Handbook of Turbomachinery Second Edition Revised - Ventech!

value as the operating value of uv/p decreases. Theoretically, this decreasing
trend of f with decreasing uv/p continues downward to a value of f ¼ 0at
uv/p ¼ 0. However, a transition value of uv/p exists below which the actual
value of f increases with decreasing uv/p. This transition signifies the onset of
metal-to-metal contact. As the value of uv/p continues to decrease, the value
of f increases very rapidly due to more and more of the load being supported
by metal-to-metal contact. This trend of increasing actual value of f with
decreasing value of uv/p continues until yet another transition value of uv/p
occurs, below which the actual coefficient of friction remains at a high value.
This second transition point signifies total loss of oil-film load-carrying
capacity, and the entire load is supported by metal-to-metal contact.
Dirt, deformations, misalignments, surface roughness, and reduced oil
flow rate all have deleterious effects on the resulting coefficient of friction.
When such considerations exist, the onset of metal-to-metal contact occurs
at higher values of uv/p. Wearing-in as opposed to wearing-out of the
bearing has a beneficial effect since hydrodynamic full film operation is
maintained to lower values of uv/p. In the full film hydrodynamic regime,
increasing clearance has a beneficial effect since bearing oil flow rates
increase, which lowers oil-film temperature and increase lubricant viscosity.
At full-speed operation, the bearing operates in a ‘‘full film friction
and lubrication’’ regime with relatively low coefficient of friction. When
running at the much lower turning gear speed, the bearing operates in a
‘‘complete boundary friction and lubrication’’ regime with very high
coefficient of friction. Using these coefficient of friction characteristics, a
schematic is depicted in Fig. 54. The solid-line curve is quite similar to field
observations in a bearing that has been scored or wiped. Solid particle
contaminants in the lubricant and/or deformations and misalignments of
the bearing members would also result in similar contact-induced transient
temperature rise of the bearing during coastdown.
Steam Turbine Lubrication Supply Systems
The lubrication system of a steam turbine generator unit performs the vital
function of supplying lubricating oil to the thrust and journal-type bearings
at all times under all operating conditions. Without proper design,
installation, operation, and maintenance of the lubrication system, reliable
performance of the turbine generator unit cannot be achieved. Failure of the
lubrication system to perform its function can cause significant damage to
the turbine generator. While lubrication system failures are not common
occurrences, they can result in extended unit outrages to repair or replace
damage turbine generator components such as blades, casings, rotor
windings, and entire shafts [111–113].
Copyright © 2003 Marcel Dekker, Inc.