Quantcast Gas turbine service practices

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TM 5-685/NAVFAC MO-912
turbine which is mounted on a common shaft with
nance program that is built around records and
observation. The program is described in the manu-
the compressor. The compressor draws in the air for
combustion and also drives the gearbox gear train.
facturer's literature furnished with each engine. It
About two-thirds of the power derived from combus-
includes appropriate analysis of these records.
b. Record keeping. Engine log sheets are an im-
tion is required to sustain combustion. The remain-
ing power is available for work purposes and drives
portant part of record keeping. The sheets must be
the output shaft.
developed to suit individual applications (i.e., auxil-
b. Speed signal. An engine speed signal, gener-
iary use) and related instrumentation.
ated by magnetic pickups (speed transducers) in the
c. Log sheet data. Log sheets should include en-
gine starts and stops, fuel and lubrication oil con-
gearbox, provides electrical signals that are propor-
tional to engine speed. The signal causes a dc volt-
sumption, and a record of the following:
age to be generated.
(1) Hours s ince last oil change.
c. Thermocouples. Thermocouples sense the tur-
(2) Hours since first put in service or last over-
bine discharge/inlet total temperature. The electri-
cal temperature sensing signal is an average of the
(3) Total ho urs on engine.
operating temperature profile.
d. Oil analysis program. Use of a Spectrometric
d. Pressure sensing. Sensing of compressor dis-
Oil Analysis Program is recommended to determine
charge static pressure and turbine discharge pres-
the internal condition of the engine's oil-wetted
sure is also required for engine speed control. These
(wear metal) components, such as bearings, gears,
pressures are combined to produce an electrical sig-
and lubrication pump.
nal equal to pressure ratio.
(1) The program should be used as a supple-
e. Computer. The three signals (speed, tempera-
ment to the regular maintenance procedure of chip
ture, and pressure ratio) are summed in an
detection and filter inspection. Normal wear causes
acceleration/deceleration computer. Computer out-
microscopic metal particles (smaller than one mi-
put functions with a governor to meter fuel required
cron) to mix with the lubricating oil and remain in
for engine operation. If required, a signal derived
suspension. Samples of oil taken from the engine
from a tachometer can be used to determine a rate-
after a shutdown will contain varying amounts of
wear-metal particles.
of-change feedback signal.
(2) Oil samples should be removed from the
3-21. Compressor.
engine at the time intervals specified by the engine
The function of the compressor is to raise the pres-
manufacturer. A sample should always be taken
sure and reduce the volume of the air as it pumps
from the same location on the engine (this may vary
it through the engine. An axial flow or centrifugal
from each engine). Refer to manufacturer's litera-
ture. See appendix C paragraph C-le(2).
flow compressor is used. Most engines use a multi-
(a) Metal content. Evaluation of the oil's
stage, axial flow compressor such as described
herein. The axial flow consists of two major sub-
wear-metal content is very important. The quantity
assemblies: the rotor assembly and the stator as-
of wear-metal in the sample as well as type (iron or
steel, silver, chromium, nickel, etc.) must be evalu-
sembly. Axial flow compressor efficiency is better
than centrifugal flow compressor efficiency. Cen-
ated and recorded.
trifugal flow compressors were first used in early
(b) failure forecast. Evaluation records are
design gas turbine engines. The main component is
intended as an aid in forecasting what components
an impeller which is mounted on a common shaft
are in danger of failing. Contamination of the oil
with the turbine. These compressors are generally
sample must be prevented to avoid false indication
used with smaller engines and have a fairly low
of engine internal conditions.
pressure ratio. The design has lower efficiency than
e. Industrial practices. Use recognized industrial
the axial-flow design but is less expensive to manu-
practices as the general guide for engine servicing.
Service information is provided in manufacturer's
literature and appendixes B through G.
3-22. Gas turbine service practices.
f. Reference Literature. The engine user should re-
fer to manufacturer's literature for specific informa-
a. Maintenance program. Service practices for
tion on individual units.
gas turbine engines consist of a complete mainte-


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