What is a flange?

What is a flange?

pipe flange is a mechanical device to join pipes. Flanges are available in multiple shapes, as welding neck (the pipe is welded to the collar of the flange), threaded (the pipe is screwed on the flange), socket weld (fillet welds connections), lap joint (for connections using stub ends), slip on, etc. The ASME B16.5 and ASME B16.47 specifications cover US standard pipe flanges, the EN 1092-1 specification covers European steel flanges.

Pipe flanges are available in dimensions between 1/8 and 80 inches and in multiple (forged) material grades as ASTM A105 (carbon steel flanges for high-temperature service), ASTM A350 (CS flanges for low temperature), ASTM A694 (high yield carbon steel flanges for line pipes), ASTM A182 F304, F316, F321 (stainless steel flanges), ASTM A182 F51 (duplex steel flanges), ASTM A182 F53/55 (super duplex flanges), and higher grades (Inconel, Hastelloy, Monel flanges). Non-ferrous pipe flanges (copper, cupronickel, and aluminum) are used in marine and aeronautical applications. The pipe and the flange material shall, of course, match.

For specific applications, flanges may be coated, painted or internally lined (with Teflon, for example) to enhance the resistance of the metal to the aggression of corrosive or erosive fluids.


A flanged joint is the connection of two lengths of pipes by using:

  • two mating flanges (the “main” and the “companion” flange)
  • one set of flange bolts (the dimension and the number of needed stud bolts and nuts are outlined by the flange bolt chart)
  • two gaskets (which can be either metallic, as spiral woundring jointjacketed and Camprofile types, or non-metallic, i.e. made of soft non-asbestos materials, depending on the flange face type, RF, FF or RTJ).

To prevent dangerous leakages in the pipeline, flanged joints shall be executed by trained personnel only (the standard TSE – TS EN 1591 Part 1-4, “Flanges and their joints” is the reference norm).

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Steel pipes can be connected to flanges by welding the pipe with the flange (welded connection, made with weld neck, socket weld, slip on and lap joint flanges) or by screwing the flange onto the pipe (threaded connection, made using threaded flanges).

Welded connections are used for pipelines and piping systems featuring high pressures and temperatures, and diameters above 2 inches. Threaded connections are used for small diameter piping systems that are not subject to mechanical forces as expansion, vibration, contraction, oscillation (conditions that would crack the threaded joints). Let’s take a close look at the welded connections types.


Weld-neck flanges have a tapered hub with a butt weld end that can be welded to a butt weld pipe. The quality of the welded joint may be examined visually or using radiography and/or ultrasounds (UT).

The welded connection between a weld neck flange and a pipe features a good fatigue and mechanical stress performance.


Socket-weld flanges are often used for high-pressure pipeworks below 2 inches (DN 50). The pipe is fillet-welded to the hub of the socket weld flange. As a radiographic examination would not be easy to execute on the fillet weld, the welder experience is key for this type of connection. However, if a connection is critical, and visual examination is not sufficient, specific tests may be executed to determine the integrity of the connection, such as magnetic particle (MP), or liquid penetrant (PT).


Slip-on flanges are generally preferred to weld-neck flanges due to their lower cost and installation speed (and ease). The downside of slip-on flanges vs. a weld neck flanges is that their strength is about 1/3 lower. For this reasons, slip-on flanges are typically used for low-pressure, non-critical services such as fire water, cooling water, and similar nonchallenging pipeworks. The pipe is welded to both the hub and the bore of the flange and MP, PT, or a simple visual examination are the approaches used to check the integrity of a slip-on connection. Slip-on flanges are used, generally, for pipe sizes greater than NPS 2¹⁄₂ (DN 65).


This type of connection is sometimes used for pipelines in high-cost materials (as stainless steel and nickel alloys) as it helps to reduce the overall cost of the required flanged connections.

A lap joint connection is made by the combination of a stub end (which is the part welded to the pipe) and a backing flange, or lapped flange.

A stainless steel stub end can be for instance used in combination with a carbon steel lap joint flange to have a reliable welded connection at a cheaper cost than a full stainless steel connection (less stainless steel is required, hence the cost is lower). Lap joint flanges have generally a raised face and are sealed with flat ring gasket.

1. Lap Joint flange 2. Stub End 3. Butt weld 4. Pipe or Fitting

Types of Flanges
There are 7 types of flanges according to the ASME B16.5 specification: welding neck, long welding neck, slip-on, socket weld, lap joint, threaded and blind. These types suit different pipe to flange connections: welding neck, slip-on, and socket weld flanges allow welded connections; threaded flanges are used for low-pressure screwed joints; lap joint flanges and stub ends are a cost-effective way to join pipes in expensive material grades.


