Reactor vessels in semi-regenerative CRU service are stand-alone items and can be either hot- or cold-shell depending upon design preference. In CCR service, the reactors are invariable of hot-shell design and are stacked to form a compartmented single vessel. In cold-shell service, carbon steel vessels are the norm, whereas hot-shell design necessitates the use of 1-11/4CrMo, or in some cases 21/4Cr1Mo, to meet the requirements for both high-temperature strength and resistance to hydrogen attack.
The vessels are welded and in hot service, it is the behavior of the various weldments that generally dictates the vessel life. Weldment performance is dominated by the detailed design and subsequent fabrication of these features. The better designs ensure that the metallurgical discontinuity, i.e. weld metal and associated heat affected zones (HAZ), is not coincident with the geometric discontinuity that can often be associated with the joining of the two discrete parts. This factor has been addressed before, as has the significance of this effect on inlet and outlet nozzle life.