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  • Nadirsha Shahabudeen

Selection of Various Types of Flanges

Updated: Mar 11

Pressure-Temperature ratings for the applicable materials listed in ASME B16.5, Table 1A shall be the maximum allowable working gage pressures at temperatures shown in Table 2 of the same standard. Within each pressure class, the dimensions are held constant, irrespective of the material. Physical properties, and thereby the allowable stress values, of different materials vary so the pressure-temperature ratings within each pressure class vary with the material.


Flange Dimensional Standards

The following dimensional standards shall apply to metallic flanges, and bolthole patterns of non-metallic companion flanges:

  • ASME B16.1, for integral cast iron flanges and blind flanges

  • ASME B16.5, Classes 150, 300, 600, 900, 1500 up to NPS 24 and class 2500 up to NPS 12. Class 400 carbon steel flanges shall not be used.

  • Flanges larger than NPS 24 shall be specified in accordance with ASME B16.47. ASME B16.47 series A for NPS 26 to NPS 60 in class 150 to 900 replaces these flange sizes in MSS SP-44. This is usually used in plants for mating certain valves.

  • Series B supersedes API 605 in sizes NPS 26 to 60. Series B is used for pipelines and is restricted to flanges used for joints.

  • MSS SP-44 shall be used for steel pipeline flanges for sizes smaller than ASME B16.47 where the material grade is not listed in ASME B16.5

  • Flanges of unlisted materials and flanges not covered by the above standards shall be designed in accordance with ASME Section VIII Div. 1, Appendix 2, and for blind flanges, in accordance with ASME Section VIII Div. 1, Section UG-34

  • Tolerances for flanges shall be in accordance with ASME B16.5, section 7 for flanges up to NPS 24, and ASME B16.47 for flanges over NPS 24.

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.


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.

  • Flanges requiring galvanizing shall be Zinc coated using hot dip galvanizing process conforming to ASTM A 153. Flange facing shall be protected suitably during galvanizing in order to avoid irregular surfaces which will lead to leakage when flanges are assembled.

  • 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. Company may specify additional requirements, for example normalizing. These special requirements shall be added to the purchase description.

  • Low-temperature Carbon steel flanges used for services from minus 29 C to -45 Deg C, shall conform to the impact-testing requirements of SES P02-S01. ASTM A 350-LF2 shall be the standard material for this service.

  • Low and Intermediate Alloy Steel Flanges Material for Low alloy steel flanges (1 1/4 Cr -1/2 Mo) shall be ASTM A 182-F11. Material for intermediate alloy steel flanges ( 5 Cr – 1/2 Mo) shall be ASTM A 182-F5.

  • Stainless Steel and Non-ferrous Flanges Flanges shall be in forged construction. When flange material is not forged, the material for flanges shall be subject to approval.

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

Flange Types


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, weld neck 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, weld neck flanges shall be forged. When pipe material is not forged, material for weld neck 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 the NACE service. Flanges for use in the NACE service shall be in accordance with NACE MR0175 special requirements. The purchase description shall specify the ‘NACE service.


Flange Types

The selection of appropriate joining methods varies with the required mechanical strength in the joint, from a minimum, as in slip-on connections, to maximum, as in integral-type flanges that are cast, integrally forged or butt-welded to the pipe.

Weldneck Flanges

This group of flanges is designed with a hub on the backside tapering to a diametre that will match the pipe to which it will be welded. These flanges are bored to match the inside diametre of the mating pipe so there will be no restriction of product flow. This prevants turbulence at the joint and reduces erosion. Welding neck flanges are preferred for use in severe service applications involving high pressure, sub-zero and or elevated temperatures. They also provide excellent stress distribution through the tapered hub and are easily radiographed for flaw detection. Welding ends of weld neck flanges shall be in accordance with ASME B16.5.

Threaded Flanges

Threaded flanges are threaded in the bore to match an external thread on the pipe. The threads are tapered to create a seal between the flange and pipe as the tapers approach the same diametre. These flanges are normally designed for low pressure, non-cyclic applications. They are also used in applications where welding is hazardous.


· 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 a 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.

Socket weld Flanges

These flanges are similar to slip-on flanges except they have a bore diametre equal to that of the matching pipe. They also have a counterbore from the hub side slightly larger than the outside diametre of the matching pipe. The counterbore provides a socket into which the end of the pipe is inserted. The flange is then attached to the pipe by a fillet weld aat the hub. The shoulder made by the difference between the bore and counter bore in the same width as the wall of the pipe, there-by providing for an unrestricted flow of product through the connection. These flanges were initially developed for use in small diameter, high-pressure lines. Internally welded socket type flanges are typically used in chemical process, hydraulic lines and distribution lines. 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

Slip-on flanges are designed to slide over the outside diametre of the pipe to which it will be welded. These flanges are attached to the pipe by filllet welding at the hub and at the end of the pipe inside the flange. Because of the low hub and method of attachment, These flanges are not normally used in high stress applications. Slip-on flanges cost less than welding neck flanges and require less accurate pipe cutting, but their strength is approximately 2/3 of weld neck flanges under internal pressure, and they have approximately 1/3 the fatigue life of weld neck 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:-

i. Severe cyclic conditions. See ASME B31.3, paragraph 300.2.

ii. Design temperatures above 230 °C, or where the corrosion allowance exceeds 3 mm

iii. ASME B16.5 Class 400 or higher rating

iv. 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.

v. 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.

Lapped Joint Flanges

Used in conjunction with a Lap joint stub end these flnges are nearly identical to a slip-on flange with the exception of a radius at the intersection of the flange face and the bore to accomodate the flanged portion of the stub end. These flanges are used in applications where the joint must be frequently disassembled for cleaning or where there is a need to facilitate bolt alignment.


· 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 bolt holes 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 weld necks, 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.

Blind Flanges

These flanges are manufactured without a bore and used as closure or seals for the ends of piping systems. They are also used to provide access covers for pressure vessels. Blind flanges are provided with or without a hub, depending on customer requirements. Under pressure and bolt loading, the blind flange is subjected to more stresses than any other flange type. However, the maximum stresses are bending stresses at the centre and are easily absorbed by the flange.

· Blind flanges shall be used as end closures on flanged ends and valves unless end caps are specified in the 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 weld necks, 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 overstressed.

Orifice Flanges

Designed for flow metering systems. Two of these flanges with bolts and jack screws are called an orifice flange union and are used in conjunction with an orifice plate. The orifice plate is a seperate commodity and is not sold as a part of the flange assembly. Each flange is provided with a pair of pressure taps for measurement of pressure drop in the flow through the orifice plate. The taps are precisely located with respect to the orifice plate. Orifice flanges may be provided in one of three types of flanges. Welding Neck, Slip on and Threaded.

· Orifice flanges shall conform to this standard and ASME B16.36.

· Orifice flanges shall be weldneck flanges.

· Orifice flanges shall have jackscrews to facilitate disassembly of the flanged joint during maintenance.

Other Standards

· Other standards, for example, AWWA C207 for hub flanges, may be required for proper mating to equipment and shall be reviewed at the time of generation of a material class.


The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.

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