General Information About Flanges

What is a flange?
Flange is a major component for connecting two pipes. Generally speaking, it connects the ends of two pipes through threading or welding. Thus, flanges are classified into two big categories according to attended modes: threaded connection (screwed) flanges and welding flanges. If two pipes are arranged to be connected, a pair of flanges, a gasket and several bolts and nuts are needed to complete the flange joint. At first, put the gasket between two sealing faces. Then, tighten the bolts. When the gasket surface pressure reaches a certain level, it would be deformed to fill the uneven parts of sealing surface to achieve the perfect leak tightness.
Mechanical components are indispensable in many mechanical types of equipment. They all cooperate to drive the industrial process to proceed in a smooth pace. Among the great variety of the mechanical fittings, flanges are popular in many situations. Generally speaking, a flange is typically an extension of a structure. It is usually used to increase the strength of a structure, to keep a particular object in place or to combine two parts together.
It should be mentioned that these flange pieces should be designed and manufactured under some regulations. Otherwise, these fitting would result in poor performance, even leading to the unexpected industrial accidents. In fact, there is two main organizations responsible for most standards. They are mainly set by the American National Standards Institution and American Petroleum Institute. As a result, the dimensions of the flanges are restrictively regulated.
And it is lucky that the suitable manufacturing materials are in easy access. They can be the stainless steel, carbon steel, aluminum and plastic. These are all in great demands. In fact, every material holds its advantages and disadvantages. For example, the stainless steel is able to be with good durability, being used in situations requiring good tolerance.
Depending on the various manufacturing materials and required dimensions, flanges can be made into a wide range of the types. In most cases, this fitting is used to attach two ends of the pipes together. At this moment, some securing bolts are needed during the connecting process. Just considering about the specific requirements, the faces of the flanges vary. Commonly speaking, the face is designed to be a tongue and groove. For example, there is a kind of the flange which is called the ring joint. It generally covers the entire face, including the internal and external portions of the bolts. The slim surface would give great benefit to the whole operating process.
In words, the great number of the possible flanges is making their own contributions to the development of the entire industrial process.

Different types of flanges

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Different types of flanges are explained below in order to let interested people get an idea quickly about flanges. We know it is a kind of pipe fittings and is of several types. This assemblage would be of use to those who would like to know more about this or people belonging to pipe fittings or flange business. Here it is.

Socket Welding Flanges

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Socket Weldings Flanges are vastly used for moderate services, particularly in the smaller sizes, due to it’s ease of fit-up and no alignment problem. Although generally welded at the flange hub only, the pole end may also be welded without having to reface the flange. The end weld can be ground to render a smooth bore. Socket weld flanges and socket weld reducing flanges are added to material classes for mating equipment, where a union will be subject to external stresses; and transitions between socket weld and flanged piping.

Slip-on flanges

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Slip-On flanges are famous for very normal service conditions because of the ease of fit-up and alignment and the extent of it’s permissible tolerance in cutting the pipe to the length.

Slip-on flanges cost less than welding neck flanges, and require less accurate pipe cutting, but their strength is approximately 2/3 of weldneck flanges under internal pressure, and they have approximately 1/3 the fatigue life of weldneck flanges.

  • Slip-on flanges shall be welded at the front and back of the hub, but not on the sealing face.
  • Slip-on flanges and reducing slip-on flanges shall not be used in the following services: Severe cyclic conditions. See ASME B31.3, paragraph 300.2.
  • Design temperatures above 230 °C, or where the corrosion allowance exceeds 3 mm
  • ASME B16.5 Class 400 or higher rating
  • Flange sizes larger than NPS 24, unless stress calculations in accordance with ASME Section VIII Div 1, Appendix 2, with thermal and other external piping loads considered, show that the slip-on flanged joint will not be over-stressed
  • In hydrogen service with a hydrogen partial pressure above 690 kPa, flanges shall have a predrilled 3 mm diameter hole to vent the space between the pipe OD and the flange bore.

Threaded Flanges

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Threaded Flanges are immensely used because for assembling no welding equipment is required and both the pipe and flange can be completely salvaged while dismantling. Precisely cut, clean, tapered pipe thread dismensioned to ANSI A2.1 Pipe threads assure strong, extremely tight joints.

  • When future material classes are generated, threaded flanges shall be added to material classes for threaded service, generally for mating equipment, and transitions between threaded and flanged piping.
  • Threaded flanges may also be used for water and air service in pipe sizes NPS 6 and less and at design temperature of 250 °F and below. Seal welding shall not be required.
  • Threaded flanges shall be limited to size NPS 2 and smaller in hazardous service.
  • Threaded flanges shall have taper type threads and shall conform to ASME B1.20.1.

Blind Flanges

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Blind flanges are generally used to stop one end of a flanged pipe, a valve orifice or the opening of a pressure vessel.

  • Blind flanges shall be used as end closures on flanged ends and valves, unless end caps are specified in design.
  • Blind flanges are forgings, and shall be manufactured to the same materials standards as other matching flanges.
  • Blind flanges shall be of the same material as the weldnecks, in all services. In corrosive atmospheres, stainless steel shall be used.
  • Blind flanges shall not be drilled for connections, for example drains and flushing, unless stress calculations in accordance with ASME Section VIII Div 1, Appendix 2 show that the flanges will not be over stressed.

Welding Neck Flanges

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Welding Necks Flanges are designed to be butt welded to the pipe. The long tapered hub reinforces the flange, permits stress- relieving, magnafluxing or x-raying the weld, when needed and opens the flange face from the heat affected zone.

ASME B16.25 weldneck flanges with tapered hub and welding end shall be the primary selection for flanged joints in metallic piping systems of NPS 2 and larger. The individual material classes show the size range for any given service.

Welding ends of weldneck flanges shall be in accordance with ASME B16.5, Figures 8 to 14.

  • ASME B16.25 weldneck flanges with tapered hub and welding end shall be the primary selection for flanged joints in metallic piping systems of NPS 2 and larger. The individual material classes show the size range for any given service.
  • Welding ends of weldneck flanges shall be in accordance with ASME B16.5, Figures 8 to 14.

Lap Joint Flanges

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lap joint flanges, while requiring a separate end connector, provides a joint in which the product does not come in contact with the flange. In addition the ability of the flange to rotate simplifies assembly and alignment of bolting on systems requiring frequent dismantling.

  • A lap joint is made up of a pair of stub ends, a pair of lap joint flanges used as a backup, and bolts and gaskets. These allow easy alignment of boltholes and flanged joints.
  • The stub end shall match the material of the pipe. Stub ends for lapped joint flanges, if fabricated by welding, shall be made with full penetration welds.
  • Advantages are that lapped joints are an economical alternative to weldnecks, and costs savings are large when the material is very expensive; dissimilar materials can be joined, provided galvanic corrosion does not occur; and spools can be rotated.
  • The disadvantage of this joint is that it is sensitive to external stress. Lapped joint flanges shall not be used in severe cyclic conditions.

Spectacle Blind Flanges

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Spectacle Blind Flanges are not real or genuine flanges but division sectors which can be inserted between two flanges to interrupt the flow of the fluid in a section line. They are manufactured in an 8-shape with one blind hole which can be completely replaced by the other to cut out the pipe section below the joint. They are only used with low nominal pressures for obvious sealing reasons.

Standards of Flanges


  • ASME B16.1 – Gray Iron Pipe Flanges and Flanged Fittings: Classes 25, 125, and 250
  • ASME B16.5 – Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard
  • ASME B16.24 – Cast Copper Alloy Pipe Flanges and Flanged Fittings: Classes 150, 300, 600, 900, 1500, and 2500
  • ASME B16.36 – Orifice Flanges
  • ASME B16.42 – Ductile Iron Pipe Flanges and Flanged Fittings: Classes 150 and 300
  • ASME B16.47 – Large Diameter Steel Flanges, NPS 26 Through NPS 60

ASME standards (U.S.)

ASME type flange on a gas pipeline
Pipe flanges that are made to standards called out by ASME B16.5 or ASME B16.47, and MSS SP-44. They are typically made from forged materials and have machined surfaces. ASME B16.5 refers to nominal pipe sizes (NPS) from ½” to 24″. B16.47 covers NPSs from 26″ to 60″. Each specification further delineates flanges into pressure classes: 150, 300, 400, 600, 900, 1500 and 2500 for B16.5, and B16.47 delineates its flanges into pressure classes 75, 150, 300, 400, 600, 900. However these classes do not correspond to maximum pressures in psi. Instead, the maximum pressure depends on the material of the flange and the temperature. For example, the maximum pressure for a Class 150 flange is 285 psi, and for a Class 300 Flange it is 740 psi (both are for ASTM A105 Carbon Steel and temperatures below 100F).

