Balancing of centrifugal compressor rotor

[OrangeNL]

I have two questions about the balancing of two centrifugal compressor rotors (Low Pressure stage and Medium/High Pressure stage) of a centrifugal compressor.

The above mentioned compressor will have a major maintenance overhaul this year and both rotors of the compressor will be balanced. They can be balanced low speed or high speed for substantial extra costs. I know this depends on whether the shafts can be considered rigid or flexible but how can one determine? I have also readt that flexible shafts can in some cases be balanced at low speed with good results.

The rotors have the following specification:

The LP rotor consists of 5 compressor stages, turns at 9400 RPM and weight is 360 kg (794 pound). First critical speed is 5460 RPM, 2nd is 18400 RPM.

MP/HP rotor consists of 6 compressor stages (3MP / 3HP), turns at 16600 RPM and weigh is 150 kg (331 pound). First critical speed is 6710 RPM, 2nd is 20960 RPM.

My question is, should these rotors be balanced high speed or could low speed also be sufficient (ISO 1940/1 class=G2.5)? And how do I know whether the rotors should be considered flexible or rigid?

Tim

 

[JJPellin]

Both rotors are flexible. This is necessary given the fact that they run above their first critical speed. If they were stiff rotors, the first critical would be above normal running speed. An at-speed balance can greatly reduce the critical response when the machines are started up and shut down. It can also improve performance if you happen upon a rotor stability problem. Since the LP rotor is running at about 50% of second critical, this could be a concern. We now have a policy where we do an at-speed (high speed) balance for all rotors in critical machines that run above first critical. It is very expensive and delays the return of our rotors given the schedule problems getting into the vacuum bunkers for the at-speed balance. But, our vibration levels are much better since we implemented this policy. Our failure rates for seals are much lower. We are getting longer runs between turnarounds. In our case, I feel it is worth the extra cost.

Johnny Pellin