Originally Posted by candy:
So JohnPA, you are saying it is necessary to do the balance check for spare rotors stored horizontally only? How about if stored vertically?
I agree that the balance check is a good practice to comply with the ABC rule of thump for rotating equipment.
I thought I was very clear that in many instances I am saying that a balance check is not necessary! Since the my reasons for making that statement seem difficult to understand, let me outline this in a scenario; please follow the timeline.
1. I have a steam turbine driven compressor and I swapped out the rotor in 2010.
2. The rotor that was removed was totally refurbished by the OEM or other reputable shop. Part of the refurbishment included re-balancing the rotor to the original specification.
3. After the total refurbishment, I store the rotor horizontally.
4. Fast forward to today and an outage. I want to install that rotor that has been stored for five years.
Is there some compelling reason to not trust the balance effort that was done when the rotor was refurbished and stored in 2010? There might be good reason to do another balancing; one reason might be a coupling that has to be replaced. But ask yourself, if it was balanced in 2010, do we need to do it again, and do it a high potential of residual bow in the rotor? If so, then make the shop doing the balance fully aware that the rotor was stored horizontally for five years and some residual bow may be present. A good balance shop at that point should know what to do.
As for vertical rotors, if the rotor went through a prior refurbishment that included a balance by a reputable shop, then why re-balance?
The key in either of these scenarios is the rotor was balanced as part of the refurbishment process prior to storage. If this was done, is another balance necessary? What does it accomplish?
Also, as I said earlier, this is most applicable to variable speed machines where the opportunity exists to slow roll the newly installed rotor as part of the startup process. Proximity probes can indicate the residual bow (at slow roll) and one can actually observe the bow "relaxing" itself. Obviously you would not necessarily do this on some machine driven by an induction motor that reaches full speed in 10 seconds.