Power Plant Engineering

9.6.3.2. Working Principle of a Lubrication System
Fig. 9.18. shows the lubrication system used in a gas turbine plant. Sufficient oil is stored in an
external tank to replace that, which may be lost during the normal duration of run. Oil is drawn from the
tank by a gear or rotor type lubrication pump. A relief valve is fitted to return the excess oil to the pump
inlet than that required for lubrication. The oil flows through a paper or metal screen filter of 10 to 40
micron filtration capacity. In the case of clogged filter, the oil pressure drop across the filter increases
which causes a by-pass valve to open and unfiltered oil to reach the bearings. This is undesirable. For
such malfunctions, various; warnings, safety and indicating devices such as oil temperature indicator
bulb, oil over temperature switch, low oil pressure switch and so on are installed in the filtered oil line.
Drain plug
Oil
Pressure
regulated
relief valve
Filter by pump
valve
Oil pressure
gauge port
Oil pump
bulb
Oil pressure
scavenging
switch
Oil pressure
gauge port
Low oil pressure
Air out
Oil
cooler
Gear
case
Fan
Dual scavenge pump
Oil return to
Tank
Filter
Oil pump 
Filter
Oil return
Oil vent return
Oil vent
to exhaust
tail pipe
Oil tank
B
C
B
Air
Fig. 9.18. Lubrication System for Gas Turbine.
The filtered oil flows to the bearings and gear case of turbine and after lubricating and cooling
the oil washed components, it is returned to the oil tank through a cooler by a scavenge pump. Scavenge
pumps have a greater displacement than the quantity of oil flowing into the cavity from which they are
pumping. This introduces a negative pressure, which draws air into the sump through any leakage paths,
precluding leakage of oil. The air-oil mixture returned through the cooler to the tank must be separated
and for this it is recirculated in a separator and the air is permitted to escape. The separator vent usually

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GAS TURBINE POWER PLANT
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connects to the tail pipe so that the hot gases flowing out of exhaust will burn off any remaining oil
vapor. This vent must be checked for obstruction regularly; otherwise the air may build high pressure in
the tank and result in its failure. The separator is nothing but a simple baffle which causes the air-oil
mixture to follow a devious path, the oil will tire to turning the corners and will find its way out through
the vent into the tank, and the air will find its way out through the vent into the exhaust system. A drain
plug of magnetic type is incorporated in the oil line before oil pump, which can collect any metal
particles circulating in the oil system. A through study is made of debris collected by the plug to decide
whether it is metal chips or else.