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.
