Function and Types of Flanges

Flanges are piping components bolted together using gasket in between two flanges as a sealing material. Flanges are used to connect pipes with each other, connect pipes to flanged valves, connect pipes to flanged fittings, connect pipes to flanged piping specialty items such as strainers and to isolate piping sections using a blind flange.

Following are the types of flanges used in piping systems:

  • Slip-on Flanges
  • Welding Neck Flanges
  • Socket Weld Flanges
  • Lap-joint Flanges with Stub-end
  • Threaded Flanges
  • Blind Flanges 

Slip-on Flanges

Slip-on flanges are installed on pipes by slipping on the flange bore over the outside diameter of pipe and securing it to the pipe by fillet welds. There is no full penetration weld between the pipe and slip-on flanges, hence the joint cannot be subject to radiography. A slip-on flange can have a single filled weld or double fillet weld. As per para 328.5.2 of ASME B31.3, if slip-on flanges are single welded the weld is required to be made at the hub. Where two welds are required, one fillet weld is made at the hub of the slip-on flange and the second fillet weld is made at the inside diameter of the flange close to the flange face. The position of pipe inside the slip-on flange should be such that the fillet weld can be carried out without damaging the flange face.The size of fillet weld on the inside of flange is equal to the lesser of pipe wall thickness or 6mm.


ASME B31.3 provides three options of weld details for double-welded slip-on flanges. A slip-on flange shall be double-welded as shown in Fig. 328.5.2B of ASME B31.3 when the service is

  • subject to severe erosion, crevice corrosion, or cyclic loading
  • flammable, toxic, or damaging to human tissue
  • under severe cyclic conditions
  • at temperatures below −101°C (−150°F)

The use of slip-on flanges should be avoided where many large temperature cycles are expected, particularly if the flanges are not insulated.

Advantages of slip-on flanges

Slip-on flange is a preferred flange for many applications due to the following:

  • lower procurement cost
  • less skill required for installing due to use of fillet welds instead of full penetration butt weld
  • weld end preparation not required on pipe ends
  • reduced accuracy in the cut length of pipe

Limitations and disadvantages of slip-on flanges

Though the initial procurement cost of slip-on flange is low, slip-on flange has the following disadvantages:

  • flange installation requires the use of two fillet welds
  • non-destructive examination is required on two welds which makes the installation cost approximately the same as weld neck flange
  • slip-on flanges are unsuitable for cyclic loading as mentioned above
  • slip-on flanges are not available in sizes 2½ inches and over in class 1500 rating and not available in any size for class 2500 rating
  • the strength of a slip-on flange is less than that of a weld neck flange due its design (less reinforcement in the hub) and installation method (fillet weld instead of butt-weld)

Welding Neck Flanges


Welding Neck Flanges is the correct terminology to be used as per ASME B31.3 and ASME B16.5. However in the industry welding neck flanges are commonly referred to as weld neck flanges or WN flanges. The Welding Neck Flange consists of a tapered hub which is secured to the pipe by a full penetration butt weld. Because of the additional reinforcement area available in the hub of weld neck flange and due to its installation by welding, the hub becomes an integral part of the pipe. This gives a weld neck flange greater strength and integrity and ability to withstand considerably higher bending moments than a slip-on flange. Since weld neck flanges are secured to pipe by butt-welding, the internal diameter of the hub end should match the internal diameter of the pipe end. The details of matching pipe ID or wall thickness are required to be specified for purchasing weld neck flanges.

Advantages of Welding Neck Flanges

Following are the advantages of weld neck flanges:

  • The butt-weld permits non-destructive examination by radiography to be carried out on the weld neck flange unlike slip-on flange which can be examined only by magnetic particle inspection or dye penetration inspection
  • The weld neck flange can be directly welded to butt-weld fittings like elbows, tees or reducers without requiring any short piece of pipe unlike slip-on flange which require a straight length of pipe for installation
  • Despite the initial procurement cost of weld neck flange being higher than a slip-on flange, the installation cost (which includes welding and inspection) reduces the differential
  • Welding neck flanges can withstand higher bending moments
  • Welding neck flanges are suitable for piping systems subject to high pressures, high cyclical loading, high temperatures as well as cryogenic services 

Socket Weld or Socket Welding Flanges


A socket weld flange consists of a socket in which a pipe is secured by a fillet weld at the hub on the outside. As per ASME B31.3 an approximate gap of 1⁄16 inch as shown in Fig. 328.5.2B and Fig. 328.5.2C is required to be present before any welding, including tack welding. If this gap is not maintained, the heat input during welding can cause the pipe to touch the bottom of socket due to differential expansion between the flange and pipe, and cause the weld to crack. The gap between the socket and the pipe is a source of crevice corrosion in the socket weld piping system.

