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.
piątek, 20 marca 2009
czwartek, 19 marca 2009
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
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
poniedziałek, 9 marca 2009
reaction turbine overhaul - part five
This part is the last one, dedicated to reaction turbine overhaul, I supervised. In last Friday I got info from our departament resposible for on site activity, that the trial run of this has been finished and no major defects were found.
Today, I would like to spend a couple of minutes, presenting bearings overhaul.
As it was not a point of outage scope and rotor's journals were not machined, the bearings could be used again.
In general, the most important features of the bearings are: geometry with right clearancess and the condition of the white metal. The clearancess depend on journals diameter. Sometimes, at small diameters, rotating machines can be operated with equal clearancess (bearing is a circle) but very often clearancess are not same on the sides and up, bottom. The bearing has lemon shape.
The another problem that can cause supply of new bearings is the lack of right adhesion between white metal and bearing casing. It is checked by US examination. Certain, % of surfaces can have a lack of adhesion.
In my case project managers wanted to have new, round bearings. NDT examination done on site showed the lack of adhesion and it was necessary to manufacture new bearings.
We used support of external supplier and it prepared bearings for us. However, I refused to accept first delivery, as geometry was out of tolerance. I required that the diameter is 0.00/+0.02 mm and the shape defect (cone) is not more than 0.02 mm - the bearings are very precision parts of the rotating machines and such requirements are nothing strange. Unfortunatelly, our supplier sent us bearings wrong machined and ... thought we did not discover it or whatever.
I asked our Quality Control to check dimensions against documentation I prepared prior the bearings were sent to supplier. It turned out that we have bigger diameter than can be and additionally they have ~ 0.20 mm of shape defect (cone) it is completly not acceptable and bearings were sent back to new white metal cast and machining.
The supplier did the mistake for the secound time, what is horibble but we were very sensitive and found it. The third time was the best. I think that after these mistakes we have right opinion.
What, I wrote above did not affect the overhaul deadline as the bearings were prepared parallel to other works. Anyway it can happen that new bearing delivery date is the critical one and "supplier's issue" can make us very nervous.
Today, I would like to spend a couple of minutes, presenting bearings overhaul.
As it was not a point of outage scope and rotor's journals were not machined, the bearings could be used again.
In general, the most important features of the bearings are: geometry with right clearancess and the condition of the white metal. The clearancess depend on journals diameter. Sometimes, at small diameters, rotating machines can be operated with equal clearancess (bearing is a circle) but very often clearancess are not same on the sides and up, bottom. The bearing has lemon shape.
The another problem that can cause supply of new bearings is the lack of right adhesion between white metal and bearing casing. It is checked by US examination. Certain, % of surfaces can have a lack of adhesion.
In my case project managers wanted to have new, round bearings. NDT examination done on site showed the lack of adhesion and it was necessary to manufacture new bearings.
We used support of external supplier and it prepared bearings for us. However, I refused to accept first delivery, as geometry was out of tolerance. I required that the diameter is 0.00/+0.02 mm and the shape defect (cone) is not more than 0.02 mm - the bearings are very precision parts of the rotating machines and such requirements are nothing strange. Unfortunatelly, our supplier sent us bearings wrong machined and ... thought we did not discover it or whatever.
I asked our Quality Control to check dimensions against documentation I prepared prior the bearings were sent to supplier. It turned out that we have bigger diameter than can be and additionally they have ~ 0.20 mm of shape defect (cone) it is completly not acceptable and bearings were sent back to new white metal cast and machining.
The supplier did the mistake for the secound time, what is horibble but we were very sensitive and found it. The third time was the best. I think that after these mistakes we have right opinion.
What, I wrote above did not affect the overhaul deadline as the bearings were prepared parallel to other works. Anyway it can happen that new bearing delivery date is the critical one and "supplier's issue" can make us very nervous.
piątek, 6 marca 2009
reaction turbine overhaul - part four
I am going to present what we did at outer upper part of the casing.
There are 5 diffuseurs and 3 of them were in conditon like photo shows. The customer gave us 2 diffuseurs (spare parts), they order third one to be manufactured by my company. Certainly, to install new diffusers is a must to remove the old ones, we did it but it is not complete scope of works prior to assembly the new diffuseurs. Firstly, is technical correct to machine the all cooperating surfaces inside the casing to get parallelism and perpendicularity. The diffuseurs are mounted with the negative allowance, in this case the diameter of diffusers should be 0.08 - 0.10 mm bigger than holes in casing, so after maching of the casing obtained hole geometry should not be bigger than customer's diffusers. Unfortunattely it was not the case. When we got right geometry of the seats, it turned out that provided diffusers are too small to install them and get right interference fit. We inform customer and got agreement to supply all new diffusers. Below we have the drawing of the new diffuser:
The first step like with the previous turbine parts was cleaning. The casing was moved to the special chamber and blasted using aluminium oxide. It is not the only one material we use. If customer have a special whishes we can use glass microballs, copper abrasive. These two are used very rearly as glass micrballs are not abrasive enough (it takes long time to get surface cleaned) and additionally round shape of the balls can make the surface too smooth, so the NDT - PT is unable to discover micorcracks and copper abrasive is too sharp and if used in wrong way can cause cavities on the surface, which especially at rotors are unwanted.
The next step is NDT examination. What we used the most at casings is MT examination but if necessary PT and UT can be applied as well.
