Page 59 - Compression Machinery for Oil and Gas

50 SECTION II Types of Equipment

FIG. 3.21 Self-equalizing tilting pad thrust bearing with directed lubrication. (Courtesy of Kings-
bury, Scan DeCamillo.)

Thrust bearings are positioned in a double-acting arrangement as shown in
Fig. 3.22 to control thrust loads in both directions. When considering a compres-
sor’s entire operational map, the thrust direction can reverse. The most severe
condition would typically involve the loss of either a balance piston seal or cen-
ter seal, at which point the bearing must survive until the unit is shutdown.
Bearing surfaces consist of a soft metal bonded to a hard metal backing. The
soft metal surface is most often an ASTM B23 grade 2 babbitt comprised of
89% tin alloyed with other metals. Babbitt provides a good bearing surface that
protects the shaft from damage. It is good for embedding hard contaminant par-
ticles and for resistance to seizure and galling. The main disadvantage of babbitt
bearing materials is their relatively low compressive, tensile, and fatigue
strengths especially at high temperature. The babbitt surface material is cast
and bonded as a thin layer to a hard metal backing. The thin babbitt layer is
typically less than 1mm thick and the hard metal backing is typically steel
or chromium copper. Steel is the most prevalent and least expensive backing
material. Chromium copper is used for its superior thermal conductivity
enabling reduced bearing metal temperature.

Aerothermodynamics
Euler’s Equations
Earlier in this chapter, we talked about the impeller which “impart[s] mechan-
ical energy to the gas” and the diffuser, where “part of the velocity is converted
into static pressure.” In this section, we describe in more detail how this works.
The Bernoulli’s law (which is strictly true only for incompressible flows, but
which can be modified for the subsonic compressible flows we find in gas