Element Generating Vibration in rotary lobe blower :
1. Blower Inherent characteristic
a. Impacting bearing loads excite component/system natural frequency.
b. Pressure pulsations set up vibration at four times running speed
2. Rotary lobe blower has very close clearances between the impeller and the housing. The impeller contact will
setup vibrations as follows :
a. Impeller to impeller frontal lobe contact-if contact is between only one set of lobes, the vibration frequency
will be 1x rpm, if both sets of lobes contact, the vibration frequency will be 2x rpm.
b. Impeller to cylinder contact-the vibration frequency will depend on the number of impeller tips contacting
the cylinder which could range from one to four times the RPM.
c. Impeller to head plate contact-the vibration frequency will be erratic and unsteady.
3. Damaged gears will generate vibrations at gear mesh frequency (GMF)
4. Damaged gear will generate vibrations at bearing frequency (BPFI,BPFO, FTF, BSF)
5. Rotor unbalance and bent shaft will generate vibrations at 1x rpm.
6. Blower/driver coupling misalignment will generate vibrations at 1x RPM and 2xRPM
7. Acoustic resonance in the blower inlet/discharge piping will generate vibrations at 4x RPM.
8. Operation of rotary blower at or near system torsional may cause impeller lobe contact and increase vibration
9. External piping if not properly isolated will transmit vibrations into the blower
10. Foundation design and method of mounting has considerable effect on blower vibrations.
Aug 12, 2010
Aug 11, 2010
Lubrication Problem (Grease)
This was happened on 15 kW motor that drive three stages centrifugal compressor. During routine measurement there where no evidence of any problem on the motor (figure 1).
figure 1
22 July 2010, compressor was tripped because motor was overloaded, mechanical inspection found leaks at compressor cooler. During the inspection period, motor solo run test was done to make sure that there is nothing wrong with the motor.
Motor was noisy, like something was rubbing especially at NDE position. TWF show different pattern than routine measurement (figure 2).
Figure 2.
It was decided to open bearing cover and re-grease the bearing. We found that bearing housing cavity was filled with grease. We recommend to clean bearing cavity and re-grease the bearing. The vibration spectrum after re-greasing is shown on figure 3 below :
Figure 3. Vibration spectrum after re-greasing.
Failed Overhung Fan Shop Balancing
Shop balancing was conducted after impeller repair. Running test was carried on the result shown on table 1 was amazed everyone.
The higest vibration was 32 mm/s rms. Dominated frequency was 1x running speed, the question was “ are they really balanced this equipment?”.
Balancing document review shows that the shop consider the impeller as centerhung fan, although residual unbalance was still on tolerance for grade 2.5.
Then it was decided to rebalanced the impeller (on site), and the final result vibration drop from 33 mm/s to 2.3 mm/s.
Note : There are different treatment in balancing procedure for centerhung and overhung rotor. Failure on recognition will cause unacceptable balancing result.
Aug 10, 2010
Tranducer Mounting Precautions
There some rules of how to mount the transducer to ensure you get representative data :
1. Vibration readings must be taken on a surface stiff enough that it is not affected by the pressure exerted by the probe or accelerometer.
2. All vibration reading should be taken with the transducer perpendicular to the surface of interest.
3. Vibration signals containing high frequencies must be taken with the transducer tightly screwed to the surface, as hand pressure can not hold it tightly enough to the surface for it to correctly follow high frequency
4. Magnetically mounting the transducer is preferable to a hand-held reading, but not as good as a hard mount as far as high frequency response. The magnet will rock if put on a curved surface, reducing its usable frequency range.
1. Vibration readings must be taken on a surface stiff enough that it is not affected by the pressure exerted by the probe or accelerometer.
2. All vibration reading should be taken with the transducer perpendicular to the surface of interest.
3. Vibration signals containing high frequencies must be taken with the transducer tightly screwed to the surface, as hand pressure can not hold it tightly enough to the surface for it to correctly follow high frequency
4. Magnetically mounting the transducer is preferable to a hand-held reading, but not as good as a hard mount as far as high frequency response. The magnet will rock if put on a curved surface, reducing its usable frequency range.
Vacuum Pump Suction Valve Blockage Identified by Infrared Camera.
Operation department reported increasing condenser vacuum. Condenser vacuum should be maintained at -710 mmHg. In the last three days, the highest vacuum was only -691 mm Hg. This condition was causing decreased turbine and overall power plant efficiency.
Condenser vacuum on this power plant were maintained by one vacuum pump ( one vacuum pump as redundant). The analysis was focused on this equipment. Infrared camera was used to identified any abnormality on the piping that connect condenser with this equipment. And it was found that some portion of the suction piping was 8 oC cooler than the other section.
