January 2010

Engineering Case Histories

Case 54: Is it motor vibration or some other cause?

It's not always the motor causing the vibration

Sofronas, A., Consulting Engineer

Vertical pump vibrations are usually written up by the operator as a motor vibration problem. It has been the writer's experience that 80% of the time it wasn't the motor vibrating at all. There are many causes for vibration problems (Fig. 1), but the first observation is usually made on the motor.

 

Vertical pump vibration reported as a vibrating motor problem.

 Fig. 1   

Vertical pump vibration reported as a vibrating motor problem.


Fig. 2 shows another reason why the motor can be reported as the vibration source. With only one measurement on the motor the value would be four mils. At 3,600 cpm, this would appear very rough. However, when data are taken on the vertical as shown, only one mil of vibration is on the motor, and none is on the foundation. All of the vibratory motion is taking place through the "I" beam structure. If this was a new installation, filling the structure with grout material could be required. However, if it is an old installation, loose anchor bolts may be the cause.

 

Motor vibration shown as problem with single measurement.

 Fig. 2   

Motor vibration shown as problem with single measurement.

Shutting the motor down and noticing how the system responds can be productive. Even with no vibration-monitoring equipment available, if the vibration disappears immediately when the power is removed this can indicate an electrical problem.

A "shudder" or vibration level increase and then decrease on shut down can reveal a resonance problem. Try to identify an external source of the vibration by repeating the startup and shut down, if safety isn't a concern.

A vibration level that drops as the system coasts down can signal several problems. Bent shafts, bad couplings or fouled impellers all cause imbalance forces that are reduced as the speed is reduced and come down as the square of the speed.1

With horizontal motors locked out, the motor and driven device can be rotated by hand. A heavy spot while turning could indicate misalignment.

Sometimes corrections haven't been made for thermal misalignment. The motor or driven equipment is supported in such a way that if the machines are not offset during cold alignment they will be out of alignment and vibrating when at operating temperature. When the hot alignment condition is not considered an indication of this is that the machine runs smoothly when first started but vibrates when hot. When a machine vibrates when it is cold but smoothes out when at operating temperature, the hot corrections were probably made.

Many times when there is motor vibration, usually on smaller motors, a "soft foot" can be the problem. This occurs when one of the motor feet pulls down more than the rest. It is generally thought that this causes internal misalignment which can result in vibration. When the suspect bolt is loosened the vibration drops significantly. Shimming of the "soft foot" is usually the remedy. The motor shouldn't be operated with the loose bolt, even if it runs smoother.

At one time the writer was on the platform with a 7,000 hp motor with vibration problems. As he contemplated the cause and leaned against the air intake filter screen, the vibration stopped. The cause was local resonance of a loose screen and shows the importance of a walk around.

There are many other causes of motor problems,2 however, a few rather unsophisticated checks may allow the plant specialist to save the time and the expense of an extended outage.  HP

LITERATURE CITED

1 Sofronas, A., Analytical Troubleshooting of Process Machinery and Pressure Vessels: Including Real-World Case Studies, (p. 35), ISBN: 0-471-73211-7, John Wiley & Sons.

2 Bloch, H. P. , Geitner, F. K., Machinery Failure Analysis and Troubleshooting, (p.343), ISBN 0-88415-662-1, Gulf Publishing Co.

 

 

 The author

Dr. Tony Sofronas, P.E., was worldwide lead mechanical engineer for ExxonMobil before his retirement. The case studies are from companies the writer has consulted for. Information on his books, seminars and consulting are available at the Web site http://www.mechanicalengineeringhelp.com.

 

 

The Author

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