When repairs are made to code pressure vessels, the National Board Inspection Code (*NBIC©) classifies them into three categories: 

• Alterations,
• Repairs
• Routine repairs.

Alterations are the most significant type of repair. Examples of alterations are adding a new nozzle, changing materials of a vessel part, or changing the configuration of a vessel. Alterations usually require calculations for the new vessel parts, and a hydrostatic test after completion. Standard repairs involve things like replacing a corroded or damaged vessel part.