A flange is a protruding ridge or rim, often used to increase strength or spread the load. For example, sectional beams often have flanges which carry most of the load and which are connected by a core which connects the flanges to each other. The pipes can also be fitted with flanges, which provide fasteners. Nuts and bolts may include flanges to spread the clamping force over an increased area.
Pressure Temperature Rating of Flanges
- The basis for establishing ratings shall be the minimum wall thickness, which shall be following ASME B16.5, Annexure D. The material groupings are based on closely matched allowable and yield strength values.
- 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. For example, a class 600 forged carbon steel (A 105) flange is rated at 1270 PSIG at 400 °F, whereas a class 600 forged stainless steel (A182-F304) flange is rated at 995 PSIG at 400 °F.
Properties of SS flanges
- Raised face flanges shall be specified in ratings up to Class 600. These Stainless Steel Flanges are supplied with a 0.06 inch raised face, which is included in the minimum flange thickness.
- The finished height of the face shall be less than the 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, the thickness of the 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 over 480 °C
- API 6A Type 6B flanges (wellhead 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.
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.
- Ductile Iron Flanges: Ductile iron flanges may be used, in proprietary systems, for example, plastic-lined steel piping, as backup flanges for lapped joints.