Condenser vacuum on this power plant were maintained by one vacuum pump ( one vacuum pump as redundant). The analysis was focused on this equipment. Infrared camera was used to identified any abnormality on the piping that connect condenser with this equipment. And it was found that some portion of the suction piping was 8 oC cooler than the other section.
The coolest section was after suction valve. It was suspected that the suction valve was blocked or partially opened and causing restricted flow of gasses.Recommendation was made to inspect suction valve opening/condition.
Inspection by mechanic verify that suction valve was only partially opened. Infrared scanning after inspection show the temperature was distributed evenly on all section and condenser vacuum rise to 705 mm Hg.
Benefit of predicitve Maintenance
Predictive maintenance is based on comprehensive measurements of mechanical condition. The ability of minimizing breakdown failure and giving direction to maintenance action and repair are the benefits of Predictivemaintenance. From the financial perspective Predictive maintenance is less costly compared with preventive maintenance and corrective/reactive maintenance. A major return in predictive maintenance result in maximizing production availability and minimizing the risks, cost and amount of production lost resulting from unexpected shutdown or failure.
Preventive maintenance also known as time base maintenance is type of maintenance that scheduled at specified time interval, it doesn’t matter whether the equipments are showing sign of defect or not . This kind of maintenance required inspection and sometimes replacement of some parts. Thus , preventivemaintenance typically result in unnecessarily replacing components in good condition. The worst part is when machine that in good condition disassembled for preventive maintenance and returned to service in poorer condition.
Reactive maintenance usually result in “fire fighting” where the problem causing failure are never corrected . Additionally outright may also lead to secondary failure. For example, misalignment problem could lead to bearing damaged which, in turn, causes costly damage to the bearing housing and shaft and possibly unscheduled shutdown.
Preventive maintenance also known as time base maintenance is type of maintenance that scheduled at specified time interval, it doesn’t matter whether the equipments are showing sign of defect or not . This kind of maintenance required inspection and sometimes replacement of some parts. Thus , preventivemaintenance typically result in unnecessarily replacing components in good condition. The worst part is when machine that in good condition disassembled for preventive maintenance and returned to service in poorer condition.
Reactive maintenance usually result in “fire fighting” where the problem causing failure are never corrected . Additionally outright may also lead to secondary failure. For example, misalignment problem could lead to bearing damaged which, in turn, causes costly damage to the bearing housing and shaft and possibly unscheduled shutdown.
Apr 1, 2010
NEW BEARING FAIL 2 DAYS AFTER INSTALATION CAUSED BY INPROPER BEARING FIT
NEW BEARING FAIL 2 DAYS AFTER INSTALLATION CAUSED BY IMPROPER BEARING FIT.
During power plant annual inspection, PdM department gave a recommendation to replace motor bearing and repair bearing housing dimension. The job was done by third party in a week and a acceptance test (no load condition) was conducted . Vibration spectrum and PV from acceptance test is shown in fig 1.
The data was taken at 7 July. From vibration spectrum, it was noted some activity at high frequency which usually corresponded to bearing defect, but since it’s magnitude was still lower than acceptance limit criteria it was decided to pass and put it into operation.
9 July , operator reported very high noise from this machine. Vibration spectrum and PV is shown in picture 2
It was noted increased noise floor at vibration spectrum and PeakVue. BPFO and BPFI also appear on vibration spectrum. Then it was decided to draw it from service and install new bearing. During inboard bearing disassembly, it was found that bearing housing diameter was 0.5 mm smaller than it should be. Heavily marked path pattern was identified in raceways. The probable cause was smaller dimension of bearing housing make bearing internal clearance get to tight and cause high load at he bearing. This is way the bearing only last for 2 days.
During power plant annual inspection, PdM department gave a recommendation to replace motor bearing and repair bearing housing dimension. The job was done by third party in a week and a acceptance test (no load condition) was conducted . Vibration spectrum and PV from acceptance test is shown in fig 1.
Fig 1. Vibration spectrum and PV from inboard bearing from acceptance test
The data was taken at 7 July. From vibration spectrum, it was noted some activity at high frequency which usually corresponded to bearing defect, but since it’s magnitude was still lower than acceptance limit criteria it was decided to pass and put it into operation.
9 July , operator reported very high noise from this machine. Vibration spectrum and PV is shown in picture 2
Fig 2. Vibration spectrum and PV after two days in service
It was noted increased noise floor at vibration spectrum and PeakVue. BPFO and BPFI also appear on vibration spectrum. Then it was decided to draw it from service and install new bearing. During inboard bearing disassembly, it was found that bearing housing diameter was 0.5 mm smaller than it should be. Heavily marked path pattern was identified in raceways. The probable cause was smaller dimension of bearing housing make bearing internal clearance get to tight and cause high load at he bearing. This is way the bearing only last for 2 days.
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