Machinery Lubrication India Jan Feb 14 10 | Page 15

thickness increases the susceptibility of
additional damage or producing a rub
during coastdown or startup.
When the turbine rotor is at normal
operating conditions, the bearing is in
hydrodynamic lubrication conditions.
However, when a turbine rotor is on
turning gear or beginning to roll at
slower speeds, the journal bearing is in
elastohydrodynamic
lubrication
conditions. These conditions are
generally indicated by some metallic
friction and wear. Very high coefficients
of friction may also be reached. At this
point, the oil film thickness between the
bearing and the turbine rotor will be at
its minimum. The film thickness will
increase as either the speed is increased,
the lubricant viscosity is increased, the
load is decreased or the geometric

shear forces on the Babbitt than would
occur with smooth, concentric and
round journals on a smoothly bored
Babbitt surface bearing. This is why
surfaces that appear to have minimal
damage can still cause bearing failure
during startup or slow-speed operation
before a fully developed hydrodynamic
film with a normal film thickness exists.

lubricity and induce corrosion. When a
lubrication or control system is shut
down, the normally wetted regions,
such as those in the lube oil drain pipes,
become exposed to air. Enlarging the
surface area of carbon steel that is
subject to ambient conditions increases
the amount of internal corrosion (rust)
a system develops.

During a plant startup, when a turbine
is on turning gear or at lower speeds,
the lubrication system may supply lube
oil with slugs of particulate matter to
the journal and thrust bearings. This is
especially damaging to journal bearings
because they carry the applied load
forces that force the journals into
contact with the bearing surface, thus
providing a very thin oil film. This form
of
lubrication
is
known
as

While a unit is operating, the amount
of water in the lubricating or control
system is normally maintained within
the manufacturer’s specification by the
plant’s oil-conditioning system. These
systems often contain some type of
demulsifying agent to remove water.
Even if the water content is well above
normally