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1 . Closing of inlet flow valves while centrifugal fan is still in working condition. So as inlet flow valve is closed the air is restricted to enter into centrifugal fan. Although centrifugal fan still tries to push the available air in casing radially outwards. So this continuous work done by fan on air causes drop in pressure at inlet while the pressure at outlet of centrifugal fan increases. This pressure difference between inlet and outlet of fan causes backflow of air (outlet to inlet). In response again the fan tries to push the air to outlet creating vacuum at inlet and again backflow of air takes place. This cycle keeps on repeating. This is also known as surge (Complete disruption in flow). This is the main reason for pulsation observed in centrifugal fan. This also leads to high level of vibration in the overall installed structure.
2 . Every mechanical system has a natural frequency. And when the excitation frequency matches with natural frequency, it can lead to high vibration. In this case as it is mentioned that rotor is balanced and also no damping system is mention to exist in component, so most likely one of the natural frequency matches with the operating speed. This seems to be the main reason leading to high vibrations
3. It is mentioned that belt drive is used to run the fan. Also in later part of the question it is mentioned that high axial and rocking motion is observed in the system. This indicated there is the possibility of angular or offset misalignment between motor pulley and fan shaft pulley. This misalignment can lead to rising and setting back of belt in the pulley. This rising and setting back of belts imparts the unbalance force on fan shaft. This unbalance is said to be causing high vibration. High vibration measurements thus gives the evidence, that it might be the probable cause. Depending upon the speed ratio between pulleys with misalignment would lead to harmonics of vibrations. (Rao & Gupta 2007)
4 . In this system, air enters axially and leaves radially. Hence this change in direction will lead to axial reaction. But the bearing used is with spherical balls which is not capable to resisting axial thrust. As a result of which failure marks are visible on the shaft near bearing Also looking at schematics, fan is overhanging (cantilever type), Hence air reaction on fan, will cause the unbalance leading to pitching and rolling modes
5 . With all the above mentioned probable causes of vibration, could lead to loosening of foundation bolts. After certain loosening, bolts are more susceptible to failure in high cycle fatigue. Also due to vibration of bolt the grout would chip-off from foundation (Rao & Gupta 2007).
6 . During run-down, speed of the shaft reduces continuously and so does the excitation frequency. Hence during this event excitation frequency will cross the natural frequency leading to high vibrations and creating the knocking sound. Also stalling of the fan can further lead to such noise during run down.
1 . Do not close the inlet valve of the centrifugal fan until it is completely shut down. Install a pressure probe to sense the pulsation in pressure along with machine tripping system. This system will automatically stop the machine when it goes into stall or surge. (Rao J, 2004)
2 . I would also check if the operating condition is matching with the natural frequency of system. To check that I will do a small test. I would recommend to install the tri-axial accelerometer and vary the speed of the centrifugal fan (Start-up) and over speed to may be 15%. With this speed sweep, I will be able to get the natural frequency and check if any of those are in zone of operating condition. If yes, I would add the damping system. Like a rubber sheet/ vibration isolators between foundation and machine. (Rao J, 2004)
3 . As mention above, belt misalignment may also lead to vibration issues. I would carefully check the angular and offset misalignment and further correct and fix if any of these issues is observed.
4 . Also if there are multiple belts used for drive. I will make sure that they are all in correct and equal tension. If some belts are tight and remaining are loose it would lead to slippage and unnecessary noise
5 . We need to also add an additional bearing in rotor system which can either take-up the thrust, or replace spherical bearing with tapered roller bearing. Tapper roller bearing can take both axial and radial forces, hence the issue of vibration might not arise if the bearings are fixed (Rao & Gupta 2007).
6 . Also looking at the current schematics the current setup looks like an overhanging system. Therefore in order to avoid the vibration issue further, I would try to add one more bearing on the free end of the shaft to make the system look like simple supported beam case.
7 . I would add appropriate washers (rubber and metal) to keep the bolt tight. Also it should be noted that failure of bolt is a caused due to high vibration of the fan. If the level of vibrations are controlled, loosening or failure of bolt will not occur under any circumstances.
8 . Finally to conclude, as mentioned earlier that natural frequency might cross during run-down leading to high vibration. So adding a damper system mentioned in point 2 will help to resolve the vibration issue. Also as this issue is transient it will not be an issue if the fan vibrates in the given scenarios with all above issues fixed.
1 . Rao, J. S., & Gupta, J., K. (2007). Introductory course on theory and practice of mechanical vibrations, Delhi IN.: NEW AGE INTERNATIONAL PUBLISHER
2 . Rao, J. S. (2004). Rotor dynamics, Delhi IN.: NEW AGE INTERNATIONAL PUBLISHER
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Uniresearchers is a leading team of researchers in the field of academic writing. With the track record of delivering 500+ high quality dissertations and 2500+ essays, assignments and coursework’s Uniresearchers has always tried to keep up with the expectations of our clients.
