The 13 Types of Flanges for Piping Explained

Which are the most common types of flanges used in piping? How do they look like? The key types of flanges are the welding neck, long welding neck, slip-on, socket weld, lap joint, threaded and blind flanges. In addition to these standard flanges, some special ones exist, called Weldoflange/Nipoflange and Elboflange, swivel flange, expander/reducer flange, and orifice flanges. 

STANDARD TYPES OF FLANGES

The type of flange to be used for a piping application depends, mainly, on the required strength for the flanged joint. Flanges are used, alternatively to welded connections, to facilitate maintenance operations (a flanged joint can be dismantled quickly and conveniently).

Let’s now dive in, showing the key types of flanges with pictures.

WELDING NECK FLANGE

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A welding neck flange (“WN”)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 piping system (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.

The dimension of the flange (NPS and the pipe schedule) shall match the dimension of the connecting pipe.

A welding neck flange is connected to a pipe by a single full penetration V-shaped butt weld. The dimension and weights of ASME weld neck flanges are shown in this article.

LONG WELDING NECK

Long 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.

SLIP ON FLANGE

A 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.

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

The dimensions and weights of slip-on flanges ANSI/ASME are available on this page.

WELD NECK VS SLIP ON FLANGE

Flanged joints made with slip-on flanges are, in the long run, a bit more fragile than connections made with welding neck flanges (in similar service conditions). This seems due to the following facts:

• a welding neck flange features a tapered hub, absent in a socket weld flange, which distributes the mechanical stress between the pipe and the flange more evenly
• a welding neck joint as only one welding area instead of two (socket weld flange).

Another advantage of the welding neck flange is that it can be connected either to pipes and fittings, whereas socket weld flanges suit pipes only.

THREADED FLANGE

Threaded flanges are joined to pipes by screwing the pipe (which has a male thread, generally NPT per pipe fittings dimension) onto the flange, without seam welds (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 used, mostly, small size piping in low pressure and low-temperature applications, like water and air utility services.

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.

Consult this article, to find about the dimension of ANSI/ASME threaded flanges.

SOCKET WELD FLANGE

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LAP JOINT FLANGE

 

SPECIAL TYPES OF FLANGES

NIPOFLANGE

A Nipoflange is used for branch pipelines at 90 degrees and is a product manufactured by combining a welding neck flange with a forged Nipolet.

However, a Nipoflange is a solid single piece of forged steel and not two different products welded together.

To install a Nipoflange, the piping staff has to weld the Nipolet part of the device on the run pipe and bolt the flanged part on the flange of the branched pipe.

Nipoflanges are available in different materials, such as carbon steel ASTM A105 (high-temperature service), ASTM A350 (low-temperature carbon steel), ASTM A182 (stainless steel grades, including duplex and super duplex) and nickel alloys (Inconel, Incoloy, Hastelloy, etc).

Nipoflanges are also manufactured in the reinforced variant, which has additional mechanical strength compared to a standard Nipoflange.

WELDOFLANGE

A Weldoflange is conceptually similar to a Nipoflange, as that they are a combination of a weld neck flange and a branch fitting connection (a Weldolet in this case). Weldoflanges are made out of a single piece of solid forged steel, not by welding separate parts together.

ELBOFLANGE AND LATROFLANGE

Other less common types of flange Olets is the so-called Elboflange (a combination of a flange and an Elbolet) and “Latroflange” (combination of a flange with a Latrolet). Elboflanges are used to branch a pipeline at 45 degrees.

SWIVEL FLANGE

Swivel ring flanges facilitate the alignment of the bolt holes between the two mating flanges, a feature that is helpful in many circumstances, such as the installation of large diameter pipelines, subsea and offshore pipelines, pipe works in shallow waters and similar environments.Swivel flanges suit oil, gas, hydrocarbons, water, chemical and other demanding fluids in petrochemical and water management applications.

In the case of a large diameter pipeline, for instance, the pipe is fitted, at one end, with a standard welding neck flange, and with a swivel flange at the other end: by simply rotating the swivel flange on the pipe, the operators can achieve a perfect alignment of the bolt holes in a way easier and faster way.

The major standards for swivel ring flanges are ASME/ANSI, DIN, BS, EN, ISO, etc. The most common standard for petrochemical application is the ANSI/ASME B16.5 or ASME B16.47.

 

Swivel flanges are available in all the standard shapes of common flanges, i.e. weld-neck, slip-on, lap-joint, socket weld etc, in all material grades and in a wide dimensional range (sizes can vary from 3/8” to 60” and pressure rating from 150 to 2500).

Swivel flanges can be manufactured in carbon steel (ASTM A105), alloy steel (ASTM A182 F1, A182 F5, A182 F9, A182 F91), and, stainless steel (ASTM A182 F304, A182 F304L, A182 F316, A182 F316L).

EXPANDING FLANGE (“EXPANDER”)

Expanding flanges, or “expander flanges”, are used to increase the bore of the pipeline from a specific point to another or to connect pipes to other mechanical devices such as pumps, compressors, and valves that have different inlets sizes.

The expanding flange represented in the picture is a welding neck flange with a larger bore on the non-flanged end.

Expanding flanges can be used to increase the run pipe bore only by one or maximum two sizes and not more (example: from 2 to 3 or maximum 4 inches).

Expander flanges are a cheaper (and lighter) solution compared to the combination of a buttweld reducer and a standard flange (which is the standard solution for pipe bore increases above 2 sizes).

The most common materials for expanding flanges are A105 (high-temp. carbon steel), A350 (LTCS) and ASTM A182 (stainless steel and above).
Pressure ratings and dimensions of expanding flanges are in accordance with the ANSI/ASME B16.5 specification and are available with raised or flat face (RF, FF).

 

The drawing of an ASME expanding flange.

 

REDUCING FLANGE (“REDUCER”)

Reducing flanges, otherwise called reducer flanges, have an opposite function than expander flanges seen above, i.e. they are used to decrease the bore of a pipeline.

The bore of the run pipe can be safely reduced by only 1 or 2 sizes (otherwise a solution based on the combination of a butt weld reducer and a standard flange has to be used).

Reducing flanges are available in most sizes and material grades, and are not generally available from stock.

Reducing flanges follow the same considerations in terms of specifications, sizes and material grades as expander flanges.

The last type of forged product that resembles the shape of a flange is the so-called spectacle blind: while not properly a flange, a blind (or a ring spacer or spade) is used in between pipes to isolate the pipeline mechanically and in a very easy way. More details are provided in another section of our Wiki for piping.