reaction turbine overhaul - part four

I am going to present what we did at outer upper part of the casing.

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: