Flanges and Fittings: Matching Pressure Class and Material

Flanges and fittings join pipes, valves and equipment. Most problems come from mismatches: a flange rated for the wrong pressure, a facing that does not suit the gasket, or a material that does not suit the fluid. A little checking up front avoids most of them.

Pressure class

Flanges are rated by class: 150, 300, 600, 900, 1500 and 2500 are the common ones in ASME B16.5, which covers sizes up to NPS 24. Larger sizes are covered by ASME B16.47. The actual pressure a flange can take depends on both its class and the temperature and material group, so always check the pressure-temperature tables rather than assuming a Class 300 flange handles a fixed pressure.

Common flange types

• Weld neck (WN). The usual choice for high-pressure, high-temperature and cyclic service. Strong and easy to inspect.

• Slip-on (SO). Lower cost and easier to align; used in lower-pressure service.

• Socket weld (SW). For small-bore lines.

• Threaded. For small lines where welding is not wanted.

• Blind. Closes the end of a line or a nozzle.

• Lap joint. Used with a stub end where frequent dismantling is expected.

Facing and gaskets

The facing must suit the gasket. Raised face (RF) is common. Ring-type joint (RTJ) uses a metal ring in a groove for high-pressure service. Flat face (FF) is used when mating with cast equipment. Bolting and gasket should be chosen along with the flange, not afterwards.

Materials

Carbon steel forgings such as ASTM A105 are widely used for general hydrocarbon service. Stainless and alloy grades are used for corrosive or low-temperature duty. For sour (H2S) service, material and hardness requirements are set by NACE MR0175 / ISO 15156, so state it in your enquiry.

Butt-weld and forged fittings

Elbows, tees, reducers and caps in butt-weld form are covered by ASME B16.9. Forged socket-weld and threaded fittings for small bore are covered by ASME B16.11. Match schedule and material to the pipe so wall thickness and weld preparation line up.

Quick checklist

1. Size, class and type

2. Facing and gasket

3. Material grade and any sour-service requirement

4. Bolting material

5. Pipe schedule (for weld-end items)

6. Standard and documentation required

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