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In Figure 4, the points shown above have been plotted on a scaled sketch of the turbo machinery train. Any convenient scale may be used. In this example, it is assumed the turbine will remain stationary and the compressor will be shimmed to achieve the desired alignment. From the “Turbine to Compressor” data, it can be seen that the centerline of the compressor shaft is .005” (one half of the TIR reading) above the centerline of the turbine shaft for the point at which the indicator readings were taken. Now referring to the “Compressor to Turbine” data, it can be seen the centerline of the turbine shaft is .012” below the centerline of the compressor shaft for the point at which the indicator readings were taken. With these two points determined, the position of the compressor shaft relative to the turbine shaft is fixed. An extension of a line through these two points gives a graphical representation of the alignment situation and indicates the shimming necessary to achieve the desired alignment. It is also at this point in the alignment procedure that the offset required due to the steam end rise be greater than the exhaust end rise must also be taken into consideration.

In this example, the vertical plane has been plotted. Alignment in the horizontal plane is obtained in the same manner. On next page more

Turbine to Compressor and  Compressor to Turbine For ease of plotting, change each pair of numbers to “zero” and a positive number. Since diametrically opposed numbers represent only a deviation from an arbitrary reference, a given amount may be added to both sides without affecting the meaning. On the “Turbine to Compressor” readings, for example, 10 is added to the vertical pair of numbers while 8 is added to the horizontal pair. The other sets are handled in a similar manner. The absence of negative numbers reduces errors in plotting.

This data obtained is adequate to determine relative shaft positions. Record the readings and indicate precisely what they represent. As they are shown above, the readings shown as “Turbine to Compressor” indicates the alignment bracket is on the turbine shaft and the indicator stem rests upon the compressor shaft.

Make any corrections for the deflection of the alignment bracket. For this example, we will assume .004” TIR deflection. To make the correction for this error, simply add the TIR reading (in mils) to the bottom number in each set.

The indicator is “zeroed” on top and the following readings are observed as the shafts are rotated at 90-degree intervals.

A similar procedure is used with the complementary bracket and the indicator yields to following:

 

Use of the “Reverse Indicator” method eliminates the requirement for removing the coupling spacer in a majority of cases. This reduces the wear and tear on the coupling. The error caused by coupling hub run-out is entirely eliminated since both shafts turn as a unit (with spacer installed), and angular misalignment is greatly magnified and more precisely diagnosed. Since face readings are eliminated, there is no concern about axial float.

 The single problem that carries over to the “Reverse Indicator” method which was present in the “Face O.D.” method is that of deflection in the alignment bracket. This problem can be readily handled by building the bracket with substantial material to prevent droop or by determining the deflection in the alignment fixture and making the appropriate corrections in the alignment data.

 

For the purpose of example, Figure 3 shows a simple turbine-compressor to be aligned. It is desired to place the compressor shaft .008” higher than the turbine to accommodate for calculated thermal growth of the two machines. The alignment bracket is clamped to the turbine shaft with the indicator stem resting upon the compressor shaft (or the rigid portion of the coupling hub).

To eliminate inaccuracies in geometry of the coupling hub, turning of both shafts simultaneously such that the indicator readings are taken always at the same place on the hub will give more precision. This precaution can be difficult on larger equipment and may not be possible.

Face measurements taken by this method must have the axial float of the two shafts accounted for. Axial movement must be taken into account when turning the shafts on equipment with hydrodynamic thrust bearings or no thrust bearings. (Small machinery utilizing ball bearings will not encounter any axial float.) The shafts must have axial positions rechecked each time a reading is taken.

 

Similarly during horizontal alignment of the turbine the off set (equivalent to the radial clearance in the input bearing of the gear box installed between driven equipment and turbine) should be kept between centre line of the  turbine shaft and the  centre line of the gear box due to the tendency of the gear box shaft movement out side under load condition.

 

One additional element that must be accounted for is the difference between the exhaust end shaft rise and the steam end shaft rise. Typically, the steam end will rise more than the exhaust end due to higher temperatures. This will result in the desire to have the coupling face “open” at the top. The calculated shaft rise for both the steam and exhaust end can be found on the outline drawing for the  turbine.

 

Care must also be taken in the brackets that are used to hold the dial indicators. “Universal” or makeshift brackets contrived on the spur of the moment can give inaccurate readings, which can lead to improper alignments. Especially when the spans between the shafts are quite long, care must be taken to assure that the bracket being utilized is stiff enough that it will not deflect under its own  weight.

 

 A step up in accuracy and the elimination of some of the problems mentioned above with the “Face O.D.” would be to utilize the “Reverse Indicator” method as illustrated in Figure 2. This method employs indicator readings taken on the outside diameter of the coupling hubs or shaft only. The sketch indicates two brackets used simultaneously, which is normally the preferred method. One bracket could be utilized by switching back and forth for each set of readings but this is far less convenient.

Many methods can be utilized to perform cold  alignment of Turbine. The method described below is that of using dial indicators. While the method of employing dial indicators is an old and well established technique, it is not without problems. Some of them are

 

1.Inaccuracies in the  geometry of  the coupling hubs.

2.Axial float of the two shafts

3.Radial clearances of the bearings.

4.Difference between exhaust end shaft rise and steam end shaft rise

5.Sag in the brackets used for holding dial indicators.

 

The most widely used of the traditional alignment methods is commonly referred to as the “Face- O.D.” method as illustrated in Figure1. As shown in the figure, a bracket is attached to one shaft and extends to the proximity of the coupling hub on the adjacent shaft. Dial indicators are affixed to this bracket with the stem of one indicator resting on the face of the coupling hub and the stem of the other indicator resting on the outside diameter of the same hub. The offset of the shafts is determined by the “O.D.” readings, while angularity of the shafts is determined by the “face” readings. It is suggested that the proper distance between the shafts be established before any alignment method is used in order to fix the coupling without disturbing the alignment. This spacing dimension can be found  on  the coupling drawing.