SRA-E Lisbon 2017

Risk-based inspection (RBI) is now routine in some industries (and regions) but not in others. The differences mostly originate from the variation in the regulatory environment, perceived risk associated with the given equipment or process, and available resources, but may also arise from the inertia of established technical tradition. Even less common is assessment of resilience to damage, failures or other unwanted incidents. Tube-and-shell heat exchangers are common across many industries, but more frequently subjected to RBI in chemical industry and refineries than in conventional power plants. In this work we consider a district heat exchanger of a power plant from the points of view of inspections, maintenance, risk of in-service failure (leak) to result in unplanned shutdown, and resilience to damage and failure as seen from the available data. The practices and criteria of inspection and maintenance planning and execution have been evaluated using the inspection and failure data from one life cycle, which is the time between periods of refurbishment (retubing), spanning in this case about 13 years and six inspections. The case was selected on the basis of sustained annual mode of operation and consistent inspection history, with the same inspection service provider using the same (ultrasonic) inspection technique. The inspection data was used to assess the evolving probability of wall thinning to critical decision levels or failure. The results indicate that although the heat exchanger tubes suffer from uneven growth rates of in-service damage, the dominating damage mechanism of external erosion-corrosion has remained similar throughout the service history. As the severely loaded tubes have shortest life, inspections concentrate on these (less than 1000) tubes that during the life cycle of the heat exchanger will be increasingly plugged to prevent in-service leaks. The applied plugging criteria are shown to be appropriate and inspections sufficient to indicate the stages of failure risk acceleration and need for refurbishment. The implications for the main phases of resilience to damage and failure are discussed for the application.