Condition-Based Inspection Grouping Policy for Boiler Heat Exchanger Tubes

Boiler heat exchanger tubes lose thickness over time, resulting in costly ruptures and losses in capacity as thin tubes are taken out of operation. To mitigate the costs associated with thickness loss, boiler tubes are inspected and can be preventively taken out of service (i.e. “plugged”) to avoid in-service ruptures. Moreover, there is an economic dependence among thousands of tubes of heat exchangers in inspection and maintenance activities where a large setup cost may be induced when inspection is conducted for any tube. This paper extends the authors’ previous study on boiler tube inspections to include a dynamic inspection and preventive maintenance grouping policy to pursue additional savings on setup costs. The inspection policy is said to be condition-based since the time for next inspection is based on the current inspected state. A heuristic thickness loss threshold for plugging is computed for each tube by balancing the risk of an in-service rupture with the lost revenue due to capacity loss and an optimal inspection grouping strategy is developed using the Markov Decision Process paradigm. The policy is applied to a case study of a boiler operating in an Australian sugar factory and bench-marked with policies that represent current practices and do not consider grouping. The results show that the proposed condition-based grouping inspection policy yields significant savings compared to tube individual inspection policies.