Machinery Lubrication India Jan Feb 14 10 | Page 14

MLI

TURBINE LUBRICATION
CRAIG JENNINGS, TRI TRANSMISSION AND BEARING CORP.

Quality
Changes During Startup
How Oil

Equipment manufacturers generally
provide guidelines on how oil should be
maintained for reliable operation.
Because components have little
tolerance for contamination or
oxidation, frequent oil sampling and
monitoring are performed while the
equipment is online in order to keep the
unit operating without any issues.
However, many times little attention is
paid to the oil quality when the
equipment is shut down for a

The congealed mass shown above is an example of particulate matter and degraded
oil buildup that can accumulate in a turbine control servo. Left unattended, this
condition could eventually cause poor turbine control response and accelerate
equipment wear.

maintenance outage or on standby.
This can eventually affect the unit’s
reliability.
For instance, when a steam turbine is
down for prolonged periods or for a
short three-week maintenance outage,
the consequences of not having hot,
circulating oil running through the
system is usually not considered. In
fact, the oil quality when placing a
turbine on turning gear or during
startup is often not the same as when
the unit came offline.

Prolonged exposure to water can rust a
control system.

Typical turbine bearings are designed
for fluid-film lubrication. Most fluidfilm bearings are intended for
hydrodynamic lubrication. This means
an oil wedge is formed in a hydrodynamic
bearing to develop separation and

12| January-February 2014 | www.machinerylubricationindia.com

maintain an oil film between the rotor
and bearing. The film thickness is a
function of rotor speed, load and oil
viscosity. Under fluid-film conditions,
an increase in viscosity or speed
increases the oil film thickness, while an
increase in load or decrease in rotor
speed reduces the oil film thickness.
In developing an oil film, the bearing
surface geometry and rotor surface are
just as important as rotor speed, load
and oil viscosity. To establish a stable
oil wedge and fluid film between the
surfaces, the rotor and bearing profiles
must be perpendicular. If either or both
surfaces become damaged by large
particles being pushed through the
bearing, the ability to form an oil wedge
is diminished, resulting in a thinner oil
film thickness. Reducing the oil film