Page 73 - Offshore Electrical Engineering Manual

60 CHAPTER 4 Prime Mover Selection Criteria

LUBE OIL SYSTEMS

On large gas turbine generator sets, lubrication is accomplished by a forced
feed lube oil system, complete with tank, pumps, coolers, filters and valves.
Lubricating oil is circulated to the main bearings, flexible couplings and gear-
boxes. A portion of the oil may be diverted to function as hydraulic oil for opera-
tion of guide vanes, etc. within the turbine. A typical arrangement is for the lube
oil pumps to take their suction from the lube oil tank and the hydraulic con-
trol valves to take their suction from a bearing header. The system may contain
between 500 and 2000 gallons of oil.
The electrical engineer’s interest in this system is that out of, for example, six
lube oil pumps on each generator set, five of them are driven by electrical motor. The
following is a description of the typical function of such pumps.

1. Main pump
The main lube oil pump is a shaft-driven positive displacement unit mounted
into the inboard wall of the lower casing of the accessory gear. It is driven by a
splined quill shaft from the lower drive gear and the pressure is 65 psig max. As
the output pressure of this pump is engine speed dependent, with certain models
of turbine, insufficient lubrication pressure is available from this pump below a
certain speed, and in this case, even when it is operating satisfactorily, this pump
may require to be supplemented by an electrically driven pump.
2. Auxiliary pump
The auxiliary pump, mounted on the oil tank cover, is a submerged centrifugal
type pump which provides lube pressure during startup and shutdown of the
generator under normal conditions. The auxiliary pump is driven by a low-
voltage 30 hp two-pole alternating current (AC) flameproof motor.
3. Emergency pump
The emergency pump is also mounted on the tank cover and is a submerged
centrifugal type which also provides lube pressure under startup or shutdown
conditions. This pump is driven by a 125 V 5 hp direct current (DC) flameproof
motor.
The three remaining pumps of our typical system are listed in the
following:
4. Main hydraulic supply pump and auxiliary hydraulic pump
Failure of these pumps and the resulting low hydraulic pressure would not
necessarily cause an immediate generator set failure, although the unit would
eventually trip because of low lube pressure.
5. Hydraulic ratchet pump
Loss of hydraulic ratcheting pressure or equipment would cause generator shaft
bowing and also lead to excessive bearing stress. It may, however, be possible
to repair the fault by, for example, replacing the motor within a few hours, i.e.,
before serious damage has been caused.

These three pumps would all be normally driven by low-voltage AC motors.