MD11Engineer From Germany, joined Oct 2003, 13602 posts, RR: 63 Posted (9 years 7 months 2 weeks 6 days 23 hours ago) and read 32767 times:
Today I´ve done a boroscope inspection of a PW 2040 engine (again!) and I´d like to post some pictures of things you people (even you glamour boys and girls in the "front office" ) probably have never seen before (except mechanics and licenced engineers of course).
The pictures were taken with a 6mm flexible boroscope. A boroscope is a close relative of a medical endoscope. It consists primarely of a long thin flexible probe with a fiber optic inside. The tip can be moved by remote control and it contains a strong light source. On this (digital) boroscope the pictures can be seen on a lcd screen and saved using a floppy disk. The idea is to be able to inspect the innards of an engine without having to dismantle it. There are so called boro plugs, holes closed with a screwed in plug, al over the engine in critical places, permitting the insertion of the probe. What I did today was a routine inspection of a healthy, but not too new PW 2040 engine of a B757. Enjoy!
Starting at the front here is a picture of the 10th stage High Pressure Compressor. You can recongnise the vanes and the rotating blades. I had to check the outside brazed joint of the vanes for cracks. The round holes are openings where bleed air is taken out of the engine. The shiny spots on the vanes are not cracks, but some vaseline rubbed off, which I used to lubricate the probe.
The next one is a shot of the combustion chamber. This engine has an annular combustion chamber made up of loosely interlocked rings (to allow for thermal expansion). At the front end are the round injection nozzles. The nozzles close to the ignitor plugs are a bit soothy, but this is normal. Only part of the air is directly mixed with the fuel to allow a chemicaly combustible mix. Additional air is added through the holes in the combustion chamber liner to cool down the hot gases (the flame is almost 2000°c hot, too much for the turbine, the max EGT on startup, measured behind the last turbine stage is 425°C).
Here is a close up of an injection nozzle, you can see the swirl vanes around the actual nozzle, which provide a vortex for good mixing of fuel and air.
Next is picture of the tip of an ignitor plug sticking into the combustor, there are two of them on each engine, each powered by a seperate system.
Now we see the rear end of the combustion chamber, where the hot gases enter the nozzle guide vanes to be dircted into the first stage of the High pressure turbine. This is the hottest and most highly stressed area of the engine. The nozzle guide vanes are cooled by "cold" (still around 400°C) bleed air being ducted through their inside. The air escapes through the multitude of small holes drilled into them and forms a protective film arounfd the vane. The vanes look a bit dirty due to residue from the combustion, and in some places the ceramic coating has flaked off a bit, but everything is within limits.
Between the vanes you can see the blades of the first stage HPT, which are not only exposed to an intense heat, but also to high centrigugal forces.
They are cooled in a similar way as the nozzle guide vanes. Here is a better picture of a blade:
Finaly we reach the second stage HPT, where the gases have cooled down quite a bit (they lost energy turning the first stage turbine). Everything is cleaner, but the blades (we look at the trailing edge) are still being cooled internaly (see the slits at the trailing edge). You can se the rub strip on the turbine casing, a soft material, which gets worn in by the turbine blades, so that the gap between the turbine and the casing will be as small as possible for efficiency reasons. The casing is also cooled from the outside with bleed air sprayed on it from a series of pipes with small holes drilled into them to shrink it controlled to keep this gap small, even under thermal expansion.
Finaly here is a selfpic taken with the boroscope:
MD11Engineer From Germany, joined Oct 2003, 13602 posts, RR: 63 Reply 4, posted (9 years 7 months 2 weeks 6 days 22 hours ago) and read 32767 times:
Things I would look for on a general hot section inspection:
Cracks, burn marks, distorted or molten parts of the combustion chamber liners, which might point to a faulty injetion nozzle (when it doesn´t eject the fuel as a fine spray, to burn immidiately, but as a burning jet, which might strike some part and overheat it, on turbine blades and vanes sulphurisation, blueish or grey discolourisations, which can happen if the fuel contains too much sulphur. It will alloy itself with the metal and make it brittle. Shingled turbine blade shrouds (if the tip shrouds of the blades are not side by side, but overlapping), blade clashing (blades touching some nonmoving part of the engine), FOD, discoloured parts (which could signify an overheating).
There is also an AD out for a continued inspection of the 10th and 11th stage HPC stator vanes one PW2040 engines of certain serial numbers. The stator vanes of thse engines tended to develop cracks in the outer tiptrailing edge region, which caused some engines to "auto shut down" in flight with spectacular results (a vane breaking off and going downstream through the engine). This will continue until this engine goes back to the shop for overhaul and the offending stator segments will be exchanged for modified ones.
Aloges From Germany, joined Jan 2006, 8522 posts, RR: 46 Reply 5, posted (9 years 7 months 2 weeks 6 days 22 hours ago) and read 32767 times:
I hope you don't mind if I repost Jan's text and add the two pics I managed to get working (hopefully):
"The next one is a shot of the combustion chamber. This engine has an annular combustion chamber made up of loosely interlocked rings (to allow for thermal expansion). At the front end are the round injection nozzles. The nozzles close to the ignitor plugs are a bit soothy, but this is normal. Only part of the air is directly mixed with the fuel to allow a chemicaly combustible mix. Additional air is added through the holes in the combustion chamber liner to cool down the hot gases (the flame is almost 2000°c hot, too much for the turbine, the max EGT on startup, measured behind the last turbine stage is 425°C)."
"Finaly here is a selfpic taken with the boroscope:"
Jan, thanks a lot for this great insight (pun fully intended), RU list for you!
