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The Most Common Flanges in Pipe Welding: A Comprehensive Guide

The most used flange types per the ASME B16.5 are Blind, threaded, lap joint, socket weld, slip-on, and welding neck flange. Here you will find a short description and definition of each type.

common flanges

Welding neck flange

The easily recognizable long tapered hub of a welding neck flange provides an essential reinforcement of the joint. It is helpful for elevated, sub-zero, or high pressures applications.

The taper on this flange type offers a smooth relocation/transition of stress from flange thickness to pipe or fitting wall thickness. It is crucial under repeated bending conditions caused by the line expansion or other variable forces.

Slip-on flange

If we compare the slip-on flange with the welding flange, likely, it is not as durable. However, it has two-thirds less calculated strength under internal pressure and around one-third of the life span.

They come at easy installation and low material cost, best suited for low-pressure applications with little risk of leaking.

A few other advantages of the slip-on flanges are that it doesn’t require a sizeable longitudinal space in the line to be mounted. Also, they are less aligned with a wide variety of diameters available and do not require accurate cuts in the pipe.

Socket weld flanges

Socket weld flanges were initially developed for high-pressure, small piping. The static strength of the socket weld flange is equal to the slip-on flanges, but their fatigue strength is 50% high compared to the double-welded slip-on flanges.

Before welding, a space between 1/16” and 1/8” needs a creation between the fitting or flange and pipe. It allows you to expand the pipe on the inner side of the weld and reduces the residual stress, helping to prevent the crack weld on the fitting.

Lap joint flange

Lap joint flanges share the same features as the other flanges. However, it does not have a raised face. They are connected to a lap joint stub end.

These flanges are nearly identical to the slip-on flange, except for a curved radius as the bore. It helps accommodate the flanged portion of the stub end, and you can slide it over the pipe. The pipe is usually welded to the stub end, allowing free movement of the lap joint flange.

The strength is similar to the slip-on flange as they have a fatigue life of around one-tenth compared to the welding neck flanges.

Stub end

As a backing flange, a stub end will be used with lap joint flange applications. However, they are also helpful with slip-on flanges.

Stub ends are available in almost all pipe diameters. There are three different kinds available A, B, and C.

  • Type A is something you can machine to fit the standard lap joint support flange.
  • Type B is designed to be used with a standard slip-on flange.
  • Type C is useful with either a slip-on or lap joint flange.

Threaded flange

The top advantage of the threaded flange is that no welding is required in connection to the pipe. It is done using corresponding threads. However, a seal weld is sometimes applicable in conjunction with the flange’s threaded connection.

Threaded fittings are used exclusively in smaller pipe sizes up to around 4”; however, most pressure ratings and sizes are still available.

A threaded fitting or flange only suits applications that use a thicker wall thickness due to the presence of the thread.


This guide offers a solid foundation of the common top flanges in welding, however, with the wide range of stainless steel flanges and other flange materials available.

You can contact us if you want assistance choosing the right welding equipment for your welding project.


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