Necessity of shims in installation of compressor

[realit]

We are going to install a centrifugal compressor. We are planning to do it without using any shims above the soleplates. My question is "is it advisable/necessary to have a minimum few shims below the foot? If yes, then what amount of shims thickness is required?"

 

[zeusfaber]

I'm assuming that these shims are there to achieve alignment between compressor and prime mover.

The ideal number of shims will depend on the extent of misalignment you get if you don't use any.

The number you can get away with will depend on the ideal number and on the degree of misalignment your compressor-coupling-prime mover combination can tolerate. They're all different.

A.

 

[zdas04]

If the mounting surface exactly matches the mounting plates, then you need zero shims. If it is a small bit off, leaving out the shims is a really bad idea. Gas turbines turn kind of briskly. You spin a mass that fast and the shaft has a tendency to become very very straight. A fraction of a mm of flexure in the frame can cause the rotor to touch the rotor with pretty dramatic effect. A goal of making the mounting surface perfect is not an unreasonable goal. Changing the definition of "perfect" to "nearly perfect" can lead to a bad outcome.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[realit]

Thanks for your replies.
The shims i am talking about are for levelling of machine skid while installing.
We will definitely be using shims if we don't get readings below 0.05mm/m in Engineer's Master Level.
But, assume that we got the required levelling without using any shims. Then also, is it advisable to have 2 or 3mm shims on every soleplates? Will shims produce any,( for that mater any )spring properties that will do away with very minor misalignments/vibrations during Normal operations?

 

[JJPellin]

You question is unclear. You initially state that the shims you are considering leaving out are above the soleplate and below the machine feet. Then you indicate that the shims are used to level the machine during installation, suggesting you are talking about shims below the baseplate. Your terminology seems to be different than I am used to.

The base-plate of a new machine needs to be installed within specifications for level prior to grouting. This can be accomplished using shims or jack-screws. We preferentially use jack-screws. Shims used for leveling the base-plate would either be trapped under the grout or would be positioned in cavities where they could be removed after the grout has set. Once the base-plate is grouted, it would be verified to be flat and level. Assuming, it is within specifications, the driven machine would be final-aligned to the driver. If your machine is skid-mounted and not grouted, it may be bolted directly to a steel structure. At that point, shims would only be needed if necessary to place the base at the correct elevation and within specifications for flatness and level.

The base-plate would normally be designed for shims and they are necessary. For machines that I am responsible for (typically API process centrifugal gas compressors), there would be shims under the driver but no shims under the compressor. API specifies a nominal shim thickness for new machines. I don’t recall if it is 0.100 inch or 0.125 inch. In any case, it would be impossible to achieve the specified coupling alignment without any shims. If your base-plate was not designed for shims, or, if it was designed for equal shims under both the driver and driven machines, I would still recommend installing them. Even if the base is perfectly flat and perfectly level today, that will not be true forever. Slight movement will occur no matter how well the base is designed, built and installed. In the future, it will be necessary to make moves to restore the specified coupling alignment. The shims will allow that to be achieved. I would place a minimum of 0.100” shim under the machine that is most likely to be the moveable machine during future coupling alignments. Depending on the base design, this may make it necessary to also install shims under the other machine (compressor or driver). Install the shims at the location specified by the equipment manufacturer. Use shims of the proper size, as specified by the equipment manufacturer.

Properly designed shims do not provide any spring effect. They do not compensate or correct for vibration. They simply allow you to achieve the necessary coupling alignment without having to field-machine your base or machine feet each time.


Johnny Pellin

 

[rotw]

Agree with JJPelin.
Centrifugal compressor is normally mounted on baseplate which is then installed on concrete/foundation (at grade or on table top), which is typical of onshore installation. Unless you have some unusual configuration. Guess offshore is again another story.

So I think the term "soleplates" is not relevant unless - for example - you are mounting the driver "directly" on the foundation, in which case you normally use soleplates. Some train configurations, especially with gearbox there can be an important torsional stiffness (for example a common baseplate will then be seriously twisted) ; in such situation it could be needed to re-inforce the structural stiffness of baseplate ; mounting the driver on soleplates for direct mounting on foundation is an alternative; on some other cases, drivers could not require any soleplates ; in fact the frame structure of the driver could serve the purpose of a soleplate and mounting directly on foundation is possible.

 

[realit]

Thank you JJPelin and rotw for your informative and clear replies. I got my doubts clear. Thanks a lot.