Alain Pellegrino, a predictive maintenance technician with Emerson’s local business partner Laurentide Controls, has a great article in ReliablePlant.com on resonance in plant equipment. The article, How to identify, correct a resonance condition, describes how resonance is a common cause in excessive machine vibration.
Resonant frequencies are something most all engineers face whether it is in electrical circuits, atomic bonds, process piping, road overpasses, or rotating machinery to name a few examples. Alain defines resonance:
…the result of an external force vibrating at the same frequency as the natural frequency of a system. Natural frequency is a characteristic of every machine, structure and even animals.
He describes techniques to identify resonant frequencies in plant equipment. The first is a simple, impact test, which is to strike the equipment being measured with a mass and measure the response. The mass delivers a small amount of force over a wide range of frequencies. The measurement occurs over the frequency range and identifies the frequencies where vibrations occur.
A more advanced test uses an “instrumented hammer.” This has an accelerometer at one end of the hammer. A second sensor is on the piece of equipment being measured. Alain explains that you measure:
…the force induced to the system by the instrumented hammer and the response at different frequencies. When the phase shifts by 90 degrees, the frequency at which it occurs is a natural frequency.
This test not only can spot resonance problems, but additional ones such as imbalance, misalignment, and looseness. All of these conditions decrease the life of the equipment and can lead to unplanned downtime.
Another test Alain explains is the “coast down peak hold” which monitors the vibration level from operating region to shutdown. Without resonance, the expected vibration