Steam turbine transient maneuvers have a significant impact on the cyclic fatigue life. Modern steam turbines are operated at high temperatures for optimal efficiency, which results in high time and space temperature gradients. A low initial metal temperature after standstill results in a high temperature difference to be overcome during the next startup and consequently a low lifetime at critical locations. To achieve the fastest possible start-up time without reducing the lifetime of the turbine components, the natural cooling must be captured accurately in calculation and the start-up procedure optimized. At the past two ASME conferences we presented three papers [1], [2], [3], about a 2D numerical procedure for the thermal regime calculation during natural cooling and startup. The analysis included the rotor, casings, valves and pipes. The main concept was to replace the thermal effect of the fluid convectivity by a fluid function K(T) called “over-conductivity”, which is calibrated vs. experimental data. The paper below shows: (a) the theoretical background of the over-conductivity function K(T) and (b) the equation of the correlation function f(T,p) between the fluid velocity and fluid temperature gradient. Both K(T) and f(T,p) are applicable for the flow within the large turbine cavities with negligible pressure gradient. The robustness of the K(T) function is verified on three different turbine configurations. For each machine a separate transient thermal model was built and the calculated temperatures were compared with the corresponding measured temperatures. At the end of the paper conclusions about the natural cooling features are presented.

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