The gasket type and bolt type are generally specified by the standard(s); however, sometimes the standards refer to the ASME Boiler and Pressure Vessel Code (B&PVC) for details (see ASME Code Section VIII Division 1 – Appendix 2). These flanges are recognized by ASME Pipe Codes such as ASME B31.1 Power Piping, and ASME B31.3 Process Piping.

Materials for flanges are usually under ASME designation: SA-105 (Specification for Carbon Steel Forgings for Piping Applications), SA-266 (Specification for Carbon Steel Forgings for Pressure Vessel Components), or SA-182 (Specification for Forged or Rolled Alloy-Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service). In addition, there are many “industry standard” flanges that in some circumstance may be used on ASME work.

The product range includes SORF, SOFF, BLRF, BLFF, WNRF (XS, XXS, STD & Schedule 20, 40, 80), WNFF (XS, XXS, STD & Schedule 20, 40, 80), SWRF (XS & STD), SWFF (XS & STD), Threaded RF, Threaded FF & LJ, with sizes from 1/2″ to 16″. The bolting material used for flange connection is stud bolts mated with two nut (washer when required). In Petrochemical industries, ASTM A193 B7 STUD & ASTM A193 B16 Stud Bolts are used as these have high tensile strength.


  • ASTM A105 / A105M – Specification for Carbon Steel Forgings for Piping Applications


  • AWWA C115 – Standard for Flanged Ductile Iron Pipe with Ductile-Iron or Gray-Iron Threaded Flanges



  • MSS SP-9 – Spot Facing for Bronze, Iron and Steel Flanges
  • MSS SP-25 – Standard Marking Systems for Valves, Fittings, Flanges, and Unions
  • MSS SP-44 – Steel Pipeline Flanges
  • MSS SP-106 – Cast Copper Alloy Flanges and Flanged Fittings Class 125,150, and 300
  • ASME B16.5 and ASME B16.47 cover pipe flanges up to NPS 60 (B16.5 from 1/2″ to 24″ and B16.47 from 26″ to 60″). ANSI B16.47 covers two series of flanges, Series A is equal to MSS SP-44-44, and Series B is equal to API 605 (API 605 has been canclled).

European and German Dimension DIN

Germany and several other countries in the European Union mainly install flanges according to standard DIN EN 1092-1 (forged Stainless or Steel Flanges). Similar to the ASME flange standard, the EN 1092-1 standard has the basic flange forms, such as weld neck flange, blind flange, lapped flange, threaded Flange (Thread ISO7-1 instead of NPT), weld on collar, pressed collars, and adapter flange such as flange coupling GD pressfittings. The different forms of flanges within the EN 1092-1 (European Norm Euronorm) is indicated within the flange name through the type.

Design According to EN Type According to DIN
Weld Neck Flange Type 11 DIN 2627 – DIN 2638
Blind Flange Type 05 DIN 2527
Threaded Flange Type 12 DIN 2558, DIN 2565 – DIN 2569
Flat Flange Type 01 DIN 2573, DIN 2576
Lapped Flange Type 02 & Type 04 DIN 2641, DIN 2642, DIN 2655, DIN 2656

Similar to ASME flanges, EN1092-1 steel and stainless flanges, have several different versions of raised or none raised faces. According to the European form the seals are indicated by different form:

Form: types of Contact Faces DIN EN 1092-1
Without Raised Face Form A
Raised Face (Rz = 160 Mechanical Turned) Form B1
Raised Face ( Rz = 40 Mechanical Turned) Form B1
Raised Face (Rz = 16 Mechanical Turned) Form B2
Tongue According to DIN2512 Form C
Groove According to DIN 2512 Form D
Male According to DIN 2513 Form E
Female According to DIN 2513 Form F
Female According to DIN 2514 Form G
Male According to DIN 2514 Form H

Flange Facings
Dimensions for facings shall be in accordance with ASME B16.5, Table 4 for flat face, raised face, and tongue and groove flanges, and Table 5 for ring joint flanges. These tables shall be used in conjunction with ASME B16.5, Figure 7.
Flat face flanges, with full-face gaskets, shall be used when one or both of the mating flanges in a joint are ASME B16.1. Class 125 gray cast iron, aluminum, or plastic, can be over-stressed by bearing against a raised face. Adapter rings may be necessary in some cases, to level off the surface, for mating equipment.
Raised face flanges shall be specified in ratings up to Class 600. These flanges are supplied with a 0.06 inch raised face, which is included in the minimum flange thickness. Finished height of the face shall be less than nominal pipe wall thickness. Flanges in classes over 600 are supplied with a 0.25 inch raised face that is additional to the minimum flange thickness.
In ring joint flanges, thickness of lap remaining after machining the ring groove shall not be less than the nominal wall thickness of the pipe used. Ring joint flanges for use with ASME B16.20 ring joint gaskets shall be used for:

