Gas Turbine Handbook : Principles and Practices

96 Gas Turbine Handbook: Principles and Practices
any accessories. To protect the gas turbine, in the event of a lubrication
oil leak, the reservoir is sized such that the holding capacity, or
volume, between the “low alarm level” and “pump suction” provides
enough time for the unit to be shutdown before the oil supply is depleted.
In cold climates a heater, built into the reservoir, warms the
oil prior to start-up. Care must be taken in sizing the heater. An oversized
heater, while it will heat the oil faster, may result in carborizing
the oil around the heater element. This will lead to deterioration of
the oil and failure of the heater. A heater that is undersized will be
ineffective at low ambient temperatures. Normally natural convection
is sufficient to circulate the oil as it is heated. When it is necessary
to heat the oil quickly, a circulating pump should be installed within
the reservoir to keep the oil in motion.
During operation the reservoir also serves as a deaerator. As the
lubrication oil circulates through the bearings it may entrap air in
the oil. This is commonly referred to as oil “foaming.” The foam must
be removed before the oil is returned to the pump or the air bubbles
will result in pump cavitation. To deaerate the oil the reservoir surface
area is made as large as possible and screens, baffles, or both
are built into the reservoir. Anti-foaming agents can also be added to
the oil. Oil reservoirs usually operate at a positive pressure of 1 to
2 inches water gauge above atmospheric pressure. Depending on the
location and the application, the oil reservoir is either vented locally
or is vented to a safe area. In either case as the vented oil mist cools
it condenses and leaves an oil film. If left to accumulate, this oil film
can become a hazard. Care should be taken to collect this oil and
route it to a safe waste dump. The inside of reservoirs should never
be painted. Where corrosion is a concern the reservoir should be of
stainless steel construction.
Pump
In a pressure circulating system, oil pressure and flow is provided
by the oil pump. A pump failure will result in severe damage
to the bearings in the gas turbine, and possibly secondary damage to
the compressor and turbine blades. Over the years two approaches
have evolved to minimize the effects of a pump failure: one is to utilize
a direct-drive or indirect-drive shaft driven pump, the other is to
install a backup (redundant) pump. Direct-drive pumps are mounted
on the accessory gearbox. Depending on the gas turbine type the