Failure analysis of the leakage at boiler bottom wall tube of steam power plant

In order to maintain the availability of electricity supply, a failure at the power plant is mandatory to be prevented. If damage has occurred, analysis of the root cause of failure must be prioritized to resolve the problem and prevent further damage. One of the important units in a power plant is a boiler that has a role as a steam generator producing the steam by changing the water in the liquid to the vapor phase. The quality of the (feed) water becomes one crucial factor in determining the lifetime of a boiler. Moreover, some factors related to the process variable control such as velocity, temperature, pressure, and flow rate of fluid should follow the specified design. Otherwise, the failure can occur, resulting in the unit shutting down, thus stopping the electric production.In this paper, the mechanism of the leakages that occurred at boiler bottom wall tubes of the steam power plant was investigated. Quality of (feed) water and several factors associated with the leakages such as temperature, pressure, and fluid characteristic are mainly considered the suspect contributing to the damage. Further to this, macro-and microstructural observations at the leaked tubes as the physical analysis were done. Analysis of chemical composition was performed to the (feed) water. Other factors concerned with the failure were also observed to validate the root cause analysis and mechanism of the leakages.Chemical analysis of (feed) water shows that the excess content of hydrazine (N2H4) as an inhibitor of corrosion is 100 times more than specified representing the low-quality level of (feed) water which significantly influences the aggressiveness of the water. In addition to this, an intentional turbulence of the flow rate of (feed) water shown by having a Reynold number of 33,000 together with the contaminated (feed) water significantly increases the risk of failure at the inner tube. Macro observation exhibits a sectional reduction on the upper curve radius at the leaked tubes indicating that a loss of product reaction between the outer tube surface with a suspicious variable occurred during the operation. Moreover, a damage to the protective magnetite layer was particularly found around the leakage denoting that a local failure happened at the inner surface tube. On the other hand, micro observation shows a local plastic deformation implying an increase of high local temperature around the leaked tubes.To conclude, the tube leakages are mainly resulted from the collaboration of suspicious factors originating from the ambient environment on both sides of the tubes. Indeed, the low level of water quality, the high turbulence of flow rate, and the increasing of local temperature during operation at inner tubes successfully caused the erosion corrosion. While the suspicious factor at outer tube, such as the fuel, led to induce the pitting corrosion. In this case, both corrosions are responsible factors to encourage the failure.