  • Flanges in Class 900 and higher ratings
  • Design temperatures in excess of 480 °C
  • API 6A Type 6B flanges (well head piping)

Hazardous fluid mediums
Tongue-and-groove facing, and male-and-female facing joints, shall not be used except in high-pressure service, or when it is necessary to match existing equipment.
Flanges shall be finished in accordance with MSS SP-6, and ASME B46.1. Table I provides acceptable ranges of contact surface finishes for each type of gasket and service. The surface finishes shall be in Ra, Roughness average, expressed in micrometers, followed by micro-inches.
Flange roughness shall be judged by visual comparison to Ra standards using GAR model S-22 Micro Finish Comparator.
Ring joint flanges shall have flat-bottom type grooves in accordance with ASME B16.20.

Bore of Welding Neck Flanges and Hub Design
Dimensions of welding ends, bevel slopes and bores shall be in accordance with ASME B 16.5, Table 6, and Figures 8 to 14. Ratings of welding neck flanges shall be based upon their hubs at the welding end having thicknesses equal to that calculated for pipe having a 40 ksi specified minimum yield strength (SMYS). When SMYS of the hub is less than that of the attached pipe, the minimum thickness of the hub at the welding end shall be at least equal to the product of the pipe wall thickness and the ratio, pipe to flange, of the specified minimum yield strengths. See MSS SP-44.
Mis-matches between pipe and flange shall be corrected during fabrication. Weldneck shall be taper bored, if specified in the purchase description. Pipe wall thickness shall be specified in the purchase description to ensure that the flange is bored within the specified tolerance.

Flange Material Limitations

Flanges and flanged fittings shall be castings, forgings, or plates.
Bolting materials shall conform to ASME B16.5, Table 1B.
The material for flanges in pipeline service shall be suitable for welding. The carbon equivalents shall match the pipe material.
Iron Flanges
Cast Iron Flanges. Gray cast iron flanges shall not be used for process piping within the battery limits of any plant. The only exception shall be for use in approved fire systems. The material shall be ASTM A 126, Class B.
Ductile Iron Flanges. Ductile iron flanges may be used, in proprietary systems, for example plastic lined steel piping, as back-up flanges for lapped joints.
ASME B16.1 Class 125 and class 250 cast iron flanges may be mated with ASME B 16.5 class 150 and 300 steel flanges respectively. However care shall be exercised to ensure that a flat-faced cast iron flange shall mate only with a flat faced steel flange, and vice versa.
Carbon Steel Flanges
Carbon steel flanges shall not be used in services over 425 °C.
General Service. Standard carbon steel material shall be ASTM A 105. Standard material shall be used between minus 29 °C and 425 °C.
Low-temperature Alloy Steel Flanges. Carbon steel flanges used for services below minus 29 °C, shall conform to the impact-testing requirements of ASME B 31.3. ASTM A 350-LF2 shall be the standard material for this service.
Low and Intermediate Alloy Steel Flanges
Material for Low alloy steel flanges (11/4 Cr – 1/2 Mo) shall be ASTM A 182-F11. Material for intermediate alloy steel flanges (11/2 Cr – 5 Mo) shall be ASTM A 182-F5.
Stainless Steel and Non-ferrous Flanges
Usually, weldneck flanges shall match the metallurgy of the pipe in any material class. Austenitic stainless steels, however, may in certain cases be interchangeable. For example, type 347 and 321 stainless steels are compatible. Flanges that are double stamped, or double graded, and are so marked. For example, low carbon grades such as 304L, and 316L may be substituted, for the ‘straight’ grade, provided that the ‘L’ grade meets the physical requirements of the application.
When pipe material is forged, weldneck flanges shall be forged. When pipe material is not forged, material for weldneck flanges shall be subject to client approval.

Pipeline Service Flanges
Flanges for pipeline service shall match SMYS, and carbon equivalency specified in ASME B31.4 and B31.8.

NACE Service Flanges
When an in plant service has water and H2S concentrations above the limits specified in NACE MR0175, that service shall be considered as NACE service. Flanges for use in NACE service shall be in accordance with NACE MR0175 special requirements. Purchase description shall specify ‘NACE service’.
High Strength Material Flanges for Pipeline Service
High strength materials are fittings for API Std. 5L pipe Grade X42 to X65.

