A part (for instance a pipe for crude) may suddenly fracture for creep, a welding zone of a steam line may cracks after a short exercise time for caustic embrittlement, or a valve body in two- phase austenitic-ferritic steel may have cracks initiated in zone reared by welding; each one of these damages leads to the Plant stop.

Repairing or replacing the damaged component is the easiest way of proceeding. Since avoiding repeats of failures is specially important for limiting useless costs, it is necessary to diagnose the cause of the failure initiation which could be due to material defects or incorrect operating.

A Plant degrades over time as components age till they are no longer economical to operate. In order to ensure reliable service for the ageing plant population advanced material damage and remaining life techniques as well as diagnostic monitoring are necessary to anticipate when components must be scheduled for repair or replacement. An up-to-date maintenance considering the "health" of the single components can lead to a life extension of a Plant.

The conventional NDT for inspecting components are RT, UT, MT, PT and ET; UT also gives some indication on the material structure, erosion and corrosion and RT can give indication on residual stress. These tests are suitable to indicate the defect location and extension but cannot help for understanding the defect causes that are very important for the anticipation of the components repairing and replacing and Plant life extension. Other NDT like visual examination by endoscope, hardness, chemical analysis, and replace are more convenient for a search of the defect cause and then for the Plant life extension.

The aim of this paper is the description and analysis of some significant application of these tests for the Plant life extension.