Experience on in-service damage in power plant components

Two principal groups of failure mechanisms appear in power plant components. On one hand, each component is usually known to suffer at least potentially from certain failure mechanisms that are expected and largely accounted for in design, manufacturing and assembly of the plant. Such mechanisms include for example creep, fatigue and corrosion under the foreseen service conditions. Even these mechanisms can contribute to real failures, because the real plant and its components may not turn out to fulfil the requirements, or because the plant is used longer or under different conditions from those initially expected. On the other hand, something unexpected can also happen in real service, so that other than expected failure mechanisms may intervene.
Examples are shown on both types of failure mechanisms. Creep damage can be expected to limit the component life in superheaters and hot steam lines.
Inspections on creep damage are widely used to indicate minimum life, or allowable time to next inspection. In general, unexpected failures are therefore not very common even after exceeding the nominal design life. However, unexpected mechanisms such as internal thermal shock cracking in the steam lines can result in failures much earlier than design life limit.
Such damage is much more difficult to accommodate in the inspection and preventive maintenance programs, because it is more difficult to observe and inherently missed from the expected mechanism list.