During the overhaul I led, no big defects were found - the most important was discoverd inside the valve chamber at diffusers, see the picture below:
There are 5 diffuseurs and 3 of them were in conditon like photo shows. The customer gave us 2 diffuseurs (spare parts), they order third one to be manufactured by my company. Certainly, to install new diffusers is a must to remove the old ones, we did it but it is not complete scope of works prior to assembly the new diffuseurs. Firstly, is technical correct to machine the all cooperating surfaces inside the casing to get parallelism and perpendicularity. The diffuseurs are mounted with the negative allowance, in this case the diameter of diffusers should be 0.08 - 0.10 mm bigger than holes in casing, so after maching of the casing obtained hole geometry should not be bigger than customer's diffusers. Unfortunattely it was not the case. When we got right geometry of the seats, it turned out that provided diffusers are too small to install them and get right interference fit. We inform customer and got agreement to supply all new diffusers. Below we have the drawing of the new diffuser:wtorek, 3 marca 2009
reaction turbine overhaul - part three
Just now I would like to put here description of rotor overhaul.
Similar material condition we met on the carriers, that is why we recommended to exchange stationary blades and rotor's blades as well.
So, we left the rotor as it is and moved to the next ovehaul step - rotor's geometry measurement. To perform this step we needed a lathe. The rotor was installed on the machine and than we measure what is the run out. The measurement sheet is too big to put here but the biggets radial run out was 0.08 mm, it means still acceptable.
Next we started to remove the seal strips, as the old ones were damaged.
There are several areas of the seal strips on this rotor:
- front steam gland seal strips,
- balance piston seal strips,
- seal strips installed between the stages,
- internal steam gland between HP (high preassure) and IP (intermediate preassure),
- rear steam gland seal strips.
The seal strips have "l" shape and are fixed to the rotor with the wire, see the drawing below:
We can see only one seal strip before rotor's stage it was done in this way as we welded the shrounds with the tanon blade in the carriers. We wanted to avoid, the strip touches the welded place on the shround during turbine work. We could put two strips, machined them to have right diameter including proper radial clearance but in this case would be good to machine the welded shround, as well. It could create some machining problems, as shround was not welded on whole circumference. Finally it was faster to put only one strip without carrier machining. It was emergency repair ... .
It was the end of the manual job. The two last step in rotor overhaul were machining and low speed balancing.The machining was not so complicted. The rotor's journals, throust collar, oil guard surfaces were polish only. The longest part of the machining was the seal strips machining as there are a lot of strips and the machined diameters have to be quite accurate. I required +0/ -0.05 mm tolerance for seal strip diameters.
The last rotor overhaul step is the low speed balancing. We reduced the amount of weights from previous balancing and changes the angles a little bit.
Usually, we start repair activity with cleaning. In this case was the same - the rotor has been blasted and moved to next known step like NDT examination is.
It depends on the ovehaul scope what kind of NDT examination should be carried out. The most frequent are: Visual Testing VT, Magnetic Particle Testing MT(UV - in Ultraviolet light), Ultrasonic Testing UT.
However it is possible to perform more sophisticated test, like: Phase Array - to check if the blade groove is free from cracks without blades disassembly , Borasonic Test - to check the rotor from the inside trough the central hole (for impulse rotors) if it is free from cracks, defects. Results of this test can be used to define the rotor life). One of the quite modern methods of rotor's testing, but not only dedicated to rotors is Metal Magnetic Memory Method.
In ovehaul, which I led, the VT, MT and UT were used. Certainly, I was not the person who did it, as I do not have right training and certificates to carry out such testing.
The VT gave us a lot of info what is the current state of the rotor - corrosion/erosion is the main problem, let's see a few pictures, startin with the general rotor's view:
erosion presence
Similar material condition we met on the carriers, that is why we recommended to exchange stationary blades and rotor's blades as well.
So, we left the rotor as it is and moved to the next ovehaul step - rotor's geometry measurement. To perform this step we needed a lathe. The rotor was installed on the machine and than we measure what is the run out. The measurement sheet is too big to put here but the biggets radial run out was 0.08 mm, it means still acceptable.
Next we started to remove the seal strips, as the old ones were damaged.
There are several areas of the seal strips on this rotor:
- front steam gland seal strips,
- balance piston seal strips,
- seal strips installed between the stages,
- internal steam gland between HP (high preassure) and IP (intermediate preassure),
- rear steam gland seal strips.
The seal strips have "l" shape and are fixed to the rotor with the wire, see the drawing below:
We can see only one seal strip before rotor's stage it was done in this way as we welded the shrounds with the tanon blade in the carriers. We wanted to avoid, the strip touches the welded place on the shround during turbine work. We could put two strips, machined them to have right diameter including proper radial clearance but in this case would be good to machine the welded shround, as well. It could create some machining problems, as shround was not welded on whole circumference. Finally it was faster to put only one strip without carrier machining. It was emergency repair ... . It was the end of the manual job. The two last step in rotor overhaul were machining and low speed balancing.The machining was not so complicted. The rotor's journals, throust collar, oil guard surfaces were polish only. The longest part of the machining was the seal strips machining as there are a lot of strips and the machined diameters have to be quite accurate. I required +0/ -0.05 mm tolerance for seal strip diameters.
The last rotor overhaul step is the low speed balancing. We reduced the amount of weights from previous balancing and changes the angles a little bit.
The very last step before dispatch was to put corrosion protection on the rotor.
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