q flange facing and surface finishraised face

Question

Q. Flange Facing and Surface Finish?

Raised face flange facings are preferred.

Flat face flanges to be used only when mating with like equipment (fabricated from brittle materials; e.g. cast iron).
Ring joint facings are an acceptable alternative for higher-pressure services.

Standard flanges shall have a surface finish of 125-250 AARH. For flanges in services requiring special consideration (e.g. hydrogen and vacuum), the gasket contact surface finish shall be 63-125 AARH. Mating flanges should have the same surface finish.

Confined Joints

For any of the following conditions, gasketed flange joint designs (main shell joints) larger than NPS 24, shall provide outer confinement of the gasket:

1. Design pressure 300 psi (2068 kPa) or higher.

2. Design temperature hotter than 500oF (260^oC).

3. Minimum design metal temperature colder than -20^oF (-29^oC).

4. Cyclic pressure or temperature service.

5. Joint requires metallic gasket. Robust metal reinforced gaskets, such as spiral-wound with inner and/or outer gauge ring, are exempted.