Usual Sizes of Flanges

Number of clamps and bolts, size of bolt, bolt hole, bolt circle, outer diameter or OD and maximum inside diameter or ID are the things to consider regarding flange dimensions. NW 16 CF/Mini has an OD of 1.33 and ID of 0.630. While its bolt hole and circle sizes are 0.169 and 1.063 respectively. An NW 25 CF/2 ½” has 0.88 and 2.11 for the ID and OD. It has bolt hole of 0.265 and a bolt circle of 0.169 as well. No clamps needed for this size and the number of bolt is four. For NW 35 CF/2 ¾”, OD and ID sizes are 2.75 and 1.500. 0.263, 2.312 and 2.75 are considered their bolt measurements. No need for clamps as well and the bolt number is 6. NW 50 CF/3 3/8″ has 3.37 and 1.88 for its OD and ID. Its bolt hole and circle sizes are 0.332 and 2.850. Eight (8) is the required bolt number.

Practical Uses

Available flange measurements indicate the different variations of its usage. Flange is utilized as guide or support. This goes for train wheel. Flanges are also used for pipe linking and construction. For camera lenses, flanges are vital parts as well. Hot water tanks also use flanges to permit proper water flow. Flange sizes and measurements, because of their practical usages, are now standardized. Since their sizes are variable, one of the best references would be the internet. There are graphic pages in which actual sizes are written for easy access and recognition. In some ‘do-it-yourself’ stores, always ask store assistants for help regarding the right sizes of flanges for practicality. Having a ‘know-how’ or something about flange sizes is another ‘plus’ in making any task or project be perfect.

Pros Of Using The Flange Nuts For Your Industrial Operations

A standard hex bolt looks like a flange nut from one side but the bottom of the nut is widened into a circular flange through creating a bell-like shape. The fastener is used widely for different applications, which depends on the style of the head. Some of its variants do not offer premium-locking capabilities but still have some engaging benefits when used properly in the applications. In addition, it provides better grip options on the surface of the joints. They act like a non spinning washer because at the end they have a wide flange. It distributes the pressure of the nut over the part being secured. They are also known from the names spin lock, tooth, or face nuts, etc.

This helps to reduce the chances of damage to the part and make it less likely to loosen throughout the application. The locking action of the fastener provides more resistance to the vibrations. They are designed with a wide flange to act like a washer and increase the bearing surface. They are fabricated with the strong material like steel, which is durable as well. Stainless steel improves the corrosion resistance and provides extra finish and protective coating for the complex applications.

Pros Of Using A Flange Nut:

They are best used for the oversized and irregularly shaped holes, since it covers all the discrepancies and serrations cover the entire bearing surface. They are used without any limitations unlike the standard lock nuts. However, checking back for the manufacturer’s instructions is a wise choice covering the torque values and any lubrication requirements.
You will not need washers with such type of the fastener as they serve the purpose of the washers well. Since, it extends the bearing surface and all those serrations, which need direct contact with the mating surface. This saves time and money spent on your applications through diminishing the need of torque when installing the fastener.
You can find the nuts in all the common finishes including plain and zinc plated forms. Stainless steel form is also available and provides optimum results against the problem of corrosion and various other damaging ailments of environment.
If you are working on the projects where nuts and washers are must used together, try considering flange nuts, as they are the simple two in one locking solution where strength and resistance is required. It provides more stability to the structure thus maintaining the holding power of the fastener.

Flange is applicable for many fields. For example, you would have noticed that they are used on the rails. By doing this, the wheels are kept in well motion and the rails are not easily worn out. There exist diverse materials that the flanges made from. Among them, the steel is the common material. The flange made from stainless would be very popular in many fields as its long service time.

Flanges do great favors to plumbing system because they enable the system to stay strong for a long time avoiding incidents such as leaks. We can take another example on the application of the pipe flanges-blinds. They are flanges with round plates but no center. It is used to determine the intensity of the flow rates of gases or fluids in any system. In fact, they are convenient to be installed and removed. As a result, they are mostly employed for systems which require constant cleaning and inspection.

There are other flange types, too. When the working environment appears to be of high pressure, it is advisable to use the welded flange. It would efficiently prevent the pipes from corrosion. While under the low pressure, it is better to use the flange with bolts as it enables the smooth flow. Some flanges are designed to prevent slipping are also available to the environment of low pressure.

Source: Network Arrangement – China Pipe Flange Manufacturer – Yaang Pipe Industry Co., Limited (

(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.)

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