Predicting the Remaining Life of High Temperature Steel Piping
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FEATURE
Predicting the Remaining Life of High Temperature Steel Piping
Published online: 31 October 2008 Ó Energy Research Center at Lehigh University 2008
As the fleet of U.S. coal-fired power plants continues to age, one of the many decisions power generation companies will need to make is when to replace high temperature and pressure steam piping connecting the boiler and turbine. Waiting too long before replacing a creep-damaged steam pipe can lead to catastrophic pipe rupture and replacing a pipe prematurely would be an unnecessary expense. A research team at the Energy Research Center led by Professor Terry Delph has developed an analysis method which provides more realistic estimates of remaining useful life than has been possible up to now. Delph explains, ‘‘Steel components used for high temperature and pressure applications undergo a degradation process referred to as creep damage, which weakens the steel and will eventually lead to mechanical failure by pipe rupture. The years of life which remain in a steam pipe depends on the years of service, operating temperature and stress level, and original creep properties of the material. There are well-defined ASME procedures which can be used for making remaining life predictions, and these require availability of creep and creep rupture test data obtained from material similar to the material used to make the steam pipe. The difficulty with this approach is that there is always relatively large scatter in creep and creep rupture test data, possibly due to differences in properties of steel from one batch to another and to random errors in creep test data. Figures 1 and 2, for example, show plots of Reprinted with permission from the ‘‘Lehigh Energy Update,’’ July 2008, published by the Energy Research Center at Lehigh University. The Center is a noncommercial, nonprofit multidisciplinary research group involving professional staff, faculty, and students working to provide solutions to the Nation’s energy and environmental problems. Subscriptions to the LEU are available upon request at www.lehigh.edu/energy.
creep rate and creep failure data for A-335 steel, a material commonly used in power plant piping systems. It’s easy to see, even on a log-log plot, that the data exhibit a great deal of scatter. Large uncertainties in the creep and creeprupture data can result in unacceptably large uncertainties in predictions of remaining life.’’ To solve the data uncertainty problem, the Lehigh team (Terry Delph, Gary Harlow and Murat Ozturk) turned to Monte Carlo simulations for obtaining predictions of remaining life. The Monte Carlo method is a numerical technique developed for making probabilistic predictions for a wide range of complex physical phenomena. In this case, the application of Monte Carlo to the steam piping problem gives the probability of failure of a steam pipe at the present time or at a certain number of years in the future. Professor Harlow continues, ‘‘We used the Monte Carlo approach to predict the remaining life for three steam pipes in a coa
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