Page 39 - Improving Machinery Reliability

Requiremerits Specification 11

For magnetic bearings, scores of turbocompressors ranging in size to 15,000 hp
and operating speeds around 10,000 RPM had either been manufactured or retrofit-
ted with such bearings by 1997.
Magnetic bearings have several advantages and disadvantages.* Two primary
advantages of magnetic bearings are the very low power consumption and very long
life. Because there is no contact between the rotor and stator, there is no wear.
Where fluid film bearings have high friction losses due to the oil shearing effects,
magnetic-bearing losses are due to some low-level air drag, eddy currents, and hys-
teresis. Also, the losses associated with oil pumps, filters, and piping are much
greater than the power associated with controls and power amplifiers. Overall, mag-
netic bearings normally have a lower power consumption than oil film bearings.
Magnetic bearings commonly have lower power consumption than rolling element
bearings. Also, rolling element bearings have finite life and DN (diameter times
RPM) limits. Because of the noncontact nature of magnetic bearings, they have
much longer expected life and higher DN limitations.
Other advantages of magnetic bearings are related to reduced dependence on envi-
ronmental conditions. Magnetic bearings do not require oil lubrication so they are
well suited to applications such as canned pumps, turbomolecular vacuum pumps,
turboexpanders, and centrifuges where oil cannot be employed. They can operate at
much higher temperatures or at much lower temperatures than oil-lubricated bear-
ings. A study of aircraft gas turbine engines indicates that the elimination of the oil
supply and associated components with magnetic bearings could reduce the engine
weight by approximately one fourth.
Among the disadvantages we find higher cost, larger size, and somewhat lower load
capacity than in conventional bearings. Nevertheless, magnetic bearings have long left
the prototype stage and may be real contenders for some equipment applications.
Getting back to our examination of Figure 1-5, we note the term "gas velocity."
Gas velocities are relevant for future compressor uprates. Nozzle sizes must be cho-
sen with future uprates in mind. This topic is further discussed later in this chapter.
The next circled item, on data sheet page 5 (Figure 1-6), deals with coupling
selection. Two broad categories of couplings are available to the user: non-lubricated
metallic disc and lubricated-gear-type couplings. Metallic disc or diaphragm COLI-
plings are engineered for maintenance-free infinite life, but proper alignment is criti-
cal; should the couplings ever fail, they may do so with little advance warning. Gear-
type couplings are sensitive to lubrication deficiencies and can experience
accelerated wear if operated with certain amounts of misalignment. Gear couplings
require more maintenance than metallic disc-type couplings. On the other hand, they
do give adequate warning of distress. So, which type should be specified? Should the
coupling incorporate torque sensing and on-stream alignment monitoring devices?
Should a promising new coupling type be specified, or would it be more prudent to

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"Couitesy of S2M America, Roanoke, Virginia