Fouling formation and under deposit corrosion of boiler firetubes

Abstract

The most severe problem in firetube boilers is fouling formation and under deposit corrosion (UDC) on the tube surfaces, leading to water or steam leakage into the fire–side and emergency shutdowns. In this work, fouling propensity and UDC mechanism on the outer surfaces of firetubes of seawater boilers are investigated. The novelties of this work are characterization of a composite fouling layer, including corrosion scales and inorganic deposits, and assessment of pH–phosphate convergence to control the fouling and UDC phenomena on the outer surfaces of firetubes of such boilers. For this aim, two industrial 3–pass seawater steam–boilers in the pressure range of 200 psi were monitored over a period of six years. After periodical shutdowns, the chemical composition of deposits on the outer surfaces of firetubes was inspected by XRF and XRD patterns. During the working period of steam–boilers, pH, phosphate concentration and dissolved oxygen level of boiler water were determined. The results revealed that the outer layer of corrosion scales has been formed mainly from carbonates and hematite minerals, while γ-FeOOH was the main component of the inner layer. Moreover, using phosphate congruent control (PCC) diagrams the risk of fouling formation and UDC propagation is decreased and the predictive maintenance of such boiler systems will be improved.