Investigation of Failed Coils from a Steam Boiler

Abstract

This case study presents results of analysis of failed coil samples from a steam-generating boiler. Scanning electron microscope images suggest that the main cause of failure was overheating. Calcium carbonate and calcium phosphate scaling salts probably derived from the water hardness were identified from the energy-dispersive x-ray (EDX) map. The combination of iron oxide and scales was likely to have created an insulating effect resulting in overheating and eventual tube failure. Calcium carbonate and phosphate scales are easily removed by chemical and mechanical methods. However, approximately 9% of the debris consisted of calcium silicate, as identified from the EDX map. Silicate scales adhere strongly to the surface and are not easily removed by chemical and mechanical methods. From the result of this analysis, it was possible to provide the prior information required for satisfactory repair and maintenance of the failed boiler coils.