The unavailability of coal fired electrical generating capacity is often attributable to corrosion in steam generators. Electricity demand grows faster than new plants come online, forcing greater use of old plants. Identifying such problems requires proper inspection tools and techniques to take the guesswork out of whether boiler tubes should be repaired or replaced.
Stress assisted corrosion (SAC) is a major concern for kraft recovery boilers in the pulp and paper industry. A water leak can cause a smelt-water explosion in the boiler. A number of carbon steel tubes showed a deep decarburized layer on the inner surface (water-touched) and also an unusually large grain size at the inner tube surface.
Failure analysis methodology is applied to the principal mechanisms by which boiler tubes fail during service. sThe failure analysis procedure, or methodology for evaluation, is provided in a step by step approach. Among the case histories discussed are: fatigue, erosion, short-term overheating, and hydrogen damage.
Boiler system is one of the most critical systems for a utility plant. A utility plant had experienced high percentages of boiler downtime owing to boiler tube sheet cracking failures. Investigations carried out revealed high stress at the tube-to-tube sheet joint in the boiler fire-side entrance. Tube-to-tube sheet joints at the boiler fire-side entrance had been fabricated by strength welding and without any expansion. The strength welded joint had created undue stress leading to cracking of the weld joint by thermal expansion. A higher quality expanded joint consisting of expanding, flaring and seal welding the fire-side entrance was implemented during the re-tube process. The utility plant has now zero downtime due to boiler tube failure. This article summarizes the description and history of failures with the boiler at the utility plant; investigations and corrective actions carried out; and the present improved condition of the boilers.