Page 57 - Compression Machinery for Oil and Gas

48 SECTION II Types of Equipment

the rotating journal from the stationary bearing. They have exceptionally long
life thereby enabling long periods of continuous operation, often in excess of
5years. In addition, these bearings possess dynamic characteristics that allow
for vibration control thereby enabling high-speed operation and traverse of rotor
critical speeds. They offer very good load capacity in a reasonably compact
design envelope, and they accommodate transient loads very well. This leads
to a robust rotordynamic characteristics. The primary disadvantages are rela-
tively high-power loss compared to other bearings and need for a reliable oil
supply system.
Hydrodynamic journal bearings can be either tilting pad or fixed geometry
design. Almost all centrifugal compressors are equipped with tilting pad journal
bearings. Fixed geometry designs (plain, elliptical, lobed, pressure dam, and
others) are seldom encountered in compressors constructed after about 1970.
Fig. 3.18 shows a tilting pad journal bearing with self-aligning pivots and chro-
mium copper pads for temperature reduction. Tilting pad journal bearing
designs consist of several pads arranged in an annular ring around the shaft with
the pads free to tilt about their respective pivots. Tilting pad journal bearings
may include several design variations such as self-aligning features to compen-
sate for misalignment, specialized materials, and special oil feed and drain con-
figurations for reducing temperature and power loss. Fig. 3.19 shows a tilting
pad journal bearing with directed lube features with oil introduced close to the
pad inlet. One particular advantage of tilting pad journal bearings is their
dynamic characteristics and inherent resistance to rotordynamic instability,
which enables passive control of vibration while traversing the rotor’s critical
speed and further enables reliable stable operation at speeds well above the first
critical speed.
Hydrodynamic thrust bearings are usually of the self-equalizing tilting pad
design. Fixed geometry designs (plain, tapered land, and compound tapered

FIG. 3.18 Tilting pad journal bearing with self-aligning spherical seat. (Courtesy of Elliott
Group.)