A 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.

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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 flanges (“LWN”) are similar to weld neck flanges, with the exception that the neck (tapered hub) is extended and acts as 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.


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.


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.

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


Threaded 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.

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1: flange; 2 threading; 3 pipe


Socket 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.


Lap joint flanges feature a flat face and are always used in conjunction with stub ends. 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.

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


Contrary 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.



An Orifice Flange is used in combination with orifice meters to measure the flow rate of oil, gas and other liquids conveyed by the pipeline. Orifice flanges are manufactured to ASME B16.36 in multiple sizes and, material grades. 

An orifice flange is used to measure the flow of the fluid conveyed by the pipeline via a flow nozzle positioned on the flange itself. Pairs of pressure tappings are machined onto the orifice flange, making separate tappings on the pipe wall unnecessary.

The traditional orifice flange assembly consists of a pair of flanges, orifice plate, bolts, nuts, gaskets, jacking screws and plugs. Jacking screws ensure the easy removal of the primary flow element.

Orifice flanges are available in all ASTM forged grades (ASTM A105, ASTM A350, ASTM A694, ASTM 182 respectively for carbon, alloy and, stainless steel flanges), dimensions (combinations of nominal sizes and pressure ratings) and, in socket weld, threaded or weld neck shape (WN is the most used).

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Notes for data shown in tables:

  • Dimensions are in millimeters (excluding bolts and bolt holes).
  • Different NPT sizes than 1/2 are available
  • Bolt lengths for RF flanges include an allowance for orifice and gasket thickness of 6 mm (0.25 in.) for NPS 1 to NPS 12. Bolt lengths for ring-type joint flanges include the allowance of 15 mm (0.62 in.) for NPS 1 to NPS 3.
  • Bore (B) has to be specified by the buyer
  • Stud Bolt lengths excl. the height of the chamfers


A swivel flange is a two-piece mechanical device combining a heavy welding hub (which is welded to the pipe) with a rotating ring with bolting holes which can be mated with another welding neck flange or any other compatible flange easily and quickly. A swivel flange features a retaining ring to hold the rotating ring in position during the installation.


Spectable Blind is a mechanical device used to blind pipelines temporarily or permanently and facilitate the execution of maintenance works. Spectable blinds are a cost-effective option to isolation valves, which would be way more expensive to install. Spectable blinds, ring spacers, and line blanks are available in multiple forged material grades, from carbon steel ASTM A105 to alloy and stainless steel ASTM A182 (in bare steel or coated).


A flange spade is a metallic ring to isolate the pipeline for maintenance works. A ring space is a bored metallic ring that replaces the flange spade when the pipeline is opened again. These two pipeline isolation devices are used instead of Spectable blinds (which combine a flange spade and a ring spacer in one single device) in case of narrow installation areas.

A flange spade, otherwise called steel “spade”, or “single-blind” or “blank”, is a metallic ring used to blind (isolate) a pipeline, similarly to spectable blinds

A ring spacer is metallic device inserted between the flanges when the spade is removed and the pipeline is, therefore, open again (i.e. it acts as a temporary placeholder).

Operators can easily understand whether a flange spade or a ring spade is installed on the pipeline at any point in time: indeed, the handlers of ring spacers feature a single indication hole (12 mm diameter), whereas flange spades have instead two indication holes (visual indication).

Flange spades and ring spacers are manufactured according to the ASME B16.48 specification and belong to the family of pipeline isolation devices.

A flange spade (right side) and a ring spacer (left side).


A spectable blind is simply the combination, in one single device, of a flange spade and a ring spacer (and for this reason, flange spades are called “single-blinds” or “blanks”). 

Spades and ring spacers are used in case a spectable blind can’t be installed due to space constraints.

Spades, ring spacers, and spectable blinds are available in multiple sizes (from 1/2 inch to 24 inches, and pressure rating from 150 to 2500#) and material grades (the same available for standard flanges) depending on the fluid conveyed by the pipeline.

The difference between spades/ring spacers and spectable blinds is shown in the image:

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Expanding and reducing flanges are used to increase or decrease the bore size of a pipeline. Expander and reducer flanges are a good alternative to butt weld reducers when the required pipeline bore reduction or increase is small (one or two sizes). For larger size changes, the use of buttweld fittings is recommended.


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 cheaper (and lighter) solution to using a combination of a butt weld 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).


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.


