I believe that once the unit has completed temp matching and has been sitting at base load for 5hours, this is adequate time for heat soaking the ST and an over speed test can then be performed.
I say this due to the fact that temp matching has completed so the unit is fully expanded, the additional 5hours at base load adds to the time the unit is heat soaking.
What I am being told is that the unit needs to be running for 2/3 days before an over speed test can be performed.
Can you enlighten me on this as to which process is correct and why?
I do not see the reasoning for 2 to 3 days of operation required before doing an overspeed test. However, I have not seen ever steam turbine in the world and there may be some special reason that the manufacturer stated.
Your unit should have a startup curve that has instruction and limitations on starting the turbine up. This is to ensure no damage occurs and also to minimize stresses in the thick metal parts throughout the turbine.
I wish I could provide some guidance, but I have never heard of a 3 day "soak" before doing an overspeed test. In fact, to perform an overpeed test, the unit will be offline (generator breaker open) and very little steam flow is needed (let's say 1-3%).
Is this 3 day requirement referenced in some plant documentation or is somebody just telling you verbally?
the reason for a heat soak run is to elevated the metal temperature through out the rotor bore above the ductile-brittle transition (DBTT). If the rotor metal is still in the ductile phase, it is possible for a rotor burst to occur with the increase centripetal force as speed exceeds rated.
the same DBTT is the reason for some pre-warming before reaching rated speed
For the units I've worked on the expected time for the bore to achieve equilibrium temperature is about 3 to 5 hours. the required temperature is usually achieved with any where from 10 to 30% rated load (depends upon steel composition used for each rotor include generator).
Some people believe that extended and higher loads are better (maybe the reason you have a 2/3 day recommendation) but in fact it will cause unnecessary stress when performing the over speed test since the throttled steam at no load will be cooler than the rotor surface and thus compressive stress will occur, expending some of the rotors cyclic life
