wtorek, 29 marca 2011

the poor vaccum in condenser

A couple days ago we had problem related to poor vaccum in steam turbine condenser.
The vaccum was on the level of 81-82 kPa. The normal level is around 90 kPa. Low vaccum creates the unit load limit that is so important keeps it as designed.
Before the vaccum drop there had been a turbine trip. The machine was taken in service again but trough the some moment the LP bypass got pressurized and the LP bypass pipeline shook.  The turbine was operated under stable condition but the vaccum did not come to previous, required level.
All vaccum pumps were taken into service but it did not help, in paralel we started to search for the reason.
The fitters found the manhole withe air ingress. The manhole is located on the LP bypass pipe just before the connection of it with condenser shell. It was sealed, the screws tightened (some of them were loose). The vaccum is improved now almost 90 kPa.

piątek, 11 marca 2011

The BFP mechanical seals coolers

The unit we operate and maintain is equipped with 3 boiler feed pumps.
Two of them are turbine driven and the third one is motor driven.
To prevent feed water going out of the pump the mechanical seal is in place.
The temperature of the feed water is 160 C deg (at full load - 600 MW it can be even 180 C deg)
The temp of the BFP casing is 136/132 C deg (lower part/upper part respectively).
The original desing of the BFP include the cooling circulation of the mechanical seals:
The reason of this cooling is simple the mechanical seals OEM put the remark in seals manual that seals working temp should not be higher than 80 C deg (at 90 the manual pump trip should be applied).
The eraction team has done some modification in the cooling system and what we have on site is:

  There is no cooling provided and the working temp. of mechanical seals is 50 C deg than it was designed. Definitely, it is going to affect seals lift time. Our proposal is to put the coolers back in place according to OEM design.
We will see what will  happen

niedziela, 27 lutego 2011

Came back

It has been long time when I put here last post.
There is a reason for this. It can be said that I am on the opposite side just now. Why?
Yeah, previously I was a part of company offering the unique and specialized services for power plants. It was great job ... . Anyway I felt that time that my knowledge is not complete. Yes, I dealt with steam turbines and a number of technical issues related to them but I was still out of the place where the steam turbines are used daily - the power plant.
Just now I work for company which run power plant, quite large ... 4 x 600 MW.
So, I am still within power industry but in the other corner. It is not the same what I did before but can almost touch the turbine while it is working.
The blog is going to change accordingly. Still it is dedicated to the steam turbines but more to their daily maintenance than overhauls but sooner or later the overhauls will come.

poniedziałek, 24 maja 2010

The blade groove crack removal

In March I wrote a couple of words about rotors which had cracks in blade groove.
The picture showed an example. Since that time these cracks have been removed.
It had not been easy to get rid off cracks located in such places like the egdes of blade groove. To do it, a special cutting tools were needed.
I am not a machining specialist but I have dealt with various machining issues and that is why I managed to create the cutting tools and proper machining technology to get rotors free of cracks.
Below, is a cut section of blade groove.
The shape in the edge 9 and 7 is what we got after machining. A lack of cracks was confirmed by NDE.
The cutting tools, we used :


The two pictures above show the tool used in first step of machining. To get shape from the first drawing the next tool was necessary


It is almost impossible to take pictures showing what is the blade groove geometry right now, anyway the machining path is visible and the rest is the same like on the drawing on the top of this post

wtorek, 6 kwietnia 2010

The calculation on rotor disc and the locking blade


During previous overhaul seasons (summer 2009) I supervised the repair of certain HP rotor. Having informed that first stage blades are cracked, the customer had decided to replace them.
It is not complicated to carry out this job but there are some steps critical which done improperly can cause disaster. Frankly speaking, reparing steam turbines, there is allways a must to think what would be the results of some "shorcuts".
That is why, the experience is very important. Especially, a blading work is one of the most sensitive. Take into account that the turbine rotor works in exceptional conditions: high temperature (~ 550 C deg), high pressure and forces created by elements weight, a rotor rotation (a couple of thousand rpm's), etc ... .
Anyway, the first stage consist of 52 blade sets, each set contains two blades welded together. Two sets are the locking sets. The locking sets are fixed to the disc using 4 pins. The core of reblading activity is to remove the pins. Usually, the pins are removed by boring them and here we have the most dangerous part of whole procedure. The pins boring have to be done in this way that the holes diameter in disc will be the same after boring - the holes clean up is acceptable but the diameter must not be bigger than certain value. A material thickness on the rotor disc in pins mounting area determines the pin holes diameters.
It can happen that after a few repairs of this rotor stage the pin holes are an ovals or were not bored perpendicular to the rotor disc.
Than it is technical correct to bore new holes but they will be bigger but as mentioned above holes can be increased only to some level as the stresses are important.
This conditions were met during a project supervised by me.
We had ovality on pin holes and some ot them were not perpendicular to the rotor disc.
I defined what would be the diameters of new holes and asked my colleague to perform stress calculation if disc material left is enough to carry forces.
He used FEA method to do it. I am not familiar with this but it is common for design dept. around the world.
Fortunatelly, it turned out that new holes diameters are ok and we act in this way.
Below, there are a couples of pictures presenting MES calculation.






środa, 17 marca 2010

IP rotor after fact finding activity

One of the IP rotor of two 150 MW units is in very bad condition.
The run- out measurement revealed the rotor curvature.
What we measured is showed on the chart above. The curve is the rotor's axis.
The rotor is bent and max run- out 0.23 mm is located behind 1 stage.
There are two most often applicable ways to get rid off curvature.
The first one is to machine the rotor and remove a "plus indications" so the rotor would be machined only at certain angle if material thickness to be removed was no more than 0.23 mm.
I have not seen such solution so far. The solution, we usually use is to define new rotor axis. In this particular case the new fix points sholud be a journal at IP-LP coupling and place when max run-out is measured. Next, the HP end of the rotor would be machined.
We have done such machinings but for the rotors which were not bent so much. At max indications ~ 0.15 mm. In fact the internal tensions are still present and can cause further axis deformation. We have noticed this process.
The right technical solution applied in this case is to perform thermal streightening and after this repair step to machine rotor.
The final decision will be taken shortly on the customer meeting.
The rotor curvature is not the only problem we found.
The rotor blade groove edges are cracked. Allways rotor cracks are very hard task to deal with.We have found different kinds of cracks.
1. The cracks inside the blade groove


2. The blade groove edges cracks.


czwartek, 11 lutego 2010

IP rotor reblading next steps

The inserts are removed, so the next step is to put out the blade which is in the middle. To do it it is necessary to move adjacent blades.

The insert was removed

The adjacent blades were moved clockwise and counterclockwise

and the middle blade is being released now


The middle blade/locking blade was removed

The middle/locking blade


The teeth are on the both side of the locking blades, on the picture above only the convex blade side teeth are shown but on the concave blade side the teeth are present as well.
When the middle/locking blade is out of the groove the next blades can be put out.
The set of locking blades is shown below. On the right hand side is the locking balde (the teeth are visible from both blde side). The blades with numbers are these ones which adhere to the locking blade form rigth and left hand side.

The locking set in removed, so just now the rest of the blades will be dissasembled. It takes some time as it has to be done piece by piece,





The rest stages were rebladed in exactly the same way.