Gas Turbine Handbook : Principles and Practices

98 Gas Turbine Handbook: Principles and Practices
Filters
Because the lube oil system is basically a closed system, the
function of the filters is to remove pump and gas turbine bearing wear
particles from the oil. However, it must be realized that this system
was not always closed, and will not always remain closed. Therefore,
during initial start-up and after major overhauls the “starting” filter
elements should be installed in place of the “running” filter elements.
Where 5 or 10 micron filters are satisfactory for “running” conditions,
1 to 3 micron filters should be installed during oil flushing prior to
start-up (initial or after overhaul).
Redundant oil filters should be installed along with a three-waytransfer
valve. In the event the primary filter clogs, the transfer valve
can easily switch over to the clean filter. During initial installation
and replacement of filter elements care should be taken to remove all
air from the filter housing. Failure to do so can result in temporary
loss of lubrication to one or more bearings.
Wear particles from the pump and gas turbine bearings will accumulate
in the filter element. Also temperature related oil degradation
and oil additives can create a sludge that will accumulate in the
filter. As the filter clogs, the differential pressure across the filter will
increase. Instrumentation normally included is a pressure differential
gauge for local readout, and a differential pressure transducer for remote
readout and alarm. A typical differential pressure alarm setting
is 5 psig. Integrity of the filter element is usually greater than 2 times
the operating pressure of the lube oil system. Therefore, filter failure
due to differential pressure should never be a concern. However,
pressure pulses in the oil, caused by a positive displacement pump,
may weaken the filter element over a period of time. Therefore, filter
elements should be replaced when the gas turbine is overhauled or at
least once a year.
Regulators
Regulators are used to maintain a constant pressure level in
the lube system regardless of which pump is running or even if both
pumps are running. Regulators make it possible for maintenance personnel
to operate the secondary pump on a preventive maintenance
schedule. While regulators are an absolute necessity for positive
displacement pumps, they are recommended and should also be used
with centrifugal pumps.