Ligament cracks were discovered during a cold survey in a main steam outlet header in a 250MW coal/gas fired unit following over 100,000 hours of service. Subsequent inspection revealed similar cracking in the same headers of two other units in the plant which had seen similar service. To ensure the safety of the headers a structural integrity programme was initiated by the operator, Genesis Energy, in order to determine their fitness for service. The maximum stress and stress range due to the temperature and pressure fluctuations during operation and starts was established by carrying out a detailed thermo-mechanical FE analysis, validated against detailed thermocouple measurements. The critical crack size was calculated and the time for the flaw to reach the critical flaw size was established by considering creep and fatigue crack growth mechanisms. Mechanical testing on material obtained during a repair of one of the headers was used to refine estimates of toughness and tensile properties. Moreover examination of the fracture face removed during the repair indicated that the crack growth was primarily due to fatigue. This was found to be due to frequent thermal cycling of the unit during nominally “steady state” operation. The crack growth rate established from the fracture surface analysis was compared with NDT measurements for validation of the fatigue crack growth model. The demonstrated predictability for the rate at which the ligament cracks are growing as a function of operation has provided Genesis with the opportunity develop a long term strategy for inspection, repair or replacement of their superheater headers.

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