reaction turbine overhaul - part three

Just now I would like to put here description of rotor overhaul.

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.