[Edited 2004-04-25 01:12:40]
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Klaus From Germany, joined Jul 2001, 21346 posts, RR: 54 Reply 9, posted (9 years 7 months 2 weeks 6 days 21 hours ago) and read 32767 times:
Most excellent post, indeed!
Regarding the compressor / turbine disks: It seems the high pressure compressor shown above is a single piece; Is that correct? And how many stages have individually removable blades and how many are "solid" (if any)?
I´ve only read about single-part compressors/turbines in context with military engines, so I don´t know about commercial ones...
I guess cost would be the limiting factor (One damaged blade would ruin the entire disk).
MD11Engineer: I had this problem with the pictures not loading quite often with posts with many pictures. I assume that the loading time takes so long that the connection times out.
No, the URLs just got mixed up in your post; After manually cleaning them up (open picture in separate window, check URL), the pictures did indeed load (at least the two Aloges included above).
And I agree, any further insight is highly welcome!
AUAE From United States of America, joined Apr 2004, 296 posts, RR: 4 Reply 14, posted (9 years 7 months 2 weeks 6 days 7 hours ago) and read 32037 times:
It has been quite a while since I worked on PW2037's, but I don't remember any of the compressor disks being one piece. As far as I can remember, all the compressor blades are individual. Some turbine vanes come in pieces of three of four vanes together. Only thing I can think of that is one piece that is interesting is the compressor spool. The rotating disk that the compressor (stages ?? to ??) is made up of several disks, inertia welded together. It looks like a beer keg, sorta.
The ablative material that blades rub away is rubber in the front section of the engine, but I believe in the picture it is ceramic type material. Is that correct? Ceramic ablatives in the tail section?
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MD11Engineer From Germany, joined Oct 2003, 13602 posts, RR: 63 Reply 15, posted (9 years 7 months 2 weeks 5 days 23 hours ago) and read 31815 times:
First, I´ve got a small correction to make: The max start EGT is 485°C, not 425°C.
Concerning the picture of the 10th stage HPC vanes, what you see on the top are the trailing edges of the 10th stage vanes. They come in segments of 5 to 6 vanes. They are also the first non variable vanes of the HPC, all vanes of the HPC forward are variable, e.g. their angle of attack can be changed. On the bottom you see the 11th stage HPC rotor blades. They come single and are dovetailed into the compressor disk.There are 74 blades in the 11th stage. The blade is only 2.050 inches long, the last 17th stage blades are only 1.175 inches long (this is important for estimating the size of possible damage, one of the hardest things when doing boroscope inspections, is it still within limits?).
The turbine blades are also fir tree jointed into the turbine disk. Just for info, the 1st stage HPT blades are 2,32 inches long.
P.S. the rub strip in the turbine is made of some soft ceramic stuff (don´t ask me, all I´ve been doing is boroscopes, I never dismanteled a PW 2040)
MD11Engineer From Germany, joined Oct 2003, 13602 posts, RR: 63 Reply 17, posted (9 years 7 months 2 weeks 5 days 22 hours ago) and read 31732 times:
The boroscope optics have only a very limited field of focus, additionaly the digital camera on this one is always focusing on what is in the centre of the picture. Sometimes it realy annoys me, e.g. if I do an AD inspection of the 5th stage LPT blades for blade clashing (one some engines the 5th stage LPT stator segments moved aft and touched the rotor, one engine quit with a loud bang and a fireball, and the maintenance crew had to pick up a bucket full of turbine blades from the runway...), I use a very old simple 4 mm boroscope without a camera, I just have to peep through the optics.
LMP737 From , joined Dec 1969, posts, RR: Reply 18, posted (9 years 7 months 2 weeks 5 days 21 hours ago) and read 31698 times:
Great pictures MD11Engineer! It's been awhile since I've done a boresope myself. Actually the last one I did was in the navy on a TF-30 checking for FOD damage on the N2. A couple months ago I did have the "honor" of re-installing the inspection plugs on a Trent 892 after inspection guys got done doing their part.
Brons2 From United States of America, joined Sep 2001, 2981 posts, RR: 5 Reply 21, posted (9 years 7 months 2 weeks 5 days 5 hours ago) and read 31290 times:
In my time I have disassembled quite a few land based reciprocal engines (auto, motorcycle, jet ski, mower) and the one thing that came to mind was, that turbine is much cleaner on the inside than any engine I have ever taken apart. With the notable exception of the injector nozzle, it looked pretty nasty compared to auto injectors that I have seen.
Also, the igniter was interesting, I was expecting something that looked more like a spark plug.
Firings, if well done, are good for employee morale.
MD11Engineer From Germany, joined Oct 2003, 13602 posts, RR: 63 Reply 22, posted (9 years 7 months 2 weeks 5 days 4 hours ago) and read 31263 times:
That ignitor head has a diametre of about 1/2 inch. The spark occurs between a recessed center electrode and the edge and has the shape of a ball. The voltages and currents used are lethal, this is why we always pull the ignition system C/Bs, disconnect the power input of the exciter box and wait for 10 minutes to let the capacitators discharge before we disconnect any of the high voltage leads.
The combustion in gas turbine engines is continous, because of this it can be much better regulated than a piston engine. Actualy only the two injection nozzles in the vicinity of the ignitor plugs are that dirty. The reason is that these injection nozzles allow a richer mix to improve starting.
Another thing about turbine engines is that it is rarely necessary to change oil. The oil in the tank after weeks of use looks just like the oil fresh out of a can. If it gets dark then there is something wrong with the engine, e.g. a bearing chamber seal is failing. Usualy we only top the oil level up to compensate for oil loss during operation.