scope:

This recommended practice presents materials and fabrication requirements for new 1 ¼Cr-½Mo steel heavy wall pressure vessels and heat exchangers for high-temperature, high-pressure hydrogen service. It applies to vessels that are designed, fabricated, certified, and documented in accordance with ASME BPVC, Section VIII, Division 1 or Division 2. This document may also be used as a resource for equipment fabricated of 1Cr-½2Mo Steel.

This document may also be used as a resource when planning to modify an existing heavy wall pressure vessel.

The interior surfaces of these heavy wall pressure vessels may have an austenitic stainless steel or ferritic stainless steel weld overlay or cladding to provide additional corrosion resistance.

For this recommended practice, the heavy wall is defined as shell thickness 2 in. (50 mm) or greater but less or equal to 4 in. (100 mm). Integrally reinforced nozzles, flanges, tubesheets, bolted channel covers, etc. can be greater than 4 in. (100 mm). At shell or head thicknesses greater than 4 in. (100 mm), 1 ¼Cr-½Mo has been shown to have difficulty meeting the toughness requirements given in this document. Although outside of the scope of this document, it can be used as a resource for vessels down to 1 in. (25 mm) shell thickness with changes defined by the purchaser.

This recommended practice is not intended for use for equipment operating above 825 °F (441 °C) or in the creep range.