Since socket weld flanges are attached to the pipe by fillet weld they are not considered as high-integrity joints. The joint being a fillet weld cannot be subject to radiography. Non destructive examination of socket weld flanges is carried out by magnetic particle examination or liquid penetrant examination. Though socket weld flanges are available up to 2½ inches in class 1500 rating, they are in most cases used for sizes up to 1½ inches and below and for non-critical applications. Socket weld flanges are not specified in class 2500 rating as per ASME B16.5. A socket welding flange is subject to the requirements for socket welds in para. 311.2.4 of ASME B31.3. This limits the use of socket welded flanges in any service where crevice corrosion or severe erosion may occur.

Lap-joint Flanges with Stub-end


A lap joint flange is a two component assembly comprising of a stub end and a backing lap-joint ring flange. The stub end face acts as a raised face of the flange as its outside diameter corresponds to the dimensions of the raised face portion of a flange. Only the stub end comes in contact with the process fluid. Hence, lap-joint flanges with stub ends are used as a cheaper alternative to slip-on flanges in piping specifications for corrosion resistant materials such as stainless steel or titanium. The cost of corrosion resistant stub end with carbon steel backing lap-joint ring flange is less than a complete slip-on flange in corrosion resistant material. The difference becomes significant for larger dimension and rating flanges.

Because the backing ring is loose fit on the stub end, it can be rotated after the welding the stub end on the pipe to align with the matching flange. This advantage reduces the installation effort during the fabrication process as no prior alignment is necessary. This feature is also useful for large diameter flanges, at locations where rotational adjustment is required and in services that require frequent dismantling for inspection and cleaning.

Stub ends are available as Type-A, Type-B and Type-C. Type-A stub end has flare radius on the outside. This radius matches the radius on the inside of the standard lap-joint flange. Type-B stud end does not have a flare radius. It is designed to be used with a standard slip-on flange as the backing ring joint flange. Note that a slip-on flange cannot be used as a backing ring flange for Type-A stub ends as the flange will not fit on the stub end due to radius on the outside. Type-C stub end is a fabricated form pipe. Figure 328.5.5 of ASME B31.3 shows design of five typical fabricated laps. Type-C lap-joint stub-end welding fittings shall not be used under severe cyclic conditions as specified in section 306.1.4 of ASME B31.3.

Threaded Flanges

What is a Threaded Flange ?

Threaded flange is a type of flange that has taper pipe threads conforming to ASME B1.20.1 in its bore and can be used in piping systems where it is not possible to carry out welding of flange on to the pipe such as highly explosive areas where welding can create a potential hazard. The threaded flange is screwed on to a pipe which has external taper pipe threads. Most Engineering Organizations limit their use for a size range from 1/2" to 4" and for utility services such as instrument air, plant air, nitrogen, potable water and raw water up to class 300 rating.

Blind Flanges


A Blind flange is installed at the end of a piping system either on a header or branch connection or on a valve and functions as a means for positive isolation of the piping system. Piping headers can be terminated at the ends using a butt-weld cap. However the cap is often substituted by a blind flange if a provision for future extension of the header is required. On drain systems blind flanges serve as important rodding points or can be dismantled for easy inspection. Blind flanges can be drilled and used as reducing slip-on flanges or the blind flange can be drilled and tapped for use as a threaded reducing flanges. Blind flanges are available in the same pressure ratings as weld neck flanges and with various gasket facings.

Applicable Standards for Flanges

  • ASME B16.1 - Cast Iron Pipe Flanges and Flanged Fittings
  • ASME B16.5 - Pipe Flanges and Flanged Fittings
  • ASME B16.24 - Cast Copper Alloy Pipe Flanges & Flanged Fittings: Class 150, 300, 400, 600, 900, 1500 & 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 Metric/Inch Standard
  • AWWA C115 - Flanged Ductile-Iron Pipe With Ductile-Iron or Gray-Iron Threaded Flanges
  • AWWA C207 - Steel Pipe Flanges for Waterworks Service—Sizes 4 In. Through 144 In. (100 mm Through 3,600 mm)
  • MSS SP 44 - Steel Pipeline Flanges
  • MSS SP 51 - Class 150LW Corrosion Resistant Flanges and Cast Flanged Fittings
  • MSS SP 65 - High Pressure Chemical Industry Flanges and Threaded Stubs for Use with Lens Gaskets