This publication applies to equipment in which hydrogen or hydrogen-containing fluids are processed. stored. loaded and unloaded. or otherwise handled in petroleum refineries and petrochemical plants. Many processing facilities consist of specialized equipment within which liquids or vapors are continuously processed in contact with hydrogen at elevated temperatures and pressures. In some cases@ the equipment operates undet conditions in which both chemical and physical changes occur in carbon steel making it unsuitable for safe operation. Alloy steels can be used under these conditions@ but there is an economic incentive to use the lowest alloy steel that is satisfactory. This publication summarizes the results of experimental tests and data from operating plants where practical operating limits for carbon and alloy steels for all degrees of severity of service have been established. Additional effects resulting from cold working or heat treating are also discussed. Hydrogenation processes. together with considerations of operating continuity. dictate standards or materials that are not warranted in other operations of the petroleum industry. Although these recommendations have been prepared for refinery equipment. it is recognized that a modern refinery includes facilities other than those traditionally associated with refining operations. Petrochemical and chemical facilities are often interrelated both physically and by process procedure with refining equipment. The practices recommended here can be applied to these additional facilities to the extent that such physical relationships or similarities in conditions exist. These practices can also be applied to hydrogenation plants not connected with refineries: the more common plants are those that manufacture ammonia. methanol@ edible oils. and higher alcohols.