What happens if machine is kept on turning gear for long period (for 3 days) before start up from cold condition?
Generally, "excessive" turning gear operation isn't a bad thing; it does consume a lot of electrical power to keep the pumps running, etc.
HOWEVER, GE now says for some units that two hours on turning gear is the equivalent of one hour of fired operation for the purposes of calculating axial compressor- and turbine rotor end-of-life.
Now, most of the units I've ever seen didn't have any timer or counter that kept track of turning gear (cooldown) operation for the rotor end-of-life calculations. So, it's likely most people never include cooldown time in their rotor end-of-life calculations.
Some units are susceptible to dovetail wear on the turbine wheels on cooldown (turning gear), so long times on turning gear are not good for those units.
There is a (mistaken) belief that every turbine must be on turning gear for several hours before being started. That's NOT true. Just about any turbine can be started from a "cold" stand-still condition, and some may experience high vibration during starting and acceleration. Some may trip on excessive vibration; others will not. Every machine, and every set of circumstances is different. Ideally, yes; it's better for a machine to be on cooldown for some period of time before starting from a "cold" stand-still condition. BUT, the unit can also be CRANKed for 15-30 minutes, and that will help with reducing vibration during starting/acceleration from a "cold" stand-still condition. And, some turbines have FIRE mode, which is also useful when several hours of cooldown can't be had before starting from a "cold" stand-still condition.
So, there are ways around this "requirement" of several hours of cooldown operation (turning gear) before starting a heavy duty gas turbine. Even just one hour of turning gear (cooldown) prior to a start from a "cold" stand-still condition is better than nothing in helping to reduce vibration during acceleration.
Hope this helps!!!