Gas Turbine Handbook : Principles and Practices

94 Gas Turbine Handbook: Principles and Practices
Figure 6-5. Typical ignitor plug types.
LUBRICATION SYSTEMS
General
Lubrication systems in gas turbines must address two requirements:
one is to provide lubrication between the rotating and stationary
bearing surfaces, and the other is to remove heat (cooling effect)
from those surfaces. Lube oil system designs must consider bearing
types, rpm, load, temperature, and oil viscosity. Bearing types fall
into two main categories: hydrodynamic bearings and anti-friction
bearings. Lubrication in a hydrodynamic bearing converts sliding
friction into fluid friction. While fluid friction is considerably lower,
it still consumes power and produces heat. Therefore, besides providing
the fluid wedge that separates the surfaces, lubrication oil also
removes the heat from that area.
Anti-friction bearings work on the principal of rolling friction.
The shaft load is directly supported by the rolling elements and races
in metal-to-metal contact. Furthermore, the high unit pressures between
the rolling element and the inner and outer races prohibit the
formation of an unbroken oil film. Therefore, the role of the lubrication
oil is slightly different. Lubrication oil is still required to remove
heat from the bearing area, and it reduces the sliding friction between
the rolling elements and the cage that maintains the position of the
rolling elements. The engine service and the type of bearing selected
by the engine designer, dictates the type of lubrication oil used in the
system.
Most heavy frame gas turbines, which incorporate hydrodynamic
bearings, use mineral oil while the aeroderivative gas turbines, which