This recommended practice (RP) covers materials and fabrication requirements for new 2¼Cr and 3Cr steel heavy wall pressure vessels for high-temperature, high-pressure hydrogen service. For this RP, "heavy wall" is defined as a shell thickness of 4 in. (100 mm) or greater, and high-temperature is considered to be operating temperatures of 650 °F (345 °C) and above. This RP applies to vessels that are designed, fabricated, certified, and documented in accordance with ASME Section VIII, Division 2, including Paragraph 3.4, Supplemental Requirements for Cr-Mo Steels, and ASME Code Case 2151, as applicable.
Materials covered by this RP are conventional steels including standard 2¼Cr-1Mo and 3Cr-1Mo steels, and advanced steels which include 2¼Cr-1Mo-¼V, 3Cr-1Mo-¼V-Ti-B, and 3Cr-1Mo-¼V-Nb-Ca steels. This document may be used as a reference document for the fabrication of vessels made of enhanced steels (steels with mechanical properties increased by special heat treatments such as ASME SA-542, Grade B, Class 4) at the Purchaser's discretion. However, no attempt has been made to cover specific requirements for the enhanced steels and they may be different than the requirements for vanadium grade.
Heat exchanger shells and channels which meet the conditions listed above are within the scope of this RP. They are included in the term "pressure vessel" for the purposes of this RP.

The interior surfaces of these heavy wall pressure vessels may have an austenitic stainless steel weld overlay lining to provide additional corrosion resistance. A stainless clad lining using a roll-bonded or explosion-bonded layer on Cr-Mo base metal may be acceptable, but is outside the scope of this document. Multilayer vessels are also outside the scope of this document.
Also outside of its scope, this RP can be used as a resource for vessels with wall thicknesses below 4 in. (100 mm), and/or operating at temperatures of less than 650 °F (345 °C), with changes defined by the Purchaser. This document may also be used as a resource when planning to modify an existing heavy wall pressure vessel.