Page 154 - Compression Machinery for Oil and Gas

142 SECTION II Types of Equipment

Bearings
Like with seals, IGCs require many more bearings than inline compressors. The
journal bearings in IGCs operate at similar loads and speed to those in inline
compressors, although the main contribution to the static load in IGCs is due
to power loads transmitted through the gear as opposed to mainly gravity loads
in an inline compressor. Since the bearing loads in IGCs vary with aerodynamic
power, the rotordynamic analysis must also consider off-design operating loads
and bearing characteristics to ensure acceptable stability at all potential running
conditions.

Service
Servicing an IGC machine can be more involved due to the greater number of
seals and bearings. In small IGCs, servicing the bearings and seals requires
removing the top cover of the gearbox. However, in large IGCs, it may be pos-
sible to inspect and replace seals and bearings without removing the top of the
gearbox or stage casings. Access to an IGC is typically more challenging than a
typical inline compressor since there is interstage piping to disassemble for
each stage.

IGC Design Topics

Design Process
As discussed by Wygant et al. [3], an engineered IGC essentially follows the
same development process regardless of manufacturer or application. The first
portion of that process optimizes the thermodynamic cycle and the aerodynamic
performance. Next, the mechanical implications are considered and are iterated
against the aerodynamic components for the optimal configuration. Fig. 4.7
shows the interconnectivity of various constraints that must be iterated upon
to achieve a satisfactory design. The importance of the aerodynamic design can-
not be overstated, as shown in the “satellite configuration” on all the aspects that
are influenced by aerodynamics. Likewise, errors or poor assumptions in other
aspects can result in problems that prevent operation of the unit altogether.
Some issues that prevent operation are bearing over-temperatures, excessive
rotor vibration, impeller fractures, excessive seal leakage, etc.
The sizing and selection of a compressor for a particular application depends
on matching aerodynamic performance to cycle requirements and integrating
mechanical limitations into the aerodynamic design. Wygant et al. [3] provides
a general description of the sizing and selection approach, which is as follows:

l Step 1: Determine the number of stages and intercoolers. The number of
stages is determined based upon the overall head or PR. Higher head
requirements can lead to increased torque requirements per pinion and result
in unacceptable bearing and gear loads.