DATASTICK PDA BASED SPECTRUM ANALYZERS
CTC VIBRATION ANALYSIS HARDWARE
SOLUTIONS TO CRITICAL FREQUENCY DISTURBANCES
OF
ROTATING MACHINERY
Abstract
The development of Variable Frequency Drives (referred to as VFD’s) brought about opportunities of cost savings and performance, but has also produced undesirable operating conditions leading to:
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Electrical and Mechanical failure resulting in "Down-Time" due to insufficient or inadequate design
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Loss of Production
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Interruption of Service
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Costs of repair of equipment (many times at costly shipping and/or premium labor rates)
CRITICAL FREQUENCIES
Before successful operation of Variable Frequency Drive units, confirmation or testing of mechanical condition and corrective measures must be taken to eliminate the cause and results of this dynamic phenomenon of “CRITICAL FREQUENCIES”
If this is done on a timely basis, you will prevent premature failure or catastrophic failure of the machinery due to critical frequency excitations producing undesirable vibration.
VFD operating programs will allow for frequency disabling or frequency rejection of these critical frequencies once they are identified.
We will attempt to cover every possible step to evaluate and eliminate these “Critical Frequency” responses in your rotating machinery, in order to provide you with safe and reliable operation.
CRITICAL SPEEDS
All turbo equipment and variable frequency driven equipment operate at certain speeds where the forcing or self-exciting frequencies are sympathetic (at synchronous or harmonic frequencies) to the resonance’s of a rotor, bearing or support structure. When this resonance occurs at a finite operating speed, it is referred to as a “critical frequency”.
Critical speed analysis is very important and can become quite complex. Severe amplification at these critical’s can wreck a rotor either from shear or reverse bending stresses or fatigue from large deflections, i.e. rotor rubs.
Equipment builders have a real challenge since there are certain percentages of design speed which are undesirable for dynamic operation and produce undo strain and wear due to various anomalies “out of spec” conditions such as:
· Dynamic unbalance,
· Poorly aligned components,
· Size of supporting structural mass or inadequate stiffness,
· Mechanical Looseness.
Conversely equipment operators have a real challenge in identifying and eliminating these operational critical speeds for machinery that was designed for one or two speed operation.
To think that one can operate these single speed designed machines in a dynamic variable speed condition without addressing the problem of critical frequency disturbances is unrealistic and dangerous to the machines’ health and machine operator.
Understanding the many things that can affect operational critical speeds is mind boggling. Many engineers specializing in rotor dynamics have spent the major part of their career studying this one area. There are so many variables—aerodynamic loading, gyroscoptic’s, skewed elements, rotor assembly friction, asymmetric bearings, bowed rotors, friction rubs, etc. Any or most any combination can greatly aggravate the situation.
Retrofit / refurbish overall structure as well as system (air, water, steam etc).
· Properly adjust belts and pulleys to correct any angular and parallel misalignment as well as proper tension to facilitate smooth driving forces.
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Check alignment with dial indicator or preferably with "precision" laser type
· Adjust isolation springs of the unit’s structural supporting base to alleviate a bouncing rhythm or grounding or hitting floor produced by the drive belts and pulleys and an extended motor base which may act as a spring board. (You may need to shim the overall base so as to negate or cancel out the spring isolation sometimes due to improper air flow).
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check flexible transitions for proper clearances
· Provide gussets on steel channel support base to dampen the fulcrum effect of the extended motor base.
· If the unit produces undesirable vibration amplitudes occurring at the rotational frequency, or RPM of the blower wheel, the blower wheel should be dynamically balanced.
Critical Frequency Rejection Program
If all the above and possibly more corrective measures are taken and the critical frequency's are not eliminated then proceed to variable frequency drive's program for the rejection of critical frequencies. (More often than not you will find you can eliminate these critical frequencies with mechanical corrections, therefore allowing full operational speed parameters and system flow).
1. The unit should be analyzed at normal or predominant operating speed. Vibration amplitudes should be recorded at the vibration checkpoints (bearing locations).
2. The checkpoints with the largest vibration amplitudes should be pinpointed for further observation during run-up and run-down tests (all planes of direction are preferred).
3. After test points are located, the accelerometer transducer should be placed in the predominant vibration amplitude plane of direction of the designated bearing.
4. The unit should be started and vibration amplitudes should be recorded at approximately 5% speed increases until 100% operational speed is accomplished. Each level of speed should be allowed for stabilization of vibration level before moving to the next speed. (Some vibration analyzers/collectors have a coast up and coast down program for recording a waterfall display for identifying disturbed or elevated frequencies).
5. The unit should then be slowed down in 5% decreases and the above process should be repeated.
6. When the preceding steps are finished, the data should be analyzed for the disturbing critical frequency.
7. After the critical frequency or frequencies are pinpointed, proceed with Critical Frequency Rejection Program to ensure safe operation.
LIBRARY
SOLUTIONS TO CRITICAL FREQUENCIES