Water Lancing of Bruce-A Unit 3 and 4 Steam Generators

Abstract

During the Bruce-A 1993 Unit 4 and 1994 Unit 3 outages, three water lancing operations were carried out along with chemical cleaning as part of the station boiler refurbishment program. The water lancing activities focused on three boiler areas: 1) support plates to clean partially or completely blocked broach holes and prevent boiler water level oscillations, 2) hot leg U-bend supports (HLUBS) to remove deposits contributing to boiler tube stress corrosion cracking (SCC) and 3) tube sheets to dislodge sludge piles that potentially threaten boiler tube integrity and to flush out postchemical cleaning insoluble residues. The combination of water lancing and chemical cleaning effectively reduced broach hole blockage from up to100% to 040% or less. As a result, boilers in Units 3 and 4 will operate for some time to come without concerns over water level oscillations. However, deposits remained in most tube support plate land areas. For Units 3 and 4, the prechemical clean lancing operations had limited success in dislodging HLUBS deposits; although some deposit from the scallop bar surfaces was removed, hour glass areas remained dirty. The postchemical clean lancing activities were more effective in removing surface deposits from the HLUBS in Unit 3 boilers. This was mainly due to the longer iron step performed during the Unit 3 boil- er chemical clean. Inspections carried out after chemical cleaning and water lancing in either unit, showed that most hour glass areas remained filled with deposit. Water lancing of the Unit 4 boiler HLUBS in 1995, dislodged much of the scallop bar deposit left behind in 1993, but did not succeed in cleaning hour glass areas. Visual inspections performed on Unit 4 HLUBS in 1995 revealed significant scallop bar degradation in Boilers 2 and 7, and relatively less P.O. Box 3000 Tiverton, Ontario NOG 2TO (51 9) 361-2673 scallop bar decay in Boilers 1, 3, 5 and 6; in Boilers 2, 3,6 and 7, degradation areas not identified during the final inspections in 1993 were found. The 1995 inspections also showed far more scallop bar decay in the Boiler 2 HLUBS compared with the other Unit 4 boilers. Tube sheet lancing activities in Unit 4 after chemical cleaning, effectively removed about 275 kg of wet sludge per boiler. The cold leg and outer hot leg areas were cleaned down to the tube sheet, with thin deposits remaining in the original location of the sludge pile, In the hot leg central areas, hard tube deposits did not allow lances to travel farther down than 12.7-30.5 cm (5-12 inches) above the tube sheet and prevented tube sheet inspections in these areas. Attempts to remove the hard, tenacious tube scale by water lancing did not succeed. In Unit 3, the tube sheet lancing operations before chemical cleaning removed about 222 kg of tube sheet sludge per boiler, while the postchemical clean activities dislodged roughly 109 kg of chemical cleaning insolubles from each boiler. As in Unit 4, the cold leg and outer hot leg areas were cleaned to the tube sheet and hard tube collars were left in the central hot leg areas; nearly all the hard tube collars present before chemical cleaning were unaffected by the chemical cleaning process. This is thought to have resulted from shielding of the tube collars by the rapid build up of insoluble residues on the tube sheets during chemical cleaning.