Turbo Impeller Damage

[ZamF16]

Started my annual today and discovered the turbo compressor impeller damaged.  One blade had a chunk missing, Others were bent or nicked.  Looks as if something was ingested as high turbo RPM.  The question is what.  We see no signs that anything could have come through/around the air filter.  That leaves the alternate air intake.  We are surmising that during run up for take off the intake opened somewhat and a small rock or something similar got kicked up and was ingested.  It is just a strange angle to have something kicked up from the nose tire into the open cowl flap and then into the alternate air intake.  It would have to be a one-in-a-million shot. 

 

Tomorrow, we are pulling the turbo (after the Mouse Milk frees the exhaust bolts).  We are going to use a borescope to make sure no foreign objects are downstream of the turbo.

 

Needless to say, I was very surprised to see the damage to the impeller.  Has anyone see this before?  Anything else I should be looking at/for?

 

- Dave

[N201MKTurbo]

Turbo rule of 600s

Compressor wheel = $600

Compressor housing = $600

Turbine wheel = $600

Turbine housing =$600

Bearing assembly = $600

Labor = $600

[M016576]

I have little/no experience with turbochargers, but I have quite a bit of experience with turbines. The principles are similar but the hardware is different (spinning stators/rotars vs. a single axial flow compressor). Typically when you see a turbine throw a blade, it damages the rest of the compressor stages as it's injested, and the damage multiplies. I'd imagine that it's possible that what you're looking at in your turbo is that one of the blades fractured, and the rest of the damage is due to that fractured piece spinning around in a high pressure, high temperature environment before being discharged. I doubt the damage was due to ingestion, just based on where the turbo charger is in the flow of the motor. Then again, like I said above, I have no experience (outside of academic) with turbochargers.

[ZamF16]

I have little/no experience with turbochargers, but I have quite a bit of experience with turbines. The principles are similar but the hardware is different (spinning stators/rotars vs. a single axial flow compressor). Typically when you see a turbine throw a blade, it damages the rest of the compressor stages as it's injested, and the damage multiplies. I'd imagine that it's possible that what you're looking at in your turbo is that one of the blades fractured, and the rest of the damage is due to that fractured piece spinning around in a high pressure, high temperature environment before being discharged. I doubt the damage was due to ingestion, just based on where the turbo charger is in the flow of the motor. Then again, like I said above, I have no experience (outside of academic) with turbochargers.

I have similar experience with turbo-fan engines and was thinking the missing corner of the one blade might be the cause of the other blade damage versus the impact of FOD.  We spent a lot of time verifying nothing could have come from the air filter section.  The more I think about it, I really think it unlikely that anything was introduced through the alternate air intake.  I am just frustrated as I have babied the turbo.  It has lasted 1800 hours; I guess it just may have been its time.  We also found some corrosion on the hot side housing, so it was going to have to be pulled anyway. 

[Awful_Charlie]

I've just had a quick look without success, but do recall there was a mod somewhere to the air filter assembly: this was to increase the size of the slots in the back of the fibreglass induction scoop and was intended to alleviate water build up in the filter, and subsequent damage to the turbo by ice ingestion. One of the previous owners of mine had experienced the ice damage, and made the mod whilst replacing the turbo

[ZamF16]

Pulled turbo today, and did a borescope of the intercooler.  No signs of any foreign objects to include the missing piece of the turbo impeller.  Also had corrosion on the hot side, so we are sending the turbo in for an overhauled exchange.  Should have her back in the air in by the end of next week.  Am anxious to get airborne to enjoy this awesome Texas fall weather.

 

- Dave

[Piloto]

Unlike piston turbos, jet engines blades are made of Titanium, so they will last much longer. Russian jet engines blades used steel alloys that only lasted 500hrs vs 5000hrs for titanium. Back in the 70s when the USSR sold hundred of airliners to South America each aircraft was sold together with spare engines because they could not be overhauled. Cruise missiles use also steel blades but the TBO required on those is no more than 100hrs and the engine is disposable after use.

 

José 

[aviatoreb]

Unlike piston turbos, jet engines blades are made of Titanium, so they will last much longer. Russian jet engines blades used steel alloys that only lasted 500hrs vs 5000hrs for titanium. Back in the 70s when the USSR sold hundred of airliners to South America each aircraft was sold together with spare engines because they could not be overhauled. Cruise missiles use also steel blades but the TBO required on those is no more than 100hrs and the engine is disposable after use.

