Centrifugal Compressors

Centrifugal compressors can be very reliable, but having said that, they can also have a miserable reliability record. It is tempting to relate some of the horror stories encountered on poorly designed, poorly applied centrifugal compressors. The reader, wanting to learn from this, is directed to the literature, as there have been many volumes written on this subject. At this writing, it can be said that improvements in reliability for this compressor over the past 30 or so years have been significant.

Probably the single largest contributor to problems in the centrifugal compressor is related to rotor dynamics. Long slender rotors are the Achilles heel of this machine. It is in this design that the term robust has a great deal of meaning. Rotors are subject to critical speeds that must be encountered during startup. The longer the rotor and the higher the speed, the more the number of critical speeds that must be dealt with. To this can be added the sensitivity to unbalance. In field operation and as time passes, the compressors do degrade, which usually manifests itself in ever-increasing levels of unbalance. All this boils down to the simple fact, the more sensitive the rotor, the shorter the runtime.

Another area of concern is rotor stability. Two factors enter into the stability considerations: flexibility, just discussed, and damping. Most of the rotor damping is generated by the bearings. The design must consist of bearings with adequate damping capability and rotor motion in the bearings to generate the damping. One of the significant destabilizing forces is aerodynamic cross coupling from the impellers. This is of greatest concern with high density applications. Squeeze film dampers may be added to the bearing to generate more damping but detract because this adds a degree of complexity. Designs are very empirical and are difficult to evaluate: when they are good they are good and when they are bad they are terrible.

Individual designs must cope with the tradeoffs or compromises necessary for good reliability and yet maintain good efficiency. Unfortunately, these forces are somewhat in the opposite direction from each other. One of the first issues is speed; high speed is generally good for efficiency but can cause problems with reliability. High speed raises stresses, which must be limited for reliability. High surface speed may cause bearing and seal problems if allowed to encroach on the limits of experience. One solution is to add more impellers, but that may lengthen the bearing span to a degree where the rotor dynamics may be compromised. Another consideration is the use of multiple cases connected in series and tandem. While this is sometimes the safe option, it does increase the parts count—more bearings, more seals, and more couplings, as well as increasing the cost.

While the previous statements tend to paint a rather bleak picture, the purpose of the material is not to discourage but to help the reader understand the aspects of the reliability decisions. Also as mentioned before, the centrifugal has progressed considerably in the last 30 years. More progress is yet in the works with new analytical and diagnostic tools. This, coupled with improvements in machine health monitoring, make the centrifugal compressor outlook much brighter. It would be safe to say thnt it is the compressor of choice whenever the application allows.

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