During start-up from a cold state the temperature of the steam turbine metal elements increases by as much as 500K or more. This is accompanied by elongations, heat deformations and increase of stresses in the metal. Relative elongations of the inner and outer casing and the rotor appear. As the clearances are reduced, friction of metal against metal can occur. Frequent changes of heat load and large heating rates lead to thermal fatigue and metal cracking. Permanent thermal deformations such as the so-called „cat back” (longitudinal deformation) can occur.

The start-up procedure for a large power turbine (30 MW, 50 Hz) can be scrutinized with the help of CFD (Computational Fluid Dynamics) and CSM (Computational Structure Mechanics) analysis. The surface heat flux and fluid/solid temperatures are first evaluated in the course of preheating the casing/intercasing space for this turbine, then further thermal and mechanical integrity calculations are made so as to validate the preheating procedure. The calculations include the geometry of the turbine outer casing, inner casing, stator blade carriers, rotor and other elements of the turbine heat-up system. The calculations are made for given parameters of the steam supplied to turbine inlet/exit pipes and given heat-and-flow balance at crucial turbine flow sections.

Industrial turbine of power 30 MW.

   

Temperature in fluid, metal and isolation after chase 4 of preheating (115 min) [K]

The expansion of the shaft and casing during turbine start-up.