Analyzing the Effect of Oxide Scale Growth on Creep Behavior and Rupture Time of Superheater Tube Boiler

Abstract

One way of increasing boiler efficiency is by increasing the working pressure and temperature of steam. But higher operating temperature and pressure leads to the formation and growth of steam side oxide scale. The growth of oxide scale reduces the heat transfer from flue gas to steam and overheating the tube. The accumulated stress and strain due to oxide growth can cause scale exfoliation or failure when the strain exceed the critical strain. The failure of oxide scale can cause obstructing tube bends, eroding the nozzle, and eroding the first stage of the turbine’s blades. Due to growth of oxide scale the metal tube overheating and it leads to creep damage of the tube. T92(SA213-T92) is one type of high alloy ferritic (Martensitic) steels and it is the preferred choice material for high temperature and pressure application of superheater boiler tube, thus the mechanism and kinetics of oxidation, the growth of oxidation, thermos-mechanical stress-strain and effect of oxide scale growth on creep behavior should be studied. Analytical and numerical approaches are applied in this research. For analytical computation, develop a mathematical model and computed by using python. The numerical is computed by numerical (finite element simulation) software of ABAQUS. The rate of oxide growth of T92 alloy steel is computed at different steam temperature (600◦C and 650◦C) and flue gas temperature (800◦C,900◦C and 1000◦C), based on the analytical and numerical result the oxide growth rate at 650◦C steam temperature is higher and the oxide scale growth is more affected by the increase of steam side temperature than flue gas temperature. Around 178µm the thermo-mechanical hoop strain become 1.503 × 10−3 and it cross the critical strain in ten sion curve, which means the oxide scale start cracking. Under constant internal pressure and elevated temperature for a long period of time the material accounts secondary creep behavior and computed by using Norton’s law (power creep law) and time-hardening rule. Based on the analytical and numerical result, the creep hoop stress and strain rate in increasing due to the increase of oxide scale, and after 10000 working hour of superheater boiler tube the maximum creep hoop strain become 169.2 MPa and the maximum creep strain rate become 8.612 × 10−1hr−1 . The creep rupture time is calculated based on LMP and with the Norton’s Law of minimum creep strain rate relation. Based on LMP prediction 73% reduction of rup ture time when the boiler tube has 242µm oxide scale thickness as compared to the oxide scale of 96µm. The result from both analytical approach and numerical (FEM) have a good agree ment with other literatures.