nishit,
The turbine and steam piping have several cases for normal operation, startup, and shutdown. The best solution would have the piping be flexible enough for the worst combination, so that forces are within allowable loads for all conditions.
First, the turbine vendor provides two sets of displacements, since the turbine casing will have two very different outlet temperatures depending on load / no load. During no-load startup or shutdown conditions, the outlet temperature will be much higher, almost the inlet steam temperature. Startup casing thermal growth will be greater than the normal condition when outlet temperature would be maybe 200 deg C less. The outlet steam piping needs to also consider the higher temperature during no-load startup/shutdown, along with normal outlet temperature of turbine.
The inlet steam piping during start up venting could be modeled with zero displacement of turbine inlet connection, but it might have only small difference from full temperature condition with casing displacement and steam piping thermal growth.
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R Yee