“I’m picking up bad vibrations. Bad, bad, bad, bad vibrations.” Oh, maybe that’s not exactly the way the Beach Boys tune goes. Now that you have that earworm, let’s get serious. My friend, John Terry with Canada Blower, has provided this excellent article on blower vibration. Enjoy!
The vibration generated by the fan is one of its most important technical characteristics. Vibration characteristics can be used to judge the quality of the design and manufacture of a product. Increased vibration may indicate improper installation of the fan, deterioration of its technical condition, etc.
For this reason, fan vibration is usually measured during mechanical run tests and during installation prior to commissioning. The vibration data of the fan is also used in the design of the fan support and the connected air ducting.
Fan vibration measurements can be expensive and sometimes far exceed the manufacturing cost of the product itself. Therefore, any restrictions on the values of individual discrete components of vibration or vibration parameters in frequency bands should be introduced only in cases where exceeding these values indicates a fan malfunction. The number of vibration measurement points should also be limited based on the intended use of the measurement results.
Increased vibration of the fan is one of the main causes of premature failure of units, parts, impeller, blades, bearing supports, couplings, destruction of the foundation and the fan itself as a whole.
Reasons for fan vibration are:
- shaft or fan wheel imbalance;
- misalignment of the drive;
- wear or damage to bearings;
- defects in the electromagnetic part of the drive or the electric motor.
The vibration level of the fans most accurately reflects the current technical condition of the fan, the quality of its assembly and installation. In other words, by monitoring the vibration level of the fan, it is possible to identify all of the above flaws and take timely measures to eliminate them, ensuring trouble-free operation of the fan.
A separate agreement may be made between the purchaser and the manufacturer for the installation conditions of the fan so that the factory test of the fan assembly takes into account the planned installation conditions in the field. In the absence of such agreement, restrictions on the type of substrate for factory tests are not established.
The fan manufacturer is responsible for balancing the fans. Balancing is usually carried out on highly sensitive specially designed balancing machines that allow an accurate estimate of the residual unbalance to be obtained.
A fan manufacturer can carry out balancing for several assembled elements at once which in addition to a wheel can include a shaft, a coupling, a pulley, etc.
If the fan is designed to operate from a variable speed drive, higher vibration levels are possible at other speeds due to the inevitable influence of resonances. It should be noted that stalling of the air flow can lead to increased levels of vibration. This is especially noticeable at large angles of the blade opening relative to the inlet air flow.
The vibration state of the fans after their installation is determined taking into account the rigidity of the support. When installed on large concrete foundations the support can usually be considered rigid and when installed on vibration isolators it is flexible. The steel frame on which fans are often installed can be considered either rigid or flexible. If in doubt about the type of fan support, calculations or tests can be performed to determine the first natural frequency of the system. In some cases the fan support should be considered rigid in one direction and flexible in the other.
The vibration of any fan on site depends not only on the quality of its balancing. Installation factors such as the mass and stiffness of the support system will have an impact, for example. Because the support system affects vibration, the fan manufacturer is normally not responsible for the vibration level of the fan at the site of operation.
The vibration of newly commissioned fans must not exceed the commissioning level. As the fan is in use, it should expect an increase in its vibration level due to wear processes and the cumulative effect of influencing factors.
When the vibration reaches the “warning” level it is necessary to investigate the reasons for the increase in vibration and determine measures to reduce it. Fan operation in this state should be under constant supervision and limited by the time required to determine measures to eliminate the causes of increased vibration.
If the vibration level reaches the “stop” level, measures to eliminate the causes of increased vibration must be taken immediately. Otherwise, the fan must be stopped. A delay in bringing the vibration level to an acceptable level can lead to bearing damage, cracks in the rotor and in the welded areas of the fan housing and ultimately destruction of the fan.
When evaluating the vibration state of the fan, the vibration level changes over time should be monitored. A sudden change in vibration level indicates the need for immediate inspection and maintenance of the fan. When monitoring vibration changes, transients such as grease changes or maintenance procedures should not be taken into account.
In addition to blower wheels, fans include other rotating elements that can affect the vibration level of the fan: drive pulleys, belts, couplings, rotors of electric motors or other drive devices. If the order requires a fan without a drive unit it may not be practical for the manufacturer to test the entire assembly to determine vibration levels. In this case, even if the manufacturer balances the fan wheel, there is no assurance that the fan assembly will run smoothly until the fan shaft is connected to the drive and the entire machine is vibration tested when the fan is put into service.
Usually, additional balancing is required after assembly to reduce vibration to an acceptable level. Vibration measurements are recommended for new fans before being put into service. This will allow you to establish a baseline and chart further maintenance actions.
The measuring instruments and balancing machines used must be verified and meet the requirements of the task. The period between verifications is determined by the recommendations of the manufacturer of measuring instruments. The condition of the measuring instruments should ensure their normal operation during the entire testing period.
The correct choice of support or fan base design is essential for smooth, trouble-free operation. A frame structure made of structural steel or a base made of reinforced concrete is used to ensure the alignment of rotating units when installing a fan. Sometimes an attempt to save money during the construction of a support leads to the inability to maintain the required alignment of the machine components. This is especially unacceptable in the case where vibration is sensitive to a change in the degree of alignment. This is particularly the case for machines consisting of separate parts connected together by metal fasteners.
The foundation on which the base is laid can also influence the vibration of the fan and motor. If the natural frequency of vibration of the foundation is close to the speed of the fan or motor the foundation will resonate during the operation of the fan. This can be found by taking vibration measurements at some distance from each other across the entire foundation, the surrounding floor and on the fan supports.
The personnel working with measuring instruments must have sufficient skills and experience to identify possible malfunctions and deterioration in the quality of the measuring instruments. The terms of the contract can stipulate that the customer be provided with a certificate of testing the fan for the quality of balancing.
Excitation of vibration can be caused by the interaction of the fan wheel with stationary structural elements; such as, guide vanes, electric motor or bearing supports, incorrectly selected clearances or improperly designed air inlet and outlet. A characteristic feature of these sources is the occurrence of periodic vibration associated with the wheel rotation frequency against the background of random fluctuations in the interaction of the wheel blades with air. If the above reasons lead to vibration of the blades its nature can be investigated by installing vibration sensors in different places of the structure.
The basic principle of vibration monitoring of machines is to observe the results of correctly planned measurements in order to be able to identify a tendency for an increase in vibration level and consider it from the point of view of possible problems. Control is applicable in those situations where damage develops slowly and the deterioration of the state of the mechanism manifests itself through physical signs available to measurement. .
The vibration of the fans which is a consequence of the development of physical defects can be monitored at certain specified intervals. If an increase in the vibration level is detected, observations should be performed more often so that a detailed analysis of the condition can be done. The reasons for the change in vibration can be detected at the same time. Analyzing the frequency composition of vibration makes it possible to determine the list of necessary measures and plan their implementation long before the damage reaches serious consequences.