Nipoflange and Weldoflange are flanged outlets used to branch pipelines at 90 degrees. They are manufactured as a combination, in one single solid piece of forged steel, of a standard flange and a branch connection (like Weldolets, Elbolets, Latrolets, etc).  Flanged outlets are manufactured from NPS 1/2″  to 12″ and rating from 150# to 2500#, RF or RTJ. The available schedules are 10S, STD, XS, 160 and XXS and can be installed on run pipes from NPS 3/4″ up to 36″.


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.


Dimensions and weights of countered body Nipoflanges (½” to 2″) – flanged outlets:


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


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.


The flange face finish concept refers to the type of roughness of a flange face. Some roughness is necessary to ensure that the flange face mates with the gaskets and the companion flange perfectly. The common flange face finishes are the stock, the concentric serrated, the spiral serrated and the smooth types.


Steel flanges are available with four basic face finish. The common objective of any type of flange face finish is to create the desired roughness on the face of a flange to ensure a strong match between the flange, the gasket, and the mating flange and thus provide a high-quality sealing.


The stock finish is the most widespread type of finish as it suits the large majority of applications. The pressure embeds the soft face of the gasket into the flange finish and results in the formation of a good seal due to the friction existing between the contacting parts.

As the mating flanges are bolted together, gaskets get “squeezed” into the flange face surface and create a very tight seal.

A stock finish face is manufactured using a phonographic spiral groove featuring a 1.6mm radius round-nose tool with a depth of 0.15mm and a feed-rate of 0.8mm per revolution. The resulting “Ra” value (AARH) for the surface ranges from 125µinch to 500 µinch (125 µm to 12.5 µm).


Spiral serrated finish is a phonographic spiral groove type that differs from the stock finish as the groove is crafted by a 90 degrees tool (instead of a round nosed one) that creates a “V” geometry with a 45-degree serration angle.

A serrated finish, concentric or spiral, has from 30 to 55 grooves per inch and a roughness between 125 to 250 µinch.


The concentric serrated flange finish features concentric grooves instead of spirals.

The grooves are crafted by the same 90-degree tool used for the spiral serrated finish, but the serrations have an even design on the face of the flange. To have concentric grooves, the tool has a feed rate of 0.039mm per revolution and a depth of 0.079mm.


Flanges with a smooth finish do not show visible tool markings at naked eye.

This type of flange finish is used with metal-facing gaskets such as the jacketed type. As per the stock finish, this is achieved by having the contact surface machined with a continuous spiral groove generated by a 0.8mm radius round-nosed tool at a feed rate of 0.3mm per revolution with a depth of 0.05mm (that creates a roughness between Ra 3.2 and 6.3 micrometers, i.e. 125 – 250 microinches).


The cold water finishes appear shiny to the naked eye and very smooth. The AARH value for these surfaces ranges between 85 µinch to 100 µinch. They are used with metal to metal seals (no gasket).


The term AARH (“arithmetic average roughness height”) refers to the flange face smoothness/roughness. The average arithmetic roughness height values are very important during the selection of flanges and gasket materials. Higher the “Ra” values depict a more rough surface, while lower values represent the smoother surface.

Every material possesses a surface roughness and sometimes surfaces are finished deliberately to have a specific roughness (small or bigger).

The “Arithmetic Average Roughness Height” (AARH) is the common indicator to measure the roughness of a surface, and it is the average height of the irregularities on the metal surface, from the mean line as shown in the following figure.

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The symbol Y1 to Y8 represent the peak heights which are measured from the mean line.

Arithmetic Average Roughness Height is usually measured in micro-inches and denoted by symbol “Ra”.

There are various standards for the roughness of surfaces, set according to their type of application. The equipment used to measure the surface roughness is the so-called “profilometer” (which are available in the contact and non-contact types).

In contact type profilometer the roughness is measured by moving the material under the profilometer stylus. However, modern equipment features non-contact measurements, leveraging the optical and ultrasonic technologies.


ASME/ANSI defined specific roughness standards for the flanges, as the flange face finish plays a pivotal role in gasket’s reliability and service life.

According to the ASME/ANSI specifications, the serrated, spiral serrated, and concentric flange face finish should have an average roughness of 125 µinch to 250 µinch (3.2 µm to 6.3 µm).

The tool used to imprint a rough finish on the flange should have a radius of 0.06 inch (1.5mm) or larger. The groove density on the flange face should be from 45 grooves per inch to 55 grooves per inch (1.8 grooves/ mm. to 2.2 grooves/ mm.).

These are the standards for semi-metallic and nonmetallic gaskets. If the average roughness of flange face is not according to the described standards, the contacting surfaces would not properly seal and the flanged joint may wear after some time working under pressure (resulting in loss of bolt joint tightness and a possible leakage).

