CREEP STRAIN, ELLIPTIC RELEASE AND ADDITIONAL STRESS OF THE ELBOWS INMAIN STEAM PIPING

Many investigators and engineers find that the ellipticity or ovality of an oval section of elbow operating at elevated temperature decrease s over time. The ellipticity produced during the formation of the elbo ws vanishes because creep deformation occurs while the elbows are oper ating at such elevated temperatures as those found in steam-piping wor king conditions in power plants. This paper gives a theoretical deriva tion and a practical tracing measurement to demonstrate the eliipticit y-releasing process. It proposes an ellipticity-time relationship whic h provides engineers with a means of taking account of the additional stress added at the elliptical sections of elbows. The additional stre ss added to oval sections of elbows is also derived as a function of t ime from the above ellipticity-time relationship. This stress-time fun ction makes it easier for engineers to estimate the additional ellipti city stress and to assess the remanent life of elbows in steam pipes.