation/transition of stress from flange thickness to fitting or wall thickness. It is beneficial under conditions of repeated bending, caused by other variable forces or line expansion.

Slip-on Flange

Slip-on flange compared to the welding neck flange is more easily installed to fit with your pipeline length restrictions. It has around two-thirds less calculated strength under internal pressure and around one-third life span to the weld neck.

However, they are easy to install and come at a low material cost which is, therefore, better suited to low-pressure applications with little risk of leaking.

Slip on Flanges

Other advantages of slip-on flanges are that they do not require a large longitudinal space in the line to be mounted. They are much less difficult to align with a wide variety of diameters available and do not require accurate cuts in the pipe.

Socket Weld Flange

Socket weld flanges are initially developed for use in high-pressure and small-size piping. With the static strength equal to the slip-on flanges, the fatigue strength is considered to be somewhere around 50% greater than double-welded slip-on flanges.

Before welding, a space of between 1/16” and 1/8” must be created between the flange or pipe and fitting. It allows the expansion of pipe on the inner side of the weld and reduces the residual stress, therefore preventing weld cracking on the fitting.

Lap Joint Flange

Lap joint flanges share the regular features as the other flanges mentioned, however, it does not have a raised face. The flanges are nearly identical to the slip-on flange, except for a curved radius at the bore and a larger hub to accommodate the stub end.

It is to accommodate the flanged portion of the stub end properly and then slide over the pipe. The pipe is usually welded to the stub end to allow free movement of the lap joint flange.

Stub End

Stub end is helpful with lap joint flange applications as a backing flange, however, they are also sometimes beneficial with the slip-on flanges. Stub ends are available in almost all pipe diameters. You can find three different types available that are A, B, and C.

  • Type A can be machined to fit the standard lap joint support flanges.
  • Type B is designed to use with a standard slip-on flange.
  • Type C is useful with either a slip-on or lap joint flange.

Dimensional tolerances and dimensions are defined in the ASME B.16.9 standard. Light-weight corrosion-resistant stub ends (fittings) are defined in MSS SP43.

Threaded Flange

The main advantage of the threaded flange is that no welding is essential with a pipe connection. It means that it is done by corresponding threads. However, sometimes a seal weld is also applied in conjunction with the connection of the flange thread.

Threaded fittings today are useful almost exclusively in smaller pipe sizes up to around 4.00”, however most pressure and size ratings are available per our catalog, and the dimensions of B16.5

A threaded fitting or flange is only suitable for applications that use a thicker wall thickness due to the presence of the thread. A pipe system with a thin wall thickness has no space for threading.


Two types of threaded flanges are available, one is useful to seal the two pipe ends with a sealing surface and lens pad. It is useful mostly in the production of ammonia until recently. The other type is a more standard setup, sealed with two flange sealing faces.

Flange welding equipment ranges from Head and tailstocks, welding positioners, and pipe rotators for positioning of pipe welding.

If you want any assistance choosing the right welding equipment for your welding project, you can contact us.

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