The soft nonmetallic materials such as PTFE may be used for more comfortable facing and better creep resistance.


The sealing performance of the flanges’ gaskets depends on the AARH, the flange dimensions and the pressure of the stud bolts. According to ASME, the adjacent imperfections should be separated by a distance of at least 4 times the maximum radial projection.

The radial projection can be evaluated by subtracting the inner radius from the outer radius. The serrations shall be at the same level, and the protrusion above them is not permitted. It can cause the adjacent serrations to lose hold of the gasket material and may result in wears and leakages.


The flange face, i.e. the metallic surface area that accommodates the gasket and creates the seal, may be flat (flat face flange, “FF”), raised (raised face flange, “RF”), grooved (ring joint, male and female, tongue & groove flanges), or lap joint. Flanges featuring different faces shall never be mated, example a flat face and a raised face flange, to prevent leakage of the joint.


The ASME B16.5 and ASME B16.47 norms mention a few different types of flange faces:

  • Flat face flange (FF)
  • Raised face flange (RF)
  • Ring joint flange (RTJ)
  • Lap joint flange
  • Male and female flange (M&F)
  • Large and small tongue-and-groove flange (T&G)


A raised face flange (RF) is easy to recognize as the gasket surface area is positioned above the bolting line of the flange. A raised face flange is compatible with a wide range of gaskets, ranging from flat to semi-metallic and metallic types (as, for example, jacketed gaskets and spiral wound gaskets), either ring or full face.

The main scope of a raised face flange design is to concentrate the pressure of the two mating flanges on a small surface and increase the strength of the seal.

The height of the raised face depends on the flange pressure rating as defined by the ASME B16.5 specification (for pressure classes 150 and 300, the height is 1.6 mm or 1/16 inch, for classes from 400 to 2500, the raised face height is approximately 6.4 mm, or 1/4 inch).

The most common flange finish for ASME B16.5 RF flanges is 125 to 250 micron Ra (3 to 6 micron Ra). The raised face is, according to ASME B16.5, the default flange face finish for manufacturers (this means that buyer shall specify in the order if another flange face is required, as flat face or ring joint).

Raised face flanges are the most sold type of flange, at least for petrochemical applications.


Flat face flanges (FF) have a contact surface having the same height of the bolting line of the flange. Full face gaskets, generally of the soft type, are used between two flat face flanges.

According to ASME B31.3, a flat face flange should never be mated with a raised face flange as the resulting flanged joint would definitely leak.


A ring joint flanges (RTJ) is used when a metal-to-metal seal between the mating flanges is required (which is a condition for high-pressure and high-temperature applications, i.e. above 700/800 C°).

A ring joint flange features a circular groove to accommodate a ring joint gasket (oval, or rectangular). As the two ring joint flanges are bolted together and then tightened, the applied bolting force deforms the gaskets inside the flange groove creating a very tight metal-to-metal seal. To make this happen, the material of the ring joint gasket has to be softer (more ductile) than the material of the flange.

RTJ flanges can be sealed by RTJ gaskets of different styles (R, RX, BX) and profiles (example: octagonal/oval for the R style).

The most common RTJ gasket is the R style with an octagonal section, as it ensures a very strong seal (the oval section is an older type). A “flat groove” design, however, accepts both RTJ gaskets having an octagonal or oval section.


A lap joint flange has a flat face, which is not used to seal the flanged joint but simply hosts the back of a stub end. The sealing surface is actually on the stub end itself and may be either flat face or raised face.


Two tongue and groove flanges (T&G face) perfectly fit one into the other: one flange has a raised ring, the other a groove and they can be mated easily (the tongue enters the groove and seals the joint). Tongue and groove flanges are standardized in both large and small types.


Similarly to tongue and groove flanges, male and female flanges (M&F face type) match one to the other as well.

One flange has an area extended beyond its face area, the male flange, the other flange has a matching depression machined on the facing surface, the female flange.

The female face is 3/16” deep, while the male face is 1/4″ high, and both of them are smooth finished.

The outer diameter of the female face retains the gasket.


A flange insulation kit protects a flanged joint from corrosion and ensures the cathodic protection of the pipeline. The working mechanism of flange isolation kits is very simple: they prevent a metal to metal contact among the elements of the flanged connection reducing the corrosion induced by static currents. A flange insulation kit consists of one insulating washer for each nut, one full-length insulating sleeve for each bolt, a 3mm thick insulating gasket (or oval ring), and thick plated steel washers for each nut.

