Metal plates that are welded to the inside of the pressure vessel wall. Normally, plates are of a more corrosion resistant or erosion resistant alloy than the vessel wall and provide additional corrosion/erosion resistance. In some instances, plates of a material of construction similar to the vessel wall are used for specific operating periods where corrosion and/or erosion rates are predictable.
Source: API 510, Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration Downstream Segment, Ninth Edition, June 2006. Global Standards