Sludge deposition and caustic embrittlement in the pre-mature failure of a waste heat boiler steel plate

Abstract

A failure investigation of a welded waste heat boiler steel shell that developed transverse cracks was carried out in 1996 to determine the cause of failure as well as propose measures to be adopted to prevent similar failures occurring in the future. The boiler had been in service for several years before two cracks across two traversing circumferential welds were discovered on the underside during routine inspection. Chemical and metallographic analyses indicated that the boiler plate was made from a conventional C-Min steel and that it was certainly suitable for the purpose. The boiler interior showed massive deposition of a calcareous sludge on the walls and tube bundles. This was attributed to an excessive use of sodium orthophosphate to reduce the hardness of the boiler feed water. The high levels of Na+ and PO 43- ions found in both the sludge and boiler feed water analyses were taken as confirmatory evidence. Feed water quality monitored data for the period January 1994 to October 1995 showed pH values that were in excess of 12.4, a figure much higher than the recommended pH range 8 to 11. The high pH is thought to have been responsible for the occurrence of cracking as a result of caustic embrittlement. This point of view is supported by the observation of intergranular separation in the microstructures adjacent to the crack paths.

Journal of Applied Science in Southern Africa Vol.5, No.1 pp. 9-24