Choosing Alloys for Pressure Vessels

## The ASME Code Framework ASME Section VIII governs pressure vessel design. The maximum allowable stress at temperature determines wall thickness: t = PR / (SE - 0.6P). ## Carbon and Low-Alloy Steels | Grade | Max Temp | Allowable Stress at 400C | Key Application | |-------|----------|--------------------------|------------------| | SA-516-70 | 425 degrees C | 115 MPa | General purpose | | SA-387-11 | 540 degrees C | 103 MPa | Moderate H2 service | | SA-387-22 | 595 degrees C | 96 MPa | Refinery reactors | | SA-387-91 | 595 degrees C | 125 MPa | Supercritical boilers | ## Stainless Steels **SA-240 304L/316L**: For corrosive service and cryogenic applications. Excellent toughness to -196 degrees C without impact testing. **SA-240 2205 Duplex**: Double yield strength allows thinner walls. ## Cryogenic Applications **9% Nickel Steel (SA-553)**: For LNG storage at -165 degrees C. 690 MPa tensile with 100+ J impact at -196 degrees C. ## Clad Vessels Carbon steel structural shell + 3 mm CRA liner (316L, Alloy 625, or Alloy 20) saves 40-60% versus solid CRA construction. ## PWHT Impact Carbon steels above 19 mm thickness require PWHT. Materials that avoid PWHT: austenitic stainless, duplex, and nickel alloys. ## Common Mistakes - Specifying SA-516-70 below its MDMT without impact testing - Using Grade 22 where Grade 91 allows thinner walls - Overlooking hydrogen attack limits per API 941