Inspection, repair, and structural integrity assessments of deaerator cracks in fossil plants

During a recent unit outage at an Ontario Power Generation (OPG) fossil plant, an axial through-wall crack was discovered in the cylindrical shell of the unit's tray-type deaerator. The crack was located in the south end of the deaerator near a single penetration weld that connects the east edge of the water box floor to the deaerator's shell. Subsequent ultrasonic and liquid penetrant inspections of the east-side and west-side connections revealed intermittent part-through-wall flaw indications at the inside surface of the shell, along the weld roots of the connections. Metallurgical analysis of sections removed from the cracked area determined that multiple cracks had initiated at the weld root of the east-side floor connection and propagated into the deaerator shell through the heat-affected zone. Some of these cracks had coalesced into a dominant surface crack that propagated in the radial direction through the thickness of the deaerator shell. The crack propagation mechanism was identified as high cycle corrosion fatigue. The through-wall crack and surface flaw indications exceeding an allowable size were repaired. This paper describes: (i) the inspection and repair management plans that were implemented to justify the return-to-service of the unit and the continued operation of deaerators of the same tray-type design in other units at the same fossil plant and other OPG fossil plants. (author)