We're starting a brand-new chapter now: Gas Turbines – Lubrication — Page 289, Lesson 376

We're starting a brand-new chapter now: Gas Turbines – Lubrication. This is a big one, and it's all about keeping that engine alive. Before we dive into the hardware, I want to give you the roadmap of what we're covering, because it's a logical progression. We begin with the fundamental question: why do we lubricate at all? Then we move into the different types of lubricating systems, starting with the pressure relief valve system and then the more common full flow system. After that, we break down the individual components: the oil tank, the oil pumps, the oil coolers, and the magnetic chip detectors. We'll also look at the centrifugal breather and vent, and then filters. Finally, we'll talk about the types of lubricating oils themselves. So, let's start with the very first item on that list: the reasons for lubrication. In a gas turbine, we're not just trying to reduce friction between moving parts, though that's a huge part of it. The oil has several critical jobs. It has to reduce friction and wear, of course, but it also acts as a coolant, carrying heat away from hot bearings and gears. It acts as a sealant in some areas, and it also helps to clean the internal surfaces by carrying away contaminants. And finally, it provides a cushion or damping between components. So it's a multi-purpose fluid, not just a simple lubricant. Now, before we get into the specific systems, I want to make sure you understand the two main categories we'll be discussing. The first is the pressure relief valve lubrication system, and the second is the full flow lubrication system. The names give you a hint at their operating principles, and we'll get into the details of each in a moment. Let's talk about the pressure relief valve system first. The core idea here is that the oil is supplied to the engine at a constant pressure. A pump pushes oil through the system, and a pressure relief valve is set to open at a predetermined pressure. When the pressure in the system reaches that set point, the valve opens and allows oil to bypass back to the pump inlet or the tank. This maintains a steady, regulated pressure regardless of engine speed. It's a simpler system, but it has a limitation: the oil flow doesn't increase with engine speed, so at high power settings, the bearings might not get the increased flow they need for cooling. That leads us to the full flow lubrication system, which is the more common design in modern turbine engines. In this system, the oil is supplied at a rate that is proportional to engine speed. The pump is driven by the engine, so as the engine spins faster, the pump delivers more oil. This means the oil flow automatically matches the cooling and lubrication demands of the engine at any given power setting. It's a more efficient and effective system for high-performance engines. Now, let's look at the components that make these systems work. We have the oil tank, which is exactly what it sounds like—the reservoir that holds the oil supply. It's designed to allow for expansion of the oil as it heats up, and to separate air from the oil. Then we have the oil pumps. In a typical system, there are two types: a pressure pump, which draws oil from the tank and pushes it through the engine under pressure, and scavenge pumps, which collect the oil after it has done its job and return it to the tank. The scavenge pumps usually have a larger capacity than the pressure pump because the oil expands and foams as it picks up air and heat. Next is the oil cooler. Because the oil absorbs a lot of heat from the engine, it needs to be cooled before it's recirculated. The oil cooler is a heat exchanger that transfers that heat from the oil to either the fuel or the air, depending on the design. A very important component for monitoring engine health is the magnetic chip detector. This is a plug that sits in the oil system, and it has a magnet on it. Its job is to catch any metallic particles that are circulating in the oil. If a bearing or gear is starting to wear or fail, it will shed tiny metal fragments. These fragments get caught on the magnetic chip detector. When you inspect it during a pre-flight or maintenance check, if you see metal chips on it, that's a warning sign of internal damage. Some detectors are even wired to an indicator in the cockpit that will alert the pilots to the presence of metal in the oil. We also have the centrifugal breather and vent. As the oil is churned by the gears and bearings, it gets mixed with air, creating foam. This foam needs to be separated. The centrifugal breather spins the oil-air mixture, and the centrifugal force throws the heavier oil outward, while the lighter air is vented overboard. This recovers the oil and prevents pressure from building up inside the engine's sumps. And finally, we have filters. These are the guardians of the system, trapping any solid contaminants in the oil before they can cause damage. They're usually located in the pressure line, so that all the oil going to the bearings is clean. So that's the overview of the entire chapter. We're going to go through each of these in detail, starting with the reasons for lubrication and the two system types. Let's begin.