Flange insulation kits are designed to control and prevent the electrolytic corrosion that may impact the flanges and spread on the pipeline, generating a severe impact on its reliability and service life. The insulation components, to have proper dielectric properties, need to be manufactured with appropriate materials, feature chemical stability and low water absorption.

Even if flange insolation kits are rather inexpensive devices, they may have a remarkable impact in extending the service life of the flanges and the connected pipes.

The isolation kits are available for most ASME and API flange sizes, from 1/2 up to 80 inches (and above).


Flange isolation kits are available in four main types, depending on the shape of the gasket element.


Type F gaskets (ring type) fit raised and full face flanges (RF). The external diameter of an F type gasket is a bit smaller than the internal diameter of the bolt circle. Special band protectors can be added to the flanged joint to further strengthen the cathodic isolation created by the flange isolation kit (foreign material may accumulate otherwise). Type F gaskets are manufactured from 1/8″ thick fabric based phenolic sheet either without coating or a with a nitrile rubber coating on the two faces. Type F gaskets are also available in non-asbestos fibers with high isolation capacity.


Type “E” gaskets (full face type) have an outside diameter matching the OD of the flange. This design helps to keep foreign material outside the flanged joint and protect the isolation properties of the kit. Type E flange insulation kits are available in phenolic, neoprene-faced phenolic, and high-temperature materials.


Type D gaskets suit the ring joint groove of RTJ flanges (R, RX, and BX). Type D gaskets are available in phenolic grade. This type of gasket is also called “API ring joint”.


Type O Gaskets feature an extra sealing element on both sides of the device, generally, PTFE, Nitrile or Viton Rubber, and are available in both ring and full face design (type E and F). Type O Gaskets are manufactured with phenolic and G-10 materials.

The materials for insulation sleeves are, by default, polyethylene or phenolic.



Gasket Material Dielectric Strength Maximum Temperature Water Absorption
Neoprene faced Phenolic 500 175 F 0.45%
Plain Phenolic 500 225 F 1.10%
Garlock 3400 630 700 F
Klinger C4401 300 750 F
G10 (NEMA) UL94 VO FR 0.01%
Insul-Seal 500 175 F 0.50%
JM940 Red Devil 2400 700 F
Phenolic RTJ Type D 500 225 F 1.00%
Teflon 600 450 F 0.01%


Sleeve Material Dielectric Strength Maximum Temperature Water Absorption
Phenolic 500 250 F 2.00%
Mylar 4000 350 F 0.22%
NEMA G10 UL94VO FR 0.01%
Minlon 450 300 F 0.90%


Washers Material Dielectric Strength Maximum Temperature Water Absorption
Phenolic 500 225 F 1.10%
Non-Asb 300 750 F
NEMA G11 UL94VO FR 0.01%
Teflon 600 450 F 0.01%

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


The following parameters have to specify to order (or request estimates) for flanges:

  • Flange type (welding neck, slip-on, threaded, lap joint flange)
  • Flange specification (example ASME B16.5, ASME B16.47 type A, EN 1092-1, JIS, UNI, DIN, etc.)
  • Nominal pipe size (NPS)
  • Flange rating (or class): flanges’ rating can range from 150 to 2500 for ASME flanges, from 5k to 30k for JIS B2220 flanges and KS1503 flanges; from PN 6 to PN 100 for European and Russian-standard flanges (DIN, UNI, EN 1092-1, GOST 12820 / 12821-80); Class 600/3, 1000/3, 1600/3, 2500/3, 4000/3 for SANS/SABS flanges (South African standard for flanges)
  • Pipe schedule (for welding neck and socket weld flanges)
  • Flange facing type (FF; RF, RTJ): according to ASME B16.5, the raised face is the standard facing for flanges (different facings, like RTJ or flat face FF, have to be ordered specifically)
  • Flange surface finish (smooth, stock, concentric serrated, etc)
  • Material grade (forged carbon, stainless, duplex, nickel-alloy steels or, non-ferrous materials as cupronickel, copper, aluminum, and bronze)
  • Quantity

Materials of flanges


The chemical composition of carbon steel flange materials ASTM A105 (high-temperature service) and ASTM A350 LF1, LF2, LF3 (low-temperature service, i.e. LTCS).


Chemical Composition of ASTM A105 (in %)
Element C Mn P S Si Cu Ni Cr Mo V
0.35 max 0.60-1.05 0.035 max 0.040
0.10-0.35 0.40
max (1)
max (1)
max (1-2)
max (1-2)


  1. The total of Cu, Ni, Niobium, Moly, and Vanadium shall not exceed 1.00%.
  2. The sum of Niobium and Molybdenum shall not exceed 0.32%.


