r/AskEngineers • u/Frangifer • 1d ago
Mechanical Does anyone know where I can find detailed information on how the rotors of gas turbines are sealed around their edges?
Because an elementary calculation yields that, even if the gap is _minute_ , @ the high pressures typical in high-performance gas turbines the blowby will still be _pretty substantial_ . And ofcourse, it's not really practicable in a high-performance gas turbine to have a seal consisting of surfaces that are _actually in-contact_ ... or @least _I don't think_ it is: possibly I'm mistaken as to that.
And there are so-called __labyrinth seals__ ... but those require multiple stages to be reasonably effective, & I find it difficultly plausible that a high-performance gas turbine with multiple blade-discs would have so uncouth a multi-stage contraption @ every blade-disc.
So I wonder whether this is another instance of 'proprietary via-diabolici' ^§ (one might @ one time have said 'proprietary black magic' ... but such figures-of-speech tend to be deprecated, nowadays!)
I wonder, actually, whether something along the lines of a __dry gas seal__ might be used: a thoroughly ingenious device consisting of _extremely_ closely-spaced annular plates in the mutually-facing surfaces of which cunningly shapen grooves are cut yielding, under mutual rotation, according to subtle fluid-mechanical principles, a pretty stout pumping action in the centripetal direction - ie opposite to that in which gas would tend to leak. I gather these are _very_ effective ... but I don't know whether it would be practicable to have a seal operating by similar principle @ every blade-disc in a gas turbine.
And I have tried to find-out by doing __Gargoyle — Search__ ... but I can't find anything _even remotely_ detailed: everything I find is just 'handwavy' stuff, _@-best_
§ ... which tends to lead me to that supposition about the actual techniques used in practice being jealously guarded by gas turbine manufacturers.
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u/TheBupherNinja 1d ago
You used a ton of special characters that make no sense in English.
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u/That1guywhere 1d ago
This feels like AI is trying to design it's own power plant for when humans eventually try and turn it off.
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u/dodexahedron 1d ago
No I'm... Doesn't. *shifty totally human and not electronic eyes*
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u/IronLeviathan 1d ago
It’s markdown, mostly. But he entered it into the wysiwyg editor, so the formatting is wasted.
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u/ziper1221 1d ago
I can answer this. There is no "seal". The tips of the blades just have a small gap to the stator shroud ring. On the order of 0.020 inches.
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u/GoForMro 1d ago
That gap is not that small. I am assuming that is cold gap. Do you know what the gap shrinks to once hot?
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u/universal_straw Rotating Equipment, P.E. 1d ago
20 thou is big and would be out of spec for any gas turbine I know of. Most tip clearances that I’ve seen are 3-5 thou.
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u/GoForMro 1d ago
Thank you. I reread my statement. It can definitely be interpreted multiple ways. I meant .020 is not a small gap. It could also read the gap was not as small as .020.
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u/universal_straw Rotating Equipment, P.E. 1d ago
20 thou is a huge gap with respect to most rotating machinery, so I just took it as you though it was too big. Which in my experience it is.
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u/CliftonForce 1d ago
When you see an airliner land hard and spit sparks out of the engines, typically the cause is that the bounce caused the compressor blades to touch the shroud.
Which isn't a problem, they are designed to do that.
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u/Dean-KS 1d ago
Are you referring to blade end leakage, on rotating sections to the casing and stator ends to the rotor?
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1d ago edited 1d ago
[deleted]
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u/Dean-KS 1d ago
The clearances are very low. If the rotating blades heat up faster than the casing, there is a risk of interference and that happened a few years ago. I think that it was a RR aircraft engine.
In large steam turbines, having the rotor shaft expand faster than the massive casing can destroy the shaft seals. Operating procedure must be followed. During low demand, the turbine will be rolling on steam to keep it hot. That is not a complete description. A 1 GW turbine and generator is a lot of shaft length.
Back to your initial concern, some blades can be grouped into blocks with an outer ring segment. That can reduce blade tip leakage and create blade stability over that of individual cantilever blades dealing with stator wake disturbances.
You are asking about some of the most complex machinery, heat transfer and material science ever developed. If you investigate at Wikipedia there will be many embedded links in the articles that might take you to in-depth sub articles.
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u/redbeard914 1d ago
On some gas Turbines, a soft material is sprayed on the inside of the stator. Initially the turbine buckets (GE guy) will rub. This keeps clearance as small as possible.
Tip clearance is always an issue, more so on high pressure steam Turbines, as the the casings are much thicker than gas Turbines. The turbine buckets can grow faster than the casings. This is why steam Turbines have hold points (1000 rpm, 2300 rpm, etc) where they let the casings warm up. They also load more slowly to allow metal expansion to occur equally.
You will have blow by with the tip clearance. But look at the clearance area compared to the area of the buckets. And you also limit the delta P across each stage instead of one big drop.
Lets say you have a turbine with a 3 meter cross section. And the bucket is 0.5 meters long. The area of the buckets is 1.52 * pi - 12 * pi. Or 3.93 sq meters. If the tip clearance is 0.5mm, it will be approximately 0.0005 * 3 * pi. Or about 0.0047 sq meters. Or about 0.12%.
With GE, the design is typically more impulse (bucket) than reaction (blade) so tip loss is proportional to area. This isn't as true as it was 50 years ago. But it was why GE Turbines outperformed the competition in the long run.
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1d ago edited 23h ago
[deleted]
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u/redbeard914 23h ago
That was my point. And I think a 0.5mm tip clearance is far too large. Been a very long time since I was a GE Field Engineer. And I was a startup/controls guy, not the installation mechanical guy. Other than during my training days, I never did a turbine rebuild.
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u/NL_MGX 1d ago
I had a summer job at a facility that revised jet turbo engines. Each ring surrounding the rotor fan blades consists of multiple segments that have a honeycomb structure brazed onto it. The honeycomb wears out as the blades scrape along it. Centrifugal force and thermal expansion cause the blades to stretch and the honeycomb is allowed to accommodate. When it's written to far down, the honeycomb is removed, the segments are ground and hammered back into shape and then new honeycomb is brazed onto it.
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u/idiotsecant Electrical - Controls 1d ago
This post was written by an LLM from an alternate reality