Learn about Flanges for Piping

07 Nov.,2022


what is a flange

  • ASTM A105 (carbon steel flanges for high-temperature service)
  • ASTM A350 (CS flange for low temperature)
  • ASTM A694 (high yield carbon steel flange for line pipes)
  • ASTM A182 F1 to F91 (alloy steel flange)
  • ASTM A182 F304, F316, F321 (stainless steel flange)
  • ASTM A182 F51, ASTM A182 F53/55 (duplex and super duplex flange), and higher grades (Inconel, Hastelloy, Monel flange).

The pipe and the flange material shall, of course, match (according to the ASTM piping materials selection chart).

Flange Materials, General Requirements

  • 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 the pipeline service shall be suitable for welding. The carbon equivalents of the used flanges shall match with the pipe material

Cast Iron and Ductile 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 fire systems applications. The material shall be ASTM A 126, Class B.
  • Ductile iron flanges may be used, in proprietary systems, for example, plastic-lined steel piping, as back-up for lapped joints flanges
  • 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 above 425 °C
  • High-temperature service: Standard carbon steel material shall be ASTM A 105, a material that can be safely used for temperatures between minus 29 °C and 425 °C.
  • Low-temperature Service: 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 low temperature applications
  • High-Yield Service: High strength carbon steel flanges ASTM A694 should fit API Std. 5L pipe Grade X42 to X65.

Low-Alloy 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.
  • Non-ferrous pipe flanges (copper, cupronickel, and aluminum) are used in marine and aeronautical applications.

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. The purchase description shall specify ‘NACE service’.

Coated, Painted and Lined Flanges

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.


The ASME B16.5 specification covers flanges for piping applications up to 24 inches in diameter, whereas the ASME B16.47 specification covers pipe flanges above 24 inches (series A and B are available).

More in detail, the following ASME standards apply to main and companion flanges in pipe works:

  • ASME B16.1: this standard applies to 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 to 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
  • Series B replaces API 605 in sizes NPS 26 to 60 (ASME B16.47 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.

Pipe flanges for European piping systems are covered by the EN 1092-1 specification  (weld neck flange type 11, plate flange type 01, blind flange type 05, threaded flange type 13).


Flanges and flanged fittings shall be marked in accordance with MSS SP-25. The following shall be included in the marking:

  • Pressure rating class
  • ASME B16 designation
  • Nominal pipe size
  • The letter ‘R’ and the corresponding ring groove number for ring joint flanges
  • The letters ‘PL’ shall precede the grade symbol followed by the material grade of the pipe
  • Type of flange facing
  • Schedule or wall thickness for weld-neck flanges


A flanged joint is a connection between a pipe and another piping equipment (for example, another length of pipe, a valve, or a pressure vessel). Hundreds, even thousands, of flanged joints are necessary for a piping system to function.

The three main components of a flanged joint are:

  • two mating flanges (the “main” and the “companion” flange).
  • a number of stud bolts for flanges (flanges of different diameter, class, and face finish require different numbers and sizes of stud bolts and nuts for proper sealing);
  • two gaskets, which can be either metallic (spiral wound, ring joint, jacketed, and Kammprofile), or non-metallic (non-asbestos gaskets).

A steel pipe can be welded on the flange (this is the case of weld neck, socket weld, slip-on, and lap joint flange) or screwed onto it (threaded flange). Welded connections between pipes and flanges are used for high pressures and temperatures applications, 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.

A Flange insulation kits can also be used for a flanged connection, to protect the corrosion on flanges, bolts, and gaskets generated by the static currents running through the pipeline (this is called “cathodic protection”).

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).