Lubricants, Fuels & Lubrication Systems
Synthetic oils and turbine fuels, the recirculatory oil system, and the fuel/oil heat exchanger.
Lubricants, fuels & lubrication, summary notes
- Gas-turbine oils are synthetic (e.g. Type II) chosen for a wide temperature range, thermal stability and load capacity; they lubricate and, importantly, COOL the bearings and gears, then give up that heat in an oil cooler.
- The oil system is usually self-contained recirculatory: a pressure pump feeds the bearings/gearbox, and scavenge pumps (greater total capacity than the pressure pump) return the aerated oil from the sumps to the tank via the cooler and a de-aerator.
- Key components: tank, pressure and scavenge pumps, filters (with bypass and impending-bypass indicators), oil cooler (fuel-cooled or air-cooled), pressure/temperature sensors and magnetic chip detectors for condition monitoring.
- Turbine fuels are kerosene-based (Jet A-1, wide-cut Jet B) with additives for anti-icing (FSII), anti-static, corrosion inhibition and anti-microbial control; key properties are calorific value, volatility, flash point and freezing point.
- Fuel is also used as a heat sink, the fuel/oil heat exchanger cools the oil and warms the fuel, helping prevent fuel-system icing.
- ⚠ Exam trap: scavenge pump capacity EXCEEDS pressure pump capacity so the sumps never flood (the returning oil is aerated and bulkier); and the oil's second job, cooling the bearings, is as important as lubrication in a gas turbine.
Why is the total capacity of the scavenge pumps deliberately greater than that of the pressure pump?
Oil returning from the hot bearing sumps is churned and full of air, so it occupies far more volume than the solid oil pumped in. The scavenge system must have surplus capacity to clear this aerated, bulkier oil quickly and keep the sumps from flooding, which would cause oil loss past the seals and overheating.
Lubrication & fuels concept map
Lubricants, Fuels & Oil Systems
Lubricants, fuels & lubrication quiz
Lubricants, Fuels & Lubrication Systems, quiz
1. The main purposes of a gas turbine oil system are to:
Lubricate and cool the bearings and carry away debrisProvide fuel to the burnersPressurise the cabin2. Besides lubricating, gas-turbine oil is essential for:
Cooling the bearings and gearsIgniting the fuelCompressing the air3. Scavenge pump total capacity is made:
Greater than the pressure pump (oil returns aerated/bulkier)Less than the pressure pumpExactly equal4. The standard turbine fuel is:
Kerosene-based (e.g. Jet A-1)Avgas 100LLDiesel road fuel5. A fuel-system anti-icing additive (FSII) is added to:
Prevent ice forming from dissolved water in the fuelIncrease the octane rating of the fuel so that it resists detonation at high powerColour the fuel so that the grade can be identified visually during the water check6. The fuel/oil heat exchanger:
Cools the oil while warming the fuelHeats the oil and cools the fuelHas no thermal effect7. A typical gas-turbine oil system is:
A self-contained recirculatory systemA total-loss systemSplash lubrication only8. An oil filter impending-bypass indicator warns that:
The filter is becoming cloggedThe oil is freshThe pump has stopped