 

José 

 

Why can't we have titanium in our piston turbines?

[M016576]

Why can't we have titanium in our piston turbines?

Price? That's my guess...

[carusoam]

Probably the neccesary testing didn't get done back in the day. Price of the part would not be terrible compared to the life expectancy.

CNC manufacturing has grown over the last decade. Load the program and raw material, come back when it's done. The old style of hire three machinists and have them work around the clock at slow Ti speeds, would be incredibly expensive.

Find out who makes the part and ask??? Honeywell???

Best regards,

-a-

[KSMooniac]

Ti wasn't even being used on the SR-71 yet when Rajay started doing their thing... Sent from my ADR6425LVW using Tapatalk 4

[N201MKTurbo]

I think the turbo wheels are investment castings. And the materials are pretty well refined. Considering what they do, I think they are amazingly reliable.

[N201MKTurbo]

I was just doing some reading about turbochargers. If the throttle is rapidly closed the airflow will stop in the compressor side of the turbo causing the compressor blades to stall. This puts tremendous stress on the compressor blades and can cause them to break. There normally is a pressure relief valve in the compressor duct to vent the pressure when this happens. If the pressure relief valve was stuck closed it could cause the problem you described.

 

I can't recall, does the 231 have a pressure relief valve?

[chrisk]

I was just doing some reading about turbochargers. If the throttle is rapidly closed the airflow will stop in the compressor side of the turbo causing the compressor blades to stall. This puts tremendous stress on the compressor blades and can cause them to break. There normally is a pressure relief valve in the compressor duct to vent the pressure when this happens. If the pressure relief valve was stuck closed it could cause the problem you described.

 

I can't recall, does the 231 have a pressure relief valve?

How fast is too fast?  I assume this would only be an issue when going from full throttle (and the turbo is spun up) to 0 throttle?  I wonder if this is the sources of the "reduce MP by 1 inch per minute" technique?   And is the pressure relief valve also called a pop off valve? 

[N201MKTurbo]

  And is the pressure relief valve also called a pop off valve? 

Yes

[N201MKTurbo]

I don't know for sure, but you need to go slow enough for the turbo to spool down. I would think if you went from full throttle to idle in 10 seconds you would be OK. In either case if the pop off valve was working it should take care of it. In automotive turbos, people go from full power to idle all the time and the turbos seem to last. I think thay all have pop off valves.

[ZamF16]

Finally have the overhauled turbo and some new clamps and gaskets for the exhaust system.  We are putting it all back together tomorrow, then hopefully checking the turbo at altitude this weekend.  I fully expected to spend some money on the turbo overhaul.  I had no idea how much the exhaust clams and gaskets would be. 

[N201MKTurbo]

The gaskets are $7.00. Why are you replacing the clamps are they broken?

[Awful_Charlie]

The V-band clamps can only be used so many times after which they are junk.  Think two or three

 

Fortunately they don't need to come off often in normal service, but they're upwards of 100 bucks apiece, some are nearer 200 - you certainly don't pull one off 'just for a look inside'

[ZamF16]

Yeah, I needed 3 clamps at $191 a piece, as the old ones had stretched over time and could no longer be torqued down tight enough.  There was one other clamp that is close to being unserviceable, however, we could only find that one direct from Lycoming.  That one was $1000!  So "close to being unserviceable" means it is still serviceable and we are putting that one back on. 

 

What I never realized was how much work it would be to get everything re-aligned to install the overhauled turbo.  It took a couple hours to loosely reassemble everything, with the turbo supported by a jack stand to get everything re-aligned.  Then we tightened everything up, and now will get the turbo installed on Monday.  I look forward to getting her back in the air!

[ZamF16]

The gaskets are $7.00. Why are you replacing the clamps are they broken?

The cheap gaskets are the ones that are between the exhaust system and the cylinders.  Unfortunately, the metal ones that go between the exhaust system and the turbo and waste gate are over $100 a piece. When you pull the turbo off you have to replace these gaskets.  These gaskets are the ones pressing into the groove of the exhaust system by the V-clamps.   

 

I have learned more than I wanted about the turbo replacement!

 

-Zam

[N201MKTurbo]

When I spoke about the gaskets I was referring to the rajay turbo used on the 231 which is the same turbo used on my conversion. I have never worked on a Bravo.