Page 166 - Compression Machinery for Oil and Gas

154 SECTION II Types of Equipment

compressors, bearing lubrication is designed for this higher load. Usually, bear-
ings with directed lubrication are used in contrast to bearings with flooded lubri-
cation. This means that special consideration is taken with regard to high oil
throughput and minimal oil retention time in the bearing. The gears and thrust
collars are lubricated and cooled with spray oil. Spray bars with oil nozzles are
located at the different gear mesh locations. It has to be ensured that the oil noz-
zles are chosen in way as to provide enough oil to the mesh without spraying so
hard that erosion can occur.

Seals
Theoretically, any type of shaft seal can be used in IGCs. Labyrinths seals, float-
ing carbon rings, and dry gas seals are most common, depending on the pressure
and the process gas, and brush seals can be used for high-temperature applica-
tions. Labyrinth seals are used for many low-pressure air compressors, espe-
cially when robustness is more important than low leakage. They are
normally made of metal, mainly aluminum, but thermoplastics can be used
for higher corrosion resistance or better rubbing behavior. With special laby-
rinth teeth design, smaller clearances can be achieved for improved leakage per-
formance. Carbon ring seals are a good compromise between low leakage and
simplicity for nonhazardous gases like air, nitrogen, or carbon dioxide. They
allow smaller clearances than labyrinths because the floating carbon rings
can follow the shaft motion when traversing critical speeds. Due to the longer
axial sealing gap, fewer carbon rings are needed than labyrinth teeth for the
same pressure difference.
The lowest leakage can be achieved with dry gas seals, which are the most
popular seals for explosive and toxic gases. They require the most complex seal
gas system to maintain operation free of particles, liquids, and back pressure.
The complexity of the seal gas systems increases with higher numbers of com-
pressor stages and can turn unfavorable compared to single-shaft compressors,
which only have two dry gas seals per casing regardless of the number of stages.
Dry gas seals also add mass to the pinion outboard of the bearings, which lowers
the natural frequencies and must be considered in the rotordynamics analysis.
Besides the shaft seals, internal sealing between discharge and suction side
of the impeller is needed in every compressor stage. For closed impellers, nor-
mally labyrinths are used as cover disc or eye seals, while brush seals are less
common. Open impellers have no specific seal, but a stator contour matching
the blade geometry maintains a small gap, which is limited by the amount of
radial and axial movement of the rotor relative to the stator.

Rotordynamics

The rotordynamic assessment of IG turbomachinery includes the same con-
siderations as inline machines: undamped critical speeds, damped eigenvalue