Misalignment is a major source of vibration in rotating machines. Plant maintenance personnel that have responsibility for these machines and their satisfactory performance will have to deal with alignment issues at some point in time. Misalignment often causes the following machine problems.
INDUSTRIAL SCREW COMPRESSOR MAINTENANCE
Precision Shaft Alignment
Precision Shaft Alignment
- Increase in energy cost.
- Increase vibration will shorten the machine’s life.
- Possible repetitive shaft seal failure.
- Repetitive motor bearing failure.
- Coupling damage.
For plant maintenance technicians, it should be noted there are many factors why a machine can develop alignment problems. Curing of pedestal concrete, piping stress, frequent starting of the compressor, or rust on the alignment shims are but a few of the reasons. Therefore, by establishing routine alignment checks into your maintenance program will help ensure longer machine operation with more reliability.
When to Check Alignment
- 3 months after new installation.
- Every time after bearing change out (Motor or compressor.)
- When experiencing premature and frequent mechanical shaft seal failure.
- When experiencing frequent motor bearing failure.
- When vibration data shows a sign of misalignment.
Alignment Procedure
Typical alignment procedures from screw compressor manufactures are posted in the Technical Information-Alignment section. (These procedures are for reference use only. Contact the manufacturer for the updated procedure.) Typical alignment procedures from screw compressor manufactures are posted in the Technical Information-Alignment section. (These procedures are for reference use only. Contact the manufacturer for the updated procedure.)
Saving Energy Cost by Precision Alignment
It has been reported that precision alignment can save power consumption by 1% to 3%. There is also a conflicting study report that finds no significant correlation between misalignment and machine efficiency. As well, there has been no study done indicating the energy savings for screw compressors used in refrigeration and process gas that have utilized precision alignment. However, it is not an overstatement that precision alignment can save energy consumption by reducing mechanical inefficiency. Below is a calculation for the annual dollar savings of a compressor with 350HP motor.
Yearly Savings = (Motor H.P. x 0.746) x (1/Efficiency) x Power Cost x Average Annual Operating hour x % saving.
For 350 HP motor running 6000 annual operating hours in San Antonio, TX, yearly savings will be calculated as below.
Yearly Savings = (350 x 0.746) x (1/0.95) x $0.0613/kwh x 6000 x 0.02=$2,021/yearly savings.
High vibration due to misalignment and machine life. The Estimated time a machine will fail due to misalignment.
| Misalignment (Mil/Inch) | Estimated time to failure |
| 0.2 | 250 months |
| 1 | 60 months |
| 5 | 10 months |
| 10 | 6 months |
| 50 | 2 months |
| 100 | 0.6 months |
| Note Misalignment is mil/Inch (Milradian.) 6″ of distance between shafts and 0.2 mil/Inch tolerance is equal to 1.2/1000″ of offset at the center of the shafts. |
|
Shaft Seal Failure and Misalignment
There is no proven study as to the relationship between shaft seal failure and misalignment for screw compressors. However, a majority of screw compressors with vibration caused by misalignment will vibrate more in axial direction. This may cause the shaft seal to fail prematurely. It is reported that more than 5/1000″ in T.I.R of misalignment will cause more frequent shaft seal failure in screw compressors.
Motor Bearing Failure Due to Misalignment
Bearing Life (L10 life) for deep grooved single row spherical ball bearing used in the motor can be calculated as follow.
L10 life= (Bearing Rating/Bearing Load)3
Therefore, if the bearing load increases due to mechanical stress by misalignment, it will reduce bearing life by factor powered by 3.e.g.
10% increase in bearing load=75.1% of bearing life left than normal load.
20% increase in bearing load=57.8% of bearing life left than normal load.
50% increase in bearing load=29.6% of bearing life left than normal load.
This calculation is based on zero internal clearance in the bearing. If the internal clearance increases due to mechanical stress, it will further reduce bearing life in accordance with the actual clearance.
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