Steam turbine rotor cracks 2

[Peckett]

Hi,

We have a few very small cracks on our steam turbine discs. Does anyone have any experience in assessing these?


Thanks.

 

[ScottyUK]

You assume we know where you are located in the world but don't tell us, so I guess you're in the US with the majority of people who don't know there are other countries than the one they live in out there.

It really does make a difference when you're looking for service providers!


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If we learn from our mistakes I'm getting a great education!

 

[eyec]

try TGAdvisers

 

[rmw]

Caustic embrittlement.

But why should I guess that based on so little information given by you.

Have you ever had any boiler priming issues?

rmw

 

[Peckett]

Hi Scotty,

Thanks for your response. I'm not in the US, but in Australia. Have you seen small cracks on rotors?

Thanks.

 

[stgrme]

We can't be much help if you don't give us much to go on. I suggest you start by providing some additional information, such as:
- Turbine manufacturer
- Turbine rating
- Turbine configuration (reheat or non-reheat, condensing or non-condensing, single casing or multiple casings, etc.)
- Inlet and exhaust steam conditions
- Year when unit was placed initial service
- Location and orientation of the cracks, including which rotor and which disc(s)

 

[rmw]

I HAVE seen small cracks on rotors - lots of them - and they were caustic embrittlement caused by a carryover problem and they were not only surface cracks but ran as much as 5/8" deep too.

But they were on the LP rotors mainly in the high/low teeth area of the diaphragm packing fits between the stages, all stages of the LP.

Does this come any where close to where your cracks are?

rmw

 

[Peckett]

Hi all,

We are working with the manufacturer to remedy this, and hence I am limited what information I can disclose at this stage. The small cracks are in the discs of the LP.

I am interested in the scale of the cracks you have found in the past, and how this limited the life of the rotor.

I would be more comfortable continuing this discussion in private.

Cheers

 

[eyec]

at this point in your postings, i would suggest you contract someone/company that can advise you.

telling your manufacturer something you gained from an internet posting (not to diminish the expertise on this forum) could cause you some embarrasement or worse.

 

[thgirw]

I can tell what is done for airplane engines when it comes to turbines, methodology should not be much of a different (I guess). Assessing such cracks means taking the responsibility of any accidents, deaths if not done without enough knowledge. Determination of limits for defects (cracks, dents, nicks, etc...) requires an extensive testing champaign in addition to usage of extensive analysis tools, and they are all defined in engine documents, technical orders, etc...
For your case, I would recommend to do nothing by yourself. Best way is to contact the owner of design. Even if you can provide data for dimensions, working conditions, it is impossible just to say it can withstand the loading, vibrations, etc... without having as much information as the design owner.

Regards

 

[MrMyers]

I don't want to sound like a salesman, but I've previously been involved with the assessment of cracks and flaws in turbine discs. We've developed some fancy procedures to assess the integrity and remaining life. Furthermore, I'm located in Brisbane.

Further to what thgirw said, you may get some support from the OEM depending on the youth of the unit but if it's an older unit, they will tend to suggest you buy a new rotor. Even though there may be plenty of life left in it.

Good luck

 

[metengr]

Hmm,,, large LP turbine, disk cracks. I would presume the cracks are either at the blade/bucket attachment location (rim) or if shrunk-on discs at the bore. In either case, this is probably stress corrosion cracking (SCC)> Just a wild a## guess.

 

[kjsmitty]

Remove the disc and or replace the rotor is what is usually done. Get your steam sodium analyzers functioning or installed once you replace the rotor. Sounds like you have problems in the boiler carrying over. Good luck Mate.

 

[Peckett]

Thanks for the latest response. We have cut off the offending disc and the machine is back in service.

Our crack was at the base/root of the disc. The rotor is a one piece forged rotor. and the crack was right around the upstream root of the stage 3 disc. The max depth was around 22mm.

Interesting project! If anyone else has seen cracks in this location in a one piece forged rotor I'm very interested in what was found in the way of causes.

We do have a history of non perfect steam chemistry , but don't have cracking elsewhere around the rotor, such as at the blade roots. Makes us think that there may be other contributing causes as well as steam chemistry.

Chris

 

[kjsmitty]

Chris,

Glad your back up & running. What are you using for chemicals. You may want to change over to all volitale water treatement. Using phosphate you may be carring over. Next ouytage inspect your dryers in both drums. Get sodium analyzers installed pronto. Good luck mate.

 

[stgrme]

I hope you have removed the stationary blades upstream of missing disc and have installed a baffle in place of the missing stage. The baffle is needed to produce a pressure drop similar to the original stage. If no baffle is installed, you could damage the upstream and/or downstream blading.

Best of luck,

 

[Peckett]

Thanks for this advice. We have left the upstream stationery blades (diaphragms) in place as recommended by the OEM. OEM has adviced that a baffle could be installed, and we would get a slight increase in efficiency, but the cost is significant.

Machine appears to operate well without stage 3.

Cheers


Chris