Comparative Study of Flange-to-Seal Contact Couplings With Bolt Relaxation Under Creep Condition

Future coal-fired steam turbines promise increased efficiency and low emissions. However, this comes at the expense of increased thermal load from higher inlet steam temperatures and pressures leading to severe creep that significantly influences the sealing behavior and high temperature strength of bolted flange-seal couplings. Flanges with different thicknesses were employed for a comparative study. The important stress/creep values in the flanges and U-type seals had been obtained for variations in flange thickness and bolt relaxation whilst maintaining other leading parameters constant. The variation of contact stresses due to creep deformation plays an important role in achieving a leak proof sealing. In this paper, a two-dimensional finite element analysis (FEA) of bolted flange-seal couplings has been carried out by taking the relaxation of bolt stress under full-loading turbine service. The creep strength of flanges and U-type seals are investigated by Cocks-Ashby (C-A) equivalent strain method. The multiaxial state of stresses is considered in this method by using C-A multiaxial coefficient. According to ASME allowable creep limit, the C-A equivalent strains of three flange-seal couplings are evaluated and compared. Furthermore, based on the results of contact stresses, the creep behavior of U-type seals is analyzed varying flange thickness. Finally, analysis shows that the thinner flange-seal coupling has larger long-term contact stress, while the U-type seal with the thicker flange has the least creep strength.