Gas Turbines - Bleed Air — Page 396, Lesson 501

Let’s pick this up right where the seals are doing their job. We’ve got two types of seals here, and I want you to understand exactly why each one exists and how it behaves. First, the intershaft hydraulic seal. This is the one that steps in when you have a shaft deflection problem. Picture two shafts spinning inside each other, and one of them deflects — bends slightly off its true centre. If you tried to seal that with a rigid contact seal, the deflection would create high temperatures and the real risk of shaft failure. That’s the dangerous situation. So the intershaft hydraulic seal is designed to handle it. It’s actually the first type of labyrinth seal we mentioned earlier in this section. The construction is simple: a fin, or several fins, rotate very close within an annulus of oil — that’s a ring-shaped cavity filled with oil. If the shaft deflects, the fin or fins move into the oil. The seal is maintained, but crucially, without generating any undue friction or heat. The oil absorbs the contact, so you don’t get the temperature spike or the mechanical stress that would otherwise destroy the shaft. Now, the interstage seal. This one has a different job entirely. It’s used to either prevent or control leakage of air between sections of the engine that are operating at different pressures. Think of the compressor as a series of stages, each at a different pressure. The interstage seal sits between them. Here’s the key relationship: the amount of pressure dropped across the seal depends on the number of fins the air must pass over. More fins, more pressure drop. And that gives you a clever way to control pressure zones. If you want a higher pressure in one zone of the engine than another, you simply pass the air over fewer fins into the high-pressure zone than into the lower-pressure zone. Less pressure is dropped before entry into the high-pressure zone, so it ends up higher. That’s the whole trick — you tune the fin count to set the pressure difference. Now, the efficiency of all these seals — both types — depends on two factors. First, the mechanical design of the seal itself. Second, the air pressure, which is absolutely essential; without sufficient air pressure, the seal won’t work at all. And this is where the operational reality bites. It’s during periods of low engine power — for example, selecting idle power during descent from high altitudes — that you suffer the greatest oil loss from a serviceable engine. Why? Because at low power, the sealing air pressure drops, so the seals aren’t as effective, and oil gets past them. At high power settings, the sealing air pressure is strong, and oil loss from a serviceable engine is almost negligible. That’s a direct cause-and-effect you need to remember: low power equals weak sealing air equals oil loss. Now, where does all this sealing and cooling air come from, and where does it go? Remember, we said earlier that the air for cooling and sealing is taken from as early as possible in the compressor — that keeps it cooler and at a manageable pressure. For the specific case of sealing air used in bearing chambers, it’s taken from the intermediate stages of the compressor. It passes through air transfer ports in the compressor rotor drum, then through communicating passages to wherever it’s needed. So the path is: intermediate compressor stage → air transfer ports in the rotor drum → communicating passages → bearing chamber. Finally, disposal. Once the cooling and sealing air has done its job, it has to go somewhere. And here’s the split: the HP air — high-pressure air — used for cooling is ejected into the exhaust stream. It just gets dumped into the hot gas flow leaving the engine. The LP air — low-pressure air — on the other hand, is fed out through its own dedicated vent pipe. So you have two separate disposal paths: HP cooling air into the exhaust, LP air out through a dedicated vent. Let me just tie the whole picture together for you. You’ve got sealing air bled from the intermediate compressor stages, routed through the rotor drum to the bearing chambers. You’ve got two seal types: the intershaft hydraulic seal for deflecting shafts, and the interstage seal for controlling pressure differences between stages. Both rely on mechanical design and air pressure, and both are weakest at low power, which is when you lose oil. And when the air’s finished its job, HP goes into the exhaust, LP goes out its own vent. That’s the complete bleed-air sealing and disposal story.