november's news

I got an info regards the turbine start up after the outage my company did.
It seems we did good job. There was no problems during start and trial period. The additional balancig activity is forseen as the dynamic state of whole machine can be improved. The overhaul agreement includes the vibration limits after assembly, so we are just now slightly above what was written down.

I am back

Short explanation why such long break.
The summer is the overhaul time, so each company in my sector is busy. I was tired after work and had no power to open my blog but I going to continue and this "diary" will be enriched with more posts.
During this weekend the start up of turbine, I described in my previous post, is planned. Coming back to office on Monday some details will be known. Certainly, I will touch this then.
In the mean time I has supervised the outage of ten rotors - 4 HP rotors, 3 IP rotors and 4 LP rotors but it is not the end as maybe fifth LP rotor will come. It is intersting story what we went trough with last two LP rotors but I will mention this later.
All rotors are the rotors of the 200 or 225 MW turbines. In fact it is the same desing but each turbine has been modernized up to certain level and that is why the turbine efficiency is different.
The first turbine is 200 MW. We got 3 rotors from this turbine for the overhaul process in the same time but very quick was obvious that the HP rotor is in worse technical condition than was foreseen. The first and secound stage were damaged. Blades and shrounds should be exchanged but no money was budgeted for such operation, so after meeting with customer it was decide to leave the rotor as it is and repair the spare one bought a several years before. Customer thoght that rotor HP no. 2 is in better technical condition. After NDT activity we did, it was clear that blades are ok but shrounds in 2 stages needed repleacement. The secound customer meeting took place to spread info and propose solution.
The power plant people agreed with us what to do and I prepared the technology charts accordingly.
The new shrounds were put on.
The IP rotor overhaul was enlarged in similar way like HP rotor case. Here the one shround was exchanged only.
It is easy to write that overhaul scope was increased but the the overhaul deadline was not changed at all. It means, we were forced to do more job in originally planned period of the time even when the scope was wider. Anyway we managed to do it. Honestly, the new repair circumstances affected the timing and we got one week more.

The secound turbine is the 225 MW turbine with different LP rotor. It is larger, heavier than previous one. It was modernized in 2001 by Westinghouse.
It made some troubles for us, as our sand blasting room is not big enough for such rotors. I found soultion. I had read about dry ice blasting. This technology seemed to be great in this particular case. It does require seperat place to clean dirt turbine part. I found 3 companies offering this technology. All of them saw our rotor and one of the did a little trial to see if it is possible to clean our "fan".
We saw results and took a challange. Dry ice blasting is very noisy and not as good as it looked like doing the trial.
Usually we need 2-3 days (with 1 shift daily) to clean the big rotor but now was necessary to devote 5 days with 2 shifts daily to get the LP rotor cleaned. Uff... dry ice blasting is not the best idea, is too slow.

to be continued ...

the rotors to be repaired

I had opportunity to supervise overhaul of a couple of rotors.

The first one and the most complicated is the the repair of the LP rotor of 100 MW turbine.
It is double flow rotor and have 10 discs mounted on it.

During the NDT technicians found the cracks in the corner of keyway at 6 discs.
Let me present the pictures taken before discs dissasembly.

"Naked" rotor shaft with no discs, the bigest ones are visible.

Next pictures present the dissasembled disc

These cracks are the beginning of possible disaster, that is why we and customer took decision to repare the discs.
We proposed the followin steps :

- dissasembly of all discs,

- NDT examination,

- machining the internal diameter of discs to remove the craced area,

- prepare the bushes,

- mount the bushes into discs with light interference,

- drill radial holes through the bush and disc and install pins with light interference.

The customer accepted our solution, so we started to collect the material and parallel prepared the discs.

I ordered the bushes at foundry and prepared the drawnigs how to machine the discs.

After 4 weeks we got row bushes then they were machined and put into discs.

The next step was to drill radial holes inside the discs. It seems to be qiute easy and in fact it is easy but we have too big angle head for our milling machine them we had a lot of problems to obtain what was designed. At bigest discs, which has the least hole I asked designer not to drill holes as deep as it was drawn.

In the same time the pins were prepared, then we put them into drilled holes. I wrote, the pins to be installed with light interference but to make the assembly quick we used nitrogen to cool them down.

This part was finalized but it is not the end. Now is final machinig. The discs are installed onto rotor with interferance ~ 0.60 mm, so the diameter is very important and tolerances are very tide.

The very last step of machining is keyway milling. We did it partialy inhouse, and using external subcontactors.

I put a couple of pictures taken at differt assembly steps

This is the end of machining and the assembly phase was started.

