FLANGES IN PIPING
Pipe flanges are available in multiple types, the standard ones are the welding neck, blind, socket weld, lap joint, threaded, etc.). There are also some special types of flanges, like the swivel flanges, the expander/reducer flanges, the “Nipoflange” / “Weldoflange”, and the orifice flanges.
As a general rule, welding neck, slip-on, and socket weld flanges are used for high-pressure applications that require long-lasting flanged joints. Threaded flanges can be used with a lower pressure piping system, and if vibrations are not present.
Lap joint flanges are used in connection with stub ends either to facilitate the alignment of the bolts of the two mating flanges or to reduce the cost of noble materials in high-grade flanged joints (example, in an Inconel piping system, the stub end connected to the pipe may be in Inconel, whereas the lap joint flange can be of a lower grade, thus saving the overall weight of the expensive Inconel material).
MATERIAL GRADES
The key material grades for pipe flanges, examined in a separate article, are:
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ASTM A105 (carbon steel flanges for high-temperature service)
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ASTM A350 (CS flange for low temperature)
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ASTM A694 (high yield carbon steel flange for line pipes)
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ASTM A182 F1 to F91 (alloy steel flange)
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ASTM A182 F304, F316, F321 (stainless steel flange)
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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
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Flanges and flanged fittings shall be castings, forgings, or plates
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Bolting materials shall conform to ASME B16.5, Table 1B.
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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
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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.
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Ductile iron flanges may be used, in proprietary systems, for example, plastic-lined steel piping, as back-up for lapped joints flanges
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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
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Carbon steel flanges shall not be used in services above 425 °C
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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.
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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
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High-Yield Service: High strength carbon steel flanges ASTM A694 should fit API Std. 5L pipe Grade X42 to X65.
