Study On The Theory And Application Of Leak-Before-Break Assessment Procedure For The Welded Pressure Pipeline

Pressure vessel and piping, as artery of nation economy, are indispensable in modern industry. And their safeties closely relates to the lives and wealth of people. Nowadays, safety assessment methods for in-service pressure vessel and piping in China are all based on fracture model. However, steady fluid leak often occurs before break of the pressured structures such as pressure vessel and piping. The dissertation aims to make the leak-before-break safety assessment systemically and comprehensively.In the dissertation, J-Resistance andδ- Resistance curves of welded metal and HAZ are established according to British Standard, BS7448. And the comparison between the two standards, National standard GB2038-91 and BS 7448, is discussed. The result indicates that there are some unreasonable aspects and limitation in the guideline of blunt line and exclude lines of GB2038-91, which leads to it difficult to measure JIC parameter. So it is suggested that GB2038-91 is amend by reference to international standard and practical experience.Some geometry parameters are extrapolated for crack stability analysis in LBB by applying the standard of'Safety Assessment of in-service Pressure Vessel with flows'implemented in 2005 and production of Electric Power Research Institute, EPRI. Based on the advanced levels of SINTAP and BS7910, tearing instability stress and plastic collapse stress are analyzed for circumferential through-wall crack of pipe to estimate the failure mode of pipe.Line elastic COA formula, which considers an amplification factor due to bulging, is deduced according to SINTAP. And its output is also compared with other known methods. Considering the effects that the plasticity at crack tip is to increase the opening of the crack and then the opening areas are greater than those in liner elastic case, available analytic methods such as Irwin and Dugdale models are investigated in the dissertation.Flow leakage rate through crack in welded pipe is computed by modern fluid finite element analysis technique. The effects of some factors, such as friction loss, crack path loss, excess weld metal and variable cross-section on leakage rate are taken into consideration during the computation.