Page 56 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 47

temperatures below 115°C( 175°F) require the use of high nickel steels for
their low-temperature toughness. Highly corrosive environments require the
use of stainless steels.
Another design method uses a stacked rotor assembly, where the impeller
disks form part of the rotor that is held together to shaft ends by a tie bolt.
Shaft journals and shaft probe areas have special requirements with respect
to their manufacture in order to limit mechanical and electrical runout. The sur-
face finish at the sensing areas observed by radial vibration probes is held within
1.0μm (32μin.) Ra by honing or burnishing. The probe sensing area is then
demagnetized to required levels.

Bearings
Proper rotor position is maintained by journal and thrust bearings. Two journal
bearings are used for radial positioning of the compressor rotor supporting the
gravitational load of the shaft as well as various dynamic forces caused by rotor
unbalance, misalignment, and other sources. Thrust bearings are used for axial
positioning of the compressor rotor supporting thrust loads that arise from gas
forces within the compressor case. The arrangement of the bearings is as shown
in Fig. 3.17 such that the bearings are external to the gas flow path outboard of
the DGSs. This keeps lubricants out of the process gas and process gas out of the
lubricants. This arrangement also allows for relatively easy access for mainte-
nance such as bearing and DGS replacement. Bearings used in centrifugal com-
pressors can be hydrodynamic, magnetic, or rolling element. Hydrodynamic
bearings are the most prevalent, whereas magnetic bearings are popular in niche
applications such as pipeline compressors and hermetically sealed subsea
compressors.
Hydrodynamic bearings are highly advantageous because they suffer little
or no wear due to the formation of a hydrodynamic wedge of oil that separates

FIG. 3.17 Bearing arrangement with bottom halves installed. (Courtesy of Elliott Group.)