Effect of sediment thickness on the remaining creep lifetime of 9Cr1Mo refinery furnace tubes

Studying the creep behavior of mechanical engineering parts and estimating their remaining life is a major concern in the processing industry, power generation, and oil refineries. These industries widely use 9Cr1Mo alloy for manufacturing the tubes of main furnaces or heat exchangers. There is also a possibility of sediment formation that changes the temperature gradient in tube walls, leading to different creep behaviors. Hence, in this paper, the creep behavior of 9Cr1Mo (P9) tubes is investigated experimentally using both new and old materials. "Old" samples have been machined from the tubes of a main refinery furnace after operating for a period. Uniaxial creep tests are carried out at different stress and temperature levels. Based on various optimization methods and using test data, coefficients of the creep constitutive equation have been calculated for both materials. Then, these equations are introduced in a finite element model to carry out numerical analyses to study the effect of sediment thickness on the creep lifetime of the tubes. The results show that at high temperature, tubes with new material have longer creep lifetime comparing with the old ones; but at the low-temperature level, this difference in lifetime reduces. Also, it has been shown that tubes with no sediment have longer creep lifetime. With increasing the thickness of the sediment, creep lifetime of the tubes reduces significantly. In general, it is concluded that 1-2 mm sediment thickness has the same deteriorating effect as the accumulation of the creep during the 15 years of service time.