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

Abstract

Future coalfired 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 flangeseal couplings. Flanges with different thicknesses were employed for a comparative study. The important stress/creep values in the flanges and Utype seals had been obtained for variations in flange thickness and bolt relaxation while 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 twodimensional finite element analysis of bolted flangeseal couplings has been carried out by taking the relaxation of bolt stress under fullloading turbine service. The creep strength of flanges and Utype 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 flangeseal couplings are evaluated and compared. Furthermore, based on the results of contact stresses, the creep behavior of Utype seals is analyzed varying flange thickness. Finally, analysis shows that the thinner flangeseal coupling has larger longterm contact stress, while the Utype seal with the thicker flange has the least creep strength.