Page 78 - Compression Machinery for Oil and Gas

68 SECTION II Types of Equipment

FIG. 3.34 API response showing separation margins and amplification factor calculation for cen-
trifugal compressors [8].

In regards to the bearings, the plain journal bearings are the least stable
design. Elliptical, axial groove, and multilobe journal bearings introduce “pre-
load” that improves the stability. Tilting-pad journal bearings possess essen-
tially no cross-coupling and are the most stable bearing design, often used
for high-speed centrifugal compressors. Titling-pad journal bearings are neces-
sary when operating at speeds well above the first critical speed. In addition,
tilting-pad bearings have several parameters, including clearance, preload,
pad offset, pad orientation, etc. that can be adjusted to achieve the desired
stiffness and damping.
The various annular seals within the compressor are a source of potential
destabilization. Much like the bearings, seal forces are modeled as a system