C% Mn% Si% S% P% Cr% Ni%
0.30 max 0.6/1.35 .15/.30 .040 max .035 max 0.30 max 0.40 max


C% Mn% Si% S% P% Cr% Ni%
0.30 max 0.6/1.35 .15/.30 .040 max .035 max 0.30 max 0.40 max

ASTM A350 LF3 

C% Mn% Si% S% P% Cr% Ni%
0.20 max 0.90 .20/.35 .040 max .035 max 0.30 max 3.3/3.7


Grade C Mn P S Si
ASTM A694 F42 0.26-0.265 1.60-1.64 0.025-0.030 0.025-0.030 0.15-0.35

ASTM A694 F52

Grade C Mn P S Si
ASTM A694 F52 0.26-0.265 1.60-1.64 0.025-0.030 0.025-0.030 0.15-0.35

ASTM A694 F60

Grade C Mn P S Si
ASTM A694 F60 0.26-0.265 1.60-1.64 0.025-0.030 0.025-0.030 0.15-0.35

ASTM A694 F65

Grade C Mn P S Si
ASTM A694 F65 0.26-0.265 1.60-1.64 0.025-0.030 0.025-0.030 0.15-0.35


Alloy steel flange materials: ASTM A182 F1, F5, F9, F11, F22, F91 chemical composition, pressure rating, and mechanical properties.

Chrome moly flange (alloy steel) are manufactured from low alloy steel that contains, as key elements, chromium, and molybdenum. Alloy steel flanges are suitable for high temperature and high-pressure applications and feature good resistance to corrosion (all these conditions are typical for power generation). ASTM A182 alloy flanges are extremely ductile, strong and tough and easy to weld and offer oxidation and scaling resistance.


Stainless steel flange materials: ASTM A182 F304, F309, F310, F316, F347 chemical composition, pressure rating, and mechanical properties.


The chemical composition of common stainless steel flange material grades. The key elements that differentiate the materials are the Nickel (Ni), Chrome (Cr), and Molybdenum content (Mo). The price for these metals fluctuates daily on the London Metal Exchange (Nickel, Moly) and on the ferroalloy market (ferrochrome).

  • LME price for Nickel (USD per tonne)
  • LME price for Molybdenum (USD per tonne)
  • LME price for Copper (USD per tonne)
  • Ferrochrome market price (USD per kg.)
ASTM A182 Stainless Steel Flanges Materials Composition, %
ASTM A182 GRADE C Mn P S Si Ni Cr Mo Nb Ti Others
F304(1) 0.08 2.0 0.045 0.030 1.0 8.0-11.0 18.0-20.0
F304H 0.04-0.10 2.0 0.045 0.030 1.0 8.0-11.0 18.0-20.0
F304L(1) 0.030 2.0 0.045 0.030 1.0 8.0-13.0 18.0-20.0
F304N(2) 0.08 2.0 0.045 0.030 1.0 8.0-10.5 18.0-20.0
F304LN(2) 0.030 2.0 0.045 0.030 1.0 8.0-10.5 18.0-20.0
F309H 0.04-0.10 2.0 0.045 0.030 1.0 12.0-15.0 22.0-24.0
F310 0.25 2.0 0.045 0.030 1.0 19.0-22.0 24.0-26.0
F310H 0.04-0.10 2.0 0.045 0.030 1.0 19.0-22.0 24.0-26.0
F310MoLN 0.030 2.0 0.030 0.015 0.40 21.0-23.0 24.0-26.0 2.0-3.0 N 0.10-0.16
F316 0.08 2.0 0.045 0.030 1.0 10.0-14.0 16.0-18.0 2.0-3.0
F316H 0.04-0.10 2.0 0.045 0.030 1.0 10.0-14.0 16.0-18.0 2.0-3.0
F316L(1) 0.030 2.0 0.045 0.030 1.0 10.0-15.0 16.0-18.0 2.0-3.0
F316N(2) 0.08 2.0 0.045 0.030 1.0 11.0-14.0 16.0-18.0 2.0-3.0
F316LN(2) 0.030 2.0 0.045 0.030 1.0 11.0-14.0 16.0-18.0 2.0-3.0
F316Ti 0.08 2.0 0.045 0.030 1.0 10.0-14.0 16.0-18.0 2.0-3.0 (3) N 0.10 max
F317 0.08 2.0 0.045 0.030 1.0 11.0-15.0 18.0-20.0 3.0-4.0
F317L 0.030 2.0 0.045 0.030 1.0 11.0-15.0 18.0-20.0 3.0-4.0
F321 0.08 2.0 0.045 0.030 1.0 9.0-12.0 17.0-19.0 (4)
F321H 0.04-0.10 2.0 0.045 0.030 1.0 9.0-12.0 17.0-19.0 (5)
F347 0.08 2.0 0.045 0.030 1.0 9.0-13.0 17.0-20.0 (6)
F347H 0.04-0.10 2.0 0.045 0.030 1.0 9.0-13.0 17.0-20.0 (7)
F348 0.08 2.0 0.045 0.030 1.0 9.0-13.0 17.0-20.0 (6) Co 0.20
Ta 0.10
F348H 0.04-0.10 2.0 0.045 0.030 1.0 9.0-13.0 17.0-20.0 (7) Co 0.20
Ta 0.10


