Modelling of the influence of iron oxide layers on microstructural damage of boiler tubes

Oxide layer growth inside boiler tubes when they are exposed to high temperatures is expected to assume increasing importance as plants are run beyond their design lifetime at maximum load and as steam temperatures and pressures increase with new plant designs. Thus, frequent inspections are required to ensure that the boilers are still safe to use and it is cost effective to keep them running for longer periods of time. There is a need to determine what measures to be taken to ascertain a safe boiler operation. This need has been recognised and addressed in the past, but the remedial measures devised have often failed to solve the problem. This research explores, through a model developed using a finite element computer simulation platform, the thermal behaviour of slender tubes under a constant working temperature of 728 °K. Our simulation results demonstrate that hematite layers up to 15µm thickness inside the tubes do not act as insulation. They clearly show the process of long term overheating on the outside of boiler tubes which in turn leads to the initiation of flaws. Understanding how the oxides’ growth can damage boiler tubes and lead to the creep process can help to establish inspection programs. These could include oxide thickness measurements, the aim of which would be to prevent failures before the creep process is initiated