Gas Turbine Handbook : Principles and Practices

42 Gas Turbine Handbook: Principles and Practices
The aero-derivative gas turbine stators or diaphragms are
constructed in one, two, or three vane segments and not the large
diaphragm segments used on most heavy industrial units. This facilitates
part replacement and keeps cost down. The skill levels for
on-site maintenance of the heavy industrials is not different from the
skill level required to remove and replace the aero-derivative engine
or to remove and replace modules from the aero-derivative engine.
Replacing components within an aero-derivative module requires a
“shop-level” skill. However, the modules can easily be transported to
a maintenance base where the skill level is available. This same skill
level is required at maintenance bases where detailed repair and replacement
of heavy industrial gas turbines is presently being carried
out.
Inspection of the compressor on the heavy industrial units requires
removal of the upper cover and the compressor diaphragms, a
72 hour (720 man-hours) operation. If more than two or three rows of
blades need to be replaced, the rotor must be removed, balanced, and
then replaced. With some heavy industrial units, in order to remove
the compressor cover, the combustor and turbine covers must also be
removed. This makes the total operation a time consuming 125 hours
(1,200 man-hours). Obviously, heavy lift equipment is also required!
In contrast, inspection and replacement of the aero-derivative
compressor will take from 6 to 13 hours (13 to 30 man-hours).
An alternative possible only with the aero-derivative gas turbine is
complete removal and replacement of the gas generator. This can be
accomplished in 6 hours (15 man-hours).
OPERATION
Starting and accelerating the aero-derivative unit is fast, and
loading time is limited only by the driven equipment. Shutdown is
also fast, about 3-5 minutes. Cool-down time approaches 2-3 hours,
but can be shortened by “motoring” the gas generator after shutdown.
Starting and accelerating the heavy industrial unit is slow (15
to 20 minutes for starting and accelerating to a minimum load). Also,
load-time is slow (about 20°F per minute) in order to keep thermal
stresses to a minimum. Shutdown time approaches 30 minutes. The