Cryogenic Flange Rating Considerations
Quick Answer: Cryogenic flange rating follows the same ASME B16.5 PT tables as ambient-temperature service, but the critical factor is material selection. The pressure rating at cryogenic temperatures equals the ambient rating (the -20 to 100°F row), so there is no pressure advantage from cold temperatures. However, the flange material must resist brittle fracture at low temperatures. Standard A105 carbon steel is limited to -20°F (-29°C) minimum. For lower temperatures, A350 LF2 (-50°F), A350 LF3 (-150°F), or austenitic stainless A182 F304/F316 (to -425°F) must be specified.
Cryogenic Flange Material Selection
| Material | ASTM Spec | Min Design Metal Temp (MDMT) | Impact Test Required | Pressure Rating |
|---|---|---|---|---|
| A105 | A105 | -20°F (-29°C) | No (above -20°F) | Per Group 1.1 |
| A350 LF2 Cl.1 | A350 | -50°F (-46°C) | Yes, Charpy V-notch | Per Group 1.1 |
| A350 LF3 | A350 | -150°F (-101°C) | Yes, Charpy V-notch | Per Group 1.2 |
| A350 LF6 Cl.1 | A350 | -60°F (-51°C) | Yes, Charpy V-notch | Per Group 1.2 |
| A182 F304/F304L | A182 | -425°F (-254°C) | Not required per ASME B16.5 | Per Group 2.1 |
| A182 F316/F316L | A182 | -425°F (-254°C) | Not required per ASME B16.5 | Per Group 2.3 |
The MDMT values above are per ASME B16.5 and ASME B31.3 guidelines. Project specifications may impose additional requirements.
When Cryogenic Flanges Are Used
Cryogenic flanges are specified for systems handling liquefied natural gas (LNG at -260°F/-162°C), liquid nitrogen (-320°F/-196°C), liquid oxygen (-297°F/-183°C), liquid ethylene (-155°F/-104°C), and other cryogenic fluids. They are also required for blowdown systems and emergency depressurization lines where Joule-Thomson cooling can reduce metal temperatures below the MDMT of standard carbon steel.
Key Rating Considerations
No pressure increase at low temperature: Unlike high-temperature service where pressure ratings decrease, cryogenic service does not increase the PT rating. The ambient-temperature row (-20 to 100°F) represents the maximum for all temperatures below 100°F.
Impact testing: Carbon and low-alloy steel flanges for cryogenic service must pass Charpy V-notch impact tests at the MDMT or lower. ASME B16.5 requires testing per the material specification (e.g., A350 requires impact testing as mandatory). Austenitic stainless steels (F304L, F316L) are exempt from impact testing per ASME B31.3 due to their inherently ductile face-centered cubic crystal structure.
“L” grades for welded joints: For flanges that will be welded to pipe (weld neck flanges), the low-carbon “L” grades (F304L, F316L) are preferred to avoid sensitization in the heat-affected zone, which could cause intergranular corrosion or reduced toughness.
| Service | Typical Temp | Recommended Material | Pressure Class |
|---|---|---|---|
| Propane refrigeration | -44°F (-42°C) | A350 LF2 | Per process design |
| Ethylene plant cold box | -155°F (-104°C) | A182 F304L | Per process design |
| LNG transfer | -260°F (-162°C) | A182 F316L | Typically 150 or 300 |
| Liquid nitrogen | -320°F (-196°C) | A182 F304L | Per process design |
For the complete PT rating tables and material group assignments, see the flange rating guide. For information on flange face finishes and their impact on gasket sealing at cryogenic temperatures, see the dedicated reference.
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