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Complete Guide to the Most Common Flanges in Pipe Welding

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Aside from welding, flange welds are the second most often utilized means of connecting pipe systems to different equipment, valves, and other components of nearly any processing system.

Allowing for quicker disassembly and better access to system components is facilitated by the use of flanges in pipe systems.

If you’re new to flange weld design, this guide will provide you with a good grounding in its fundamentals.

Neck Flange Welding

A welding neck flange features a unique long tapered hub to enhance the union’s strength. However, while working under high pressure or at temperatures below zero, the joint will be put under a great deal of stress.

Flanges with taper are used to make the transition between pipe and fitting wall thicknesses easier. If the lines grow or other fluctuating pressures cause recurrent bending, this tool is important to have on hand.

In addition to reducing joint erosion and joint stress, the inner bore of these flanges, which is matched to the ID of the mating pipe or fitting, also has this effect. Turbulence is avoided at the junction because the flow is unrestrained. As a consequence of this method, erosion is reduced.

Radiographs are excellent for detecting defects in neck flange welding.

One full penetration V weld (Butt-weld) is all that is needed to join this flange type to a pipe or fitting (Tapered hub or high hub flanges).

There is also the long weld neck flange, which looks like the ordinary weld neck flange but is longer. It may be used in a variety of ways, including a barrel or column.

Slip-On flange

In terms of durability, slip-on flanges are inferior to neck flanges. This means that its predicted strength under internal pressure and life expectancy is significantly reduced.

Low-pressure applications where leaking isn’t a big issue might benefit from affordable and easy-to-install slip-on flanges.

Slip-on flanges have the added advantage of not necessitating a substantial amount of line longitudinal space to be placed.

Additionally, they are simpler to align thanks to the availability of a large variety of pipe diameters, and they do not require precision pipe cutting.

The connection to the pipe must be double-welded if there is no neck on which to rest it. Two filet welds are used on both the outside and inside of the flange.

The slide-on flange has the disadvantage of requiring both the pipe and the fitting to be welded before it can be put into use.

In order to employ flange and elbow or elbow/tee combinations, the straight end of the flange is required.

Use in high-pressure or hazardous material applications is not advised.

Socket Weld Flange

Socket Weld flanges were originally developed for high-pressure, small-diameter pipe systems.

It has the same static strength and fatigue strength as slip-on flanges, but it is 50 percent stronger than double-welded slip-on flanges in this regard.

Before arc or MIG welding, the flange or fitting must be separated from the pipe by at least 1/16′′ to 1/8′′.

By reducing the residual stress and enabling the pipe to expand on the inner side of the weld, this method keeps the fitting’s weld from cracking.

Flanges and pipes are connected by a single fillet weld that is visible from the outside.

Per ASME B31.1 1998 127.3, the pipe or tube must be inserted to its greatest length into the socket before welding. It’s time to get ready for welding! (E). After that, it’s taken out of the socket by about 1/16″ (1.6 mm) from the pipe’s end and disposed of.

Because the weld is on the exterior of the pipe, socket weld flanges are less likely to damage the pipe’s inside.

Threaded flange leakage is significantly minimized because of the lack of bevel prep required for socket weld flanges before welding.

This flange’s drawbacks include its need for a gap to be constructed.

  • Welders with specialized training and welding machines with advanced control, such as Dekopro dual-voltawge welder, are needed to guarantee that the 1/16′′ gap is achieved.
  • The second reason is that the expansion gap encourages cracking, particularly in anti-corrosive pipes like those made of stainless steel.
  • Cracks between the pipe and the flange will lead to corrosion, which will need more maintenance.
  • This flange is also prohibited in several procedures.

Lap Joint Flange

Lap joints have many similarities to other flanges; however, they do not have raised faces.

  • They are joined together by a “Lap Joint Stub End.”
  • These flanges resemble Slip-On flanges except in that the bore has a curved radius.
  • Because of their flanged ends, stub ends are meant to be slid over the pipe.
  • In order to allow the lap joint flange to move freely, the pipe’s stub end is then welded to the pipe.

Thus, they are suitable for low-stress circumstances.

Lap joint flanges provide a variety of advantages.

  • The pipe may rotate freely around the flanges while dismantling for inspection and cleaning, making it simpler to align the opposing flange bolt holes.
  • Because they are not in contact with the fluid, carbon steel flanges and corrosion-resistant pipes may be utilized, which might save a lot of money in the long run.
  • Flanges may be saved from systems that are quickly degrading or corroding and repurposed.

Stub End

As a backing flange for lap joints, Stub Ends are often employed. However, they may also be used for slip-on applications.

There are stub ends available for purchase in most pipe diameters. The three choices are A, B, and C.

  • The support flange for the lap joint may be machined to fit Type A
  • In the case of Type B, a normal slide-on flange is needed
  • Whereas Type C may make use of lap or slip joints

Size and tolerance requirements are specified in the ASME B16.6-9 standard. All Stub Ends must meet MSS SP43 specifications.

Threaded Flange

As a result, no welding is required in order to attach the pipe to its threaded flange.

This is accomplished by the use of corresponding threads.

  • Using a seal weld in conjunction with the flange’s threaded connection isn’t always essential.
  • Threaded fittings are still available in a broad variety of diameters and pressure ratings for use in smaller pipe sizes up to 4.00′′.
  • In situations where the thickness of the wall is critical, a threaded flange or fitting is suitable because of the thread.
  • Small wall thicknesses make threading difficult.

Threaded steel pipe used in steam or water systems with pressures more than 100 psi and temperatures greater than 220°F must be seamless and meet the requirements of ASME B36.10 schedule 80.

The two pipe ends may be sealed with a lens pad and sealing surface in addition to the two types of threaded flanges.

A threaded flange is mostly used in the production of ammonia.


Flange weld dimensions are more important than a flange’s functional design when it comes to influencing a pipe system’s construction, maintenance, or upgrade.

However, you must take into account how the flange is attached to the pipe and the gaskets used to guarantee adequate size.

One style of flange weld does not meet all situations. Choosing the right flange design for your pipe system and intended use can assure dependable operation, a long service life, and the most cost-effective option.

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