Gas Turbines - Lubrication — Page 300, Lesson 378

I want to walk you through the lubrication system of a gas turbine engine. This is a big topic, so let's start with the fundamentals. First, the oil itself. Because of the high temperatures inside a gas turbine, the oil used is invariably synthetic. That's a hard requirement, not a preference. Now, here's a key difference from a piston engine: the oil is not changed on a regular basis. Gas turbine engines actually use more oil, because of the nature of the air seals, but the synthetic oil doesn't break down and oxidize like mineral oils do. So instead of changing the oil, the filters are removed, washed out, and refitted at regular intervals. The purpose of that is to examine any debris collected and evaluate the wear rate of the engine. So the filter inspection is your window into how the engine is wearing. Now let's look at the first type of system, the Pressure Relief Valve Lubrication System. In this system, a spring loaded valve limits the pressure in the feed line, and that controls the flow of oil to the bearing chambers. The pressure is restricted to a value that the engine designer considers correct for all conditions the engine might encounter. The spring loaded valve opens at the pressure generated by the oil pressure pump at engine idling speed. That gives a constant feed pressure over the whole of the engine speed and oil temperature ranges. So once the valve opens at idle, it holds the feed pressure steady no matter what the engine is doing. Let me walk you through the flow path. The oil is drawn through a suction filter to the oil pressure pump. The suction filter protects the pump from damage should any debris enter the tank. So it's a protective screen on the inlet side. Then the oil is passed through the pressure filter to the pressure relief valve, which maintains the oil pressure to the feed jets in the bearing chambers constant. So you have two filters in series: suction filter first, then pressure filter, then the relief valve. From there, the oil passes to the feed jets through internal drillings and external oil pipes. In this particular engine, the hollow interior of the compressor/turbine shaft is used to transfer oil from the front of the engine, where it's used in the pitch control mechanism, all the way through to the rear, where it's used in the turbine bearings. So the shaft itself is a conduit for oil — that's a clever use of the hollow shaft to get oil from front to back. Let me show you the schematic of this system. That's the pressure relief valve method with the basic components for a turboprop engine lubrication system. The key takeaway is the constant feed pressure maintained by that spring loaded valve, and the two-stage filtration protecting the pump and the bearings.