Gas Turbine Handbook : Principles and Practices

Accessories 97
accessory gearbox may be located at the front of the gas generator
(some heavy and lightweight industrial machines), under the gas generator
(some aeroderivative units), or on the power turbine accessory
gearbox (some aeroderivative units). In any of the above locations,
these pumps cannot be serviced, removed, or replaced while the gas
turbine is in operation. However, a second (redundant) pump may be
installed, usually electric motor driven ( hydraulic and steam driven
systems have also been used). This second pump also serves as the
pre- and post-lube pump. In cold climates the second pump may be
used to circulate oil in order to maintain heat in the oil and the gas
turbine.
Indirect-drive pump systems rely on a hydraulic pump or an
electrical generator mounted on the accessory gearbox. The hydraulic
pump (or electrical generator) provides the motive force to drive the
lube oil pump through a separately mounted hydraulic motor (or electric
motor). This configuration allows either pump to be the primary
unit. Through judicious control techniques, the secondary pump can
replace the primary pump, in the event of its failure, without disrupting
the operation of the gas turbine. The redundant lube oil pump can
then be serviced with the gas turbine in operation.
Lube oil pumps may be either centrifugal or positive displacement
type pumps. The positive replacement pumps are usually rotary
( gear, lobe, screw or vane) pumps 3 , which are self-priming and can
be used in various suction lift applications. These pumps lend themselves
to being directly mounted on the accessory gearbox, which
invariably will be some distance from the reservoir.
The redundant (alternate, standby) pump, either centrifugal
or positive displacement type may be mounted within the reservoir.
This type of installation eliminates the concern about the pump’s net
positive suction head (NPSH). However, servicing a pump in this type
of installation presents another set of problems. Monitoring pump
operation, temperature, and vibration is further complicated by its
installation in the reservoir, and the area above the reservoir must
be clear to enable removal of the pump.
The arrangement most widely used is to install both pumps at
the same level as the reservoir and as close as possible to the reservoir.
In this arrangement the pumps are identical and both pumps
are electric motor driven. The only difference is that the primary
pump motor is powered by the shaft driven electrical generator.