Section 15.9 / 15.10

Lubricants, Fuels & Lubrication Systems

Synthetic oils and turbine fuels, the recirculatory oil system, and the fuel/oil heat exchanger.

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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.

Solved examples

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.

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