Gas Turbines - Lubrication — Page 305, Lesson 391

Let's start with the filters, because that's where we left off in the lubrication system. Both the pressure filter and the scavenge filter are built in a tubular form. They're made from either a very finely woven wire cloth, or from resin that's been impregnated with fibres. So you've got two construction options there: fine wire mesh, or a resin-fibre composite. Now, some of these filters have a differential pressure switch fitted across them. That switch measures the pressure difference between the upstream and downstream sides of the filter. If the filter starts to clog, the pressure drop across it increases, and the switch senses that. Alternatively, some filters are fitted with a 'pop up indicator' — that's a small button you can see protruding from the filter casing. When the filter is partially blocked, that button pops up to give you a visual warning. So you have two ways of detecting a partially blocked filter: an electrical differential pressure switch, or a mechanical pop-up button. Now let's move on to the types of lubricating oils. This is where the requirements get interesting. Gas turbine engine oils have to satisfy two opposing demands. They need a high enough viscosity for good load carrying ability — that means the oil film has to be thick and strong enough to keep metal surfaces apart under load. But they also need a sufficiently low viscosity to ensure good flow at low temperatures — for example, when you're starting the engine after prolonged cold soaking. Cold soaking means the aircraft has been sitting in the cold for a long time, so everything is at ambient temperature. The oil has to flow easily enough to get around the system at start-up. Early gas turbine engines used the same oils that had been used in petrol engines for years. Those oils were mineral based. But here's the problem: at the higher temperatures and speeds at which gas turbine engines operate, mineral oils burnt, scummed, and oxidized. They simply couldn't handle the conditions. So synthetic oils had to be developed to achieve the properties I mentioned. Let me walk you through the qualities these synthetic oils have, because each one addresses a specific operational need. First, low volatility. That's to prevent evaporation at high altitudes. At altitude, the air pressure is low, and oil can evaporate more readily. Low volatility keeps the oil where it belongs. Second, high flash point. The flash point is the temperature at which the oil vapours will ignite if near a flame. You want that temperature to be high, so the oil doesn't catch fire easily. Third, high film strength. This is the ability of the oil molecules to stick together under compression loads, and to adhere to surfaces under centrifugal loads. So under compression, the molecules hold together to form a strong film; under centrifugal loads — like in a spinning bearing — they stick to the surface instead of being thrown off. Fourth, a wide temperature range. Most gas turbine lubricating oils have a temperature range of -45°C to +115°C. That's the operating envelope they're designed for. Fifth, low viscosity. This increases the ability of the oil to flow under low temperature conditions. Remember the cold soaking start — low viscosity helps there. Sixth, a high viscosity index. The viscosity index is an indication of how well the oil retains its viscosity when heated to its operating temperature. So a high viscosity index means the oil doesn't thin out too much as it gets hot — it stays effective. Now, here's an important point about why low viscosity oil is acceptable in a gas turbine. The use of a low viscosity oil is enabled because of the absence of reciprocating parts and heavy duty gearing. In a piston engine, you have reciprocating parts — pistons going up and down — and heavy gearing, both of which need a thicker oil. A gas turbine doesn't have those, so it can get away with a thinner oil that flows better at low temperatures. Let me show you the fuel-cooled oil cooler, because that's how the oil temperature is managed in the system. And here's the magnetic chip detector — this is how you detect impending bearing failure. The magnet collects iron filings, and when you see them, that's evidence of trouble coming. And finally, the centrifugal breather and vent. It rotates at high speed, and as the oil mist enters, it's thrown outward — that's how it separates the oil from the air being vented. So to tie it together: the filters protect the system from contamination, with two ways to warn you of blockage. The oil itself is synthetic, chosen for specific properties — low volatility, high flash point, high film strength, wide temperature range, low viscosity, and high viscosity index. And the reason a thin oil works is that a gas turbine has no reciprocating parts or heavy gearing. That's the core of gas turbine lubrication.