In contrast to new fabrications where all aspects can be controlled, the maintenance of plant and pipework etc. is a case of having to use what is already in place. The materials will probably be to old specifications, these may not be so readily weldable. The pipes may be 'live' [in service], and it may not be practicable to shut them down. The pipes etc may be contaminated with the contents of the pipes, for instance wax deposits. There may be corrosion of the components which reduces the wall thickness significantly. There may be other defects which have 'grown' in service.
The maintenance of plant and pipes should not be undertaken lightly. As much history of the components as possible should be studied and adequate NDT examinations of the materials should be carried out which should include an extensive visual examination. Only then is it possible to formulate repair procedures.
Special techniques may have to be used to ensure both the safety of the actual repair and also the continuing performance of the repaired component. In some cases new procedures must be developed to cover new or novel circumstances
If defects are found which are outside the original fabrication then extensive analysis may be necessary to determine if the defects can be accepted as 'fit for purpose' or must be repaired or replaced. Revalidation testing on completion of the works will provide confidence in the anticipated performance of the plant and pipes.