So JohnPA, are you saying it is not quite necessary or mandatory to store spare rotors in vertical position and in inert gas/nitrogen metal compartment?
Not at all! I am solely questioning the practice of removing a rotor from a machine, then having it totally refurbished including balancing (often by the OEM I might add), then after storing it for some extended period, upon removal the first thing done is a balance check. If the rotor was stored horizontally, it is almost certain to be out of balance due to residual bow and unless the balance shop is aware of this the corrections applied will be compensating for the residual bow induced by storage. Then, once installed and the machine is started, the residual bow relaxes often resulting in a rotor that is out of balance.
If the rotor prior to storage is refurbished to a near new condition, including a shop balance, why not trust that balancing years later when you remove that rotor from storage? If you chose not to trust the balance work done when the rotor was refurbished, then at least advise the balance shop doing the "check" balance that the rotor was stored horizontally and they need to take appropriate measures to eliminate residual bow.
All this is more practical on variable speed machines where in operation you have the ability to slow roll the newly installed rotor at some slow RPM and work out any residual bow present.
I saw this on many occasions at refineries in Philadelphia. Refurbished rotors would be installed after having a check balance accomplished and on startup high vibration would be present. If the check balance records were available on some occasions it was possible to plot the balancing corrections and vibration vectors and make a correlation.
I guess this is one instance where at least in some cases, I'm saying trust, but don't verify.