Power Plant Engineering

284 POWER PLANT ENGINEERING

3. Filter and strainer,


4. Relief valve,


5. Oil cooler,


6. Oil and pipe line,


7. Magnetic drain plug,


8. By-pass, valve, and


9. Warning devices.

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 pumpbulb

Oil pressure

scavengingswitch

Oil pressure

gauge port

Low oil pressure Air out

Oil

cooler

Gearcase

Fa n

Dual scavenge pump

Oil return toTa n k

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