Is pressure class alone enough to pick a flange for 540°C steam?

No. ASME B16.5 Class 600 may be the same pressure rating across multiple material groups, but creep resistance at 540°C differs sharply between A105 carbon steel and A182 F22 (2.25Cr-1Mo). Always specify the material group AND the temperature simultaneously, and confirm the mill test certificate (MTC 3.1) shows the heat-specific tensile + creep data.

When is RTJ facing required instead of RF?

Ring-Type Joint (RTJ) is required when (a) Δp exceeds 100 bar, (b) temperature exceeds 400°C, or (c) the service is hydrogen-rich (refinery hydrocracker, catalytic reformer). RTJ metal-to-metal seal survives thermal cycling better than RF with spiral-wound gasket.

What should be witnessed during inspection for high-temperature service?

At minimum: dimensional check, hardness test (≤ 22 HRC if sour service), PMI (Positive Material Identification), 100% UT for the bore area, hydrostatic test 1.5× design pressure for 30 minutes, MTC 3.1 with full chemistry + mechanical + creep data. For boiler / main steam: third-party witness by IBR / TÜV / BV is recommended.

High-Temperature & High-Pressure Pipe Flanges — ASME B16.5 / ASME B16.34

High-Temperature & High-Pressure Pipe Flanges — Engineering Guide · ASME / EN / NACE · Hebei Yajiada

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