Gas Turbines - Lubrication — Page 305, Lesson 388

Let’s pick up with the centrifugal breather and vent, because this is the part of the lubrication system that keeps the engine from either blowing itself apart or running dry. The problem we’re solving: inside the gearbox and the bearing chambers, air pressure builds up. If we didn’t do anything, that pressure would get excessive. So the interior of the engine has to be vented to atmosphere. But here’s the catch — the air inside isn’t clean air. It’s full of tiny oil droplets, which forms what we call an oil mist. If that mist were allowed to escape straight to atmosphere, we’d lose oil very quickly — the engine oil contents would be depleted rapidly. So the design routes that oil mist to the gearbox, and before it can reach the atmosphere, it must pass through the centrifugal breather. That’s Figure 19.8. Here’s how it works. The centrifugal breather is rotated at high speed. As the oil mist enters, it’s thrown outwards by centrifugal force. Around the inside periphery of the breather are what we call de-aerator segments. The oil is separated from the mist there, and it’s eventually flung back into the gearbox, where a scavenge pump picks it up and returns it to the system. Now, the air — because it has less inertia than the oil — makes its way out of the centre of the rotating portion of the breather, and it goes to atmosphere, but by then all the oil has been removed from it. So the net effect: the centrifugal breather minimizes oil loss in the gas turbine engine. That’s the whole point of this component. Now let’s move to filters. The oil’s job includes cleaning, and to help it do that, a number of filters and strainers are positioned throughout the lubrication system. Their purpose is to prevent debris and foreign matter from being continuously circulated around with the oil. There are several distinct filters, each with its own job and its own position. Let me walk you through them in order. First, the suction filter. The oil is drawn through this before it goes into the pressure pump. The suction filter takes the form of a coarse strainer. Its job is to prevent debris from being drawn into the pump and damaging it. So it’s the first line of defence, protecting the pump itself. Next, at the outlet of the pressure pump, we have the pressure filter. This one is a very fine mesh filter. It will retain any small particles which might otherwise block the oil feed jets. So while the suction filter protects the pump, the pressure filter protects the jets downstream. Then there are the thread type filters. These perform the function of a ‘last chance’ filter, and they’re positioned immediately prior to the oil jets. So the name tells you the job — it’s the final barrier before oil reaches the jets, catching anything that got past the earlier filters. Finally, each return oil line contains a scavenge filter, and it’s positioned just downstream of the magnetic chip detector. These scavenge filters collect any debris returning from the lubricated component. So as the oil comes back from wherever it was doing its job, this filter catches whatever it picked up along the way. So to summarise the whole filter layout: suction filter protects the pressure pump, pressure filter protects the feed jets, thread type filters are the last chance right before the jets, and scavenge filters catch debris on the return side, just after the magnetic chip detector. That’s the breather and the filter system. Next we’d look at the magnetic chip detector itself, which is the component that gives you early warning of impending failure.