Centrifugal Compressor Alignment - Best Practices

[planck121]

Hello,
I have heard conflicting opinions on machinery alignment best practices of centrifugal machinery. Is it acceptable to shim under a centrifugal compressor if one needs too or should the alignment be corrected at the driver (motor or turbine or gearbox if there is one)when normal misalignment concerns arise during operation. API 686 recommends that shims should typically be placed on the movable machine but does not really explicitly mention not to use shims under the non movable machine (compressor). It seems that as per 5.4.2.5 in API 686 centerline mounted machinery can be shimmed as well.

To me it seems if the requirement arises we are free to shim under the compressor (non movable machine) without any explicit mention to NOT do so....

Thoughts are much appreciated.

 

[Artisi]

Shim under the motor.
There is a valid technical reason for this - check the pump centreline to mounting pad tolerance and do the same for motor - all should become clear.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[MintJulep]

Not to mention that wires are more flexible than pipes.

 

[JJPellin]

In my industry (oil refining) the response would tend to depend on the configuration and manufacturer. For a Motor-Gearbox-Compressor arrangement, the typical recommendation from the manufacturer would be to shim the compressor to align it to the gearbox and shim the motor to align it to the gearbox. Since the compressor is hotter on one end than the other, it will require an angularity when aligned cold. You generally do not want to shim a gearbox unevenly. This would distort the gear-case and cause tooth contact problems. If any shims need to be installed under the gearbox, they would necessarily be uniform all the way around.

We have about forty large compressor trains. I believe that all of them are made to accommodate shims under the compressor. There are specific requirements depending on the foot arrangement. Some have large key-blocks below the center of the shaft with sleeved bolts to allow for thermal growth. Only install shims where the manufacturer intended and make appropriate adjustments to the bolt sleeves or other alignment mechanisms.

As noted in the replies above, pipe strain is a concern. It would be preferred to complete the alignment with the compressor piping disconnected. After the initial alignment is complete, the movement of the machine would be monitored as the piping is reconnected. Our specifications do not allow for any more than 0.002 inch movement at the coupling hub in any direction. If the movement is greater than that, then the pipe strain is excessive and must be corrected.

For a Turbine-Compressor arrangement, it is preferred to move the turbine. But, since the compressor is probably an arrangement that can be sold with a motor driver, it is also probably designed to accommodate shims. But, as noted above, it would be better to make the moves under the turbine. It is likely that the compressor is doweled or keyed which can make the moves more complicated.

This question is best asked of the manufacturer of the specific compressor. I know that shim moves are commonly allowed for Elliott, Dresser-Rand, Clark, Carrier, Roots and others.


Johnny Pellin