  1. Grades F304, F304L, F316, and F316L shall have a maximum Nitrogen of 0.10%.
  2. Grades F304N, F316N, F304LN, and F316LN shall have a Nitrogen of 0.10 to 0.16%.
  3. Grade F316Ti shall have a Titanium content five times above the Carbon plus Nitrogen and not more than 0.70%.
  4. Grade F321 shall have a Titanium content five times above the Carbon and not more than 0.70%.
  5. Grade F321H shall have a Titanium content four times above the Carbon and not more than 0.70%.
  6. Grades F347 and F348 shall have a Niobium content ten times above the Carbon and not more than 1.10%.
  7. Grades F347H and F348H shall have a Niobium content above than eight times the Carbon and not more than 1.10%.


F11 CL2 F22 CL3 F5 F9
CARBON 0.10-0.20 0.05-0.15 0.15 MAX 0.15 MAX
MANGANESE 0.30-0.80 0.30-0.60 0.30-0.60 0.30-0.60
PHOSPHORUS MAX 0.040 0.040 0.03 0.030
SULFUR MAX 0.040 0.040 0.03 0.030
SILICON 0.50-1.00 0.50 MAX 0.50 MAX 0.50-1.00
CHROMIUM 1.00-1.50 2.00-2.50 4.00-6.00 8.0-10.0
MOLYBDENUM 0.44-0.65 0.87-1.13 0.44-0.65 0.90-1.10


F11 CL2 F22 CL3 F5 F9
TENSILE STRENGTH PSI (MPA) 70,000 (485) 75,000 (515) 70,000 (485) 85,000 (585)
YIELD STRENGTH PSI MIN 40,000 (275) 45,000 (310) 40,000 (275) 55,000 (380)
ELONGATION 2” % MIN 20 20 20 20
REDUCTION AREA % MIN 30 30 35 40
HARDNESS (HB) MAX* 143 ~ 207 156 ~ 207 143 ~ 217 179 ~ 217
A182 F1 C-1/2 Mo Low Alloy Steel Non-corrosive applications Including water, oil and gases at temperatures between: -29 ~ 593ºC* (Not prolonged use > 470ºC).
A182 F2 0.75% Ni; Mo; 0.75% Cr Low Alloy Steel Non-corrosive applications including water, oil and gases at temperatures F2:-29ºC ~ 538ºC, WC5: -29ºC ~ 575ºC
A182 F11 1 1/4% Chrome; 1/2% Moly Low Alloy Steel Non-corrosive applications including water, oil, and gases at temperatures between -30ºC (-20ºF) and +593ºC (+1100ºF).
A182 F22 2 1/4% Chrome Low Alloy Steel Non-corrosive applications including water, oil and gases at temperatures between -30ºC (-20ºF) and +593ºC (+1100ºF).
A182 F5/F5a 5% Chrome; 1/2% Moly, Medium Alloy Steel Mild corrosive or erosive applications as well as non-corrosive applications at temperatures between -30ºC (-20ºF) and +650ºC (+1200ºF).
A182 F9 9% Chrome; 1% Moly, Medium Alloy Steel Mild corrosive or erosive applications as well as non-corrosive applications at temperatures between -30ºC (-20ºF) and +650ºC (+1200ºF).
A182 F91 9% Chrome; 1% Moly; V-N, Medium Alloy Steel Mild corrosive or erosive applications as well as non-corrosive applications at temperatures between -30ºC (-20ºF) and +650ºC (+1200ºF).


Duplex flange materials: ASTM A182 UNS S31803 and UNS S32205 (2205) chemical composition, pressure rating, and mechanical properties.

Duplex steel (ASTM A182 2205) is an extremely corrosion resistant, work hardenable stainless steel, whose microstructure consists of a mixture of austenite and ferrite phases.

Due to this particular chemical and physical composition, duplex stainless steel UNS S31803 features the properties characteristic of both types of stainless steel materials (ferritic and austenitic).

Generally speaking, duplex stainless steel is way tougher than ferritic stainless steels, has a superior strength than austenitic steels (series 300 and 400) and has a superior resistance to corrosion when compared to SS304 and SS316 (high intragranular corrosion, also in chloride and sulfide environments).

Whilst austenitic stainless steels are non-magnetic, duplex stainless steel shows magnetic properties.



Duplex 2205
(ASTM A182 UNS S31803 – UNS S32205)

Super Duplex
ASTM A182 UNS S32750 – 32760)



0.03 max



























The most widely used grade for Duplex flange is 2205, due to its superior resistance to corrosion and mechanical strength. The designation “2205” is related to the chemical composition of this material, which features 22% of chromium and 5% of nickel.

Super Duplex flange show an even superior strength and a higher corrosion resistance than standard duplex steel (and austenitic grades, of course). The main difference between a duplex and a super duplex grade is the addition of copper to the alloy (in addition to increased amounts of Chromium, Moly, and Nickel).

The addition of copper gives Super Duplex Stainless Steel an improved resistance to hot chlorides and strong reducing acids, like sulphuric acid, compared to a standard Duplex grade.

Whilst UNS S31803, UNS 32205 (duplex) and UNS S32750, UNS S32760 are standard designations, most manufacturers of superalloys attribute proprietary names to these steels (example UR52N+ is a Usinor/Arcelor Super Duplex steel, Ferralium, Zeron, Sandvik SAF 2205 22Cr, etc).



Mechanical Properties

Duplex 2205
(ASTM A182 UNS S31803 – UNS S32205)

Super Duplex
ASTM A182 UNS S32750 – 32760)

Tensile Strength (in MPa)



Proof Stress 0.2% (in MPa)



A5 Elongation (in %)




Physical Attribute

Duplex 2205
(ASTM A182 UNS S31803 – UNS S32205)

Super Duplex
ASTM A182 UNS S32750 – 32760)

Density (g.cm3)



Modulus of Elasticity (GPa)



Electrical Resistivity (Ω.m)



Thermal Conductivity (W/m.K)

19 at 100°C

17 at 100°C

Thermal Expansion (m/m.K)

13.7×10-6 to 100°C

13.5×10-6 to 200°C

The significant addition of Chromium in Duplex Steel grades, that protects the alloy against corrosion, is a source of steel embrittlement at temperatures over about 300°C. However, at lower temperatures duplex steels show better ductility properties than ferritic and martensitic stainless steels (they can easily used at temperature below -50 C°).

Rating of fange

The flange pressure rating is the maximum pressure (in psi or bars) that a flange of a given class and grade withstands at increasing temperatures. As the temperature increases, the maximum pressure that a flange can bear decreases according to a predefined curve (variable material by material). According to the ASME B16.5 specification, flanges may have seven pressure ratings, class 150, 300, 400, 600, 900, 1500, and 2500. The terms “class”, “#”, “Lb” or “Lbs” are equivalent pressure rating designations.

Flanges with a higher rating (i.e. class) can withstand higher pressures than flanges with lower ratings at the same temperature, as they are bigger, bulkier and more robust (as shown in the image). Example a flange 6 inches class 300 can withstand more pressure than a 6 inches flange class 150 at 600 degrees F° (570 psi vs 140 psi, to be specific).

It goes without saying that two flanges of the same bore size but with different pressure classes (for example a 6-inch class 300 vs a 6-inch class 1500 flange) require a different number, length, and diameter of stud bolts (as indicated by the ASME flange bolt chart). It is also evident that flanges of different materials feature different pressure-temperature rating curves as shown in the ASME B16.34 specification for ASTM materials.

How to read the pressure-temperature rating charts:

  • A carbon steel flange class 150 withstands up to 285 psi for temperatures below 100 F°, 200 psi for temperatures below 400°, and 20 psi at high temperatures as 1000 F°
  • A carbon steel flange class 300 withstands up to 740 psi (instead of 285) at temperatures below 100 F° and of 635 psi at temperatures below 400 F° (instead of 200 psi)
  • stainless steel flange class 150 withstands up to 270 psi for temperatures below 100 F°, 190 psi for temperatures below 400°, and 20 psi at high temperatures as 1000 F°

Source: China Pipe Flange Manufacturer – Yaang pipe fitting Industry Co., Limited (www.ugsteelmill.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@ugsteelmill.com

Please notice that you might be interested in the other technical articles we’ve published:


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