Below we have the hanging rotor shaft a few secounds before 1st disc assembly

It took several hours to heat up the disc and get enough internal diameter. The interference is 0.6 mm. To mount discs on the rotor we decided to heat them untill the clerance is 0.5 mm.
We needed almost one week to put 10 discs on the rotor but it was not the end as the steam gland bushes and finally couplings had to be assembled as well.
We managed to it and after run out measurement it turned out that some surfaces should be machined when done the very last step - low speed balancing.
I will write a few words about turbine start up but wait some time.

welding repair of the rotor

In my previous post I mentioned about the rotor welding repair. Let me add more info.
During the NDE it was found that it has crack placed next to journal under thrust collar. On the picture below (arrow) it is visible - thrust collar is diassembled.

Firstly, the rotor end was cut and shaft chamfered. The stub shaft was sent to us by our colleagues and welded by TIG to the main part of the rotor. The most important activity was carried out next - putting the material by SAW. It took several days and was splited into a few phases. After welding certain material thickness we machined it and did NDE to be sure if weld is free from any mistakes.
When welding was done I took over the responsibility. The two last steps remain. The rotor PWHT and machining.
To perform PWHT the rotor had to be positioned vertically. It is not usual to see rotor set in vertical position and to do it the special equipment was necessary - based on former solution I designed some devices.
The pictures below show the rotor uplift:

- rotor after rough machinig before PWHT

- the rotor uplift

The rotor is hanging and will be fixed for PWHT

The rotor PWHT is starting ...

Original rotor design was changed. Just now the thrust collar is an integral part of the rotor shaft when previously collar was a seperate part and mounted on the shaft with negative allowance.

The repair last step carried out in our workshop and supervised by me was the final machining. I got the drawing, but the technology chart prepared on my own. As usually not all info was put on the drawing but I manged to get the missing data and below we can see the rotor end after final machining:

Once machinig was finished the rotor was moved to the our subcontractor - for high speed balancing.

I know that the customer has more rotors (2 pcs.?) with similar crack and maybe the repair process will done again... who knows...

outage season

The Spring has come already and as usually this time during the year, a couple of new overhaul projects have been started. I have taken part in some of them:
- reapair and machining of the axial bearing,
- assembly and machining of the modernized steam glands (retractable seals),
- welding and machining of the rotor.

This last one is especially intersting as the rotor after welding has to be fixed in vertical position for PWHT. I think, such advanced rotor repair is uniqe and my company is the only one in this part of world, able to perform it ( Am I wrong? Please advice).
I wanted to post complete report, I did after small repair back prassure turbine - too much work just now but having more time I will do it.

small back preassure turbine repair

The next project, I was responsible for, is the small turbine overhaul.
It is 6 MW machine. The majority of such turbines were installed in Sugar factories.
The scope of works was not complicated:
1. Upper part of external casing.
- cleaning,
- hand processing of casing surface,
- NDT examnation,
- elimination of discoverd crakcs and welding repair if neccesary,
- corrosion protection.

2.The Steam glands.
- cleaning,
- NDT examionation,
- dimensional checking,
- corrosion protection.

3. The rotor.
- cleaning,
- NDT examination,
- geometry checking,

4.The Diaphragms
- cleaning,
- NDT examination,
- dimensional checking,
- corrosion protection.

5. The diaphragms carriers
- cleaning,
- NDT examination,
- dimensional checking

I am going to post the complete report I did after this overhaul.

generator's bearing machining.

The next task I supervised was the machining of the generator's bearing. My company is not specialized in generators repair but a review of the mechnical part of this machine very often is included in overhaul scope.
I am going to present only small element of whole turboset outage.
I do not remeber what was wrong exactly with generator's rotor but its journal was machined, and a diameter was decreased. It such case is a technical correct to decrease the bearing's clerances which is imposible without supply of the bearing with new, smaller internal diameter.
We used support of bearing suuplier described in my previous post but in this case they were responsible only for rough machining as we wanted to do rest of operations on our own.
Firstly, just after we received rough bearing my colleagues carried out the US NDT, to check if the "white metal" adheres good enough to bearing casing.
The next step was the hand processing of the bearing's surface. The lower and upper part have to fit to each other with special conditions - no clearance and the fitting trace has to be visible on ~ 80% of the surface. This demand can be obtain by grinding.
Than we put a metal plate between both parts of the bearing. I mentioned in my previous posts that internal shape of the bearing is lemon/ellipse shape, this is why the plate is needed. After bearing machining it will be removed and we will get, what we want to.
Next is the machining of the internal diameter, when done the surface metal plate are removed and again bearing is assembled on the lathe to machine oil seals and the external diameter.
Now, we can start the last part of the maching - the internal details for lube oil inlet and lift up oil. The major is done on lower part of the bearing.
The last activity was to clean holes for vibration and temperature sensors.
This bearing's ovehauls was done under the client preassure , so we worked during the weekend to send the part as quick as possible.
We managed to do it and a day after I had a phone call with our site supervisor to discuss some dimensions but a couple hours later he confirmed that turbine is closed and ready to be launched. I have not had any news related to this turbine, which means it is ok.
It was quite fast job under the preassure of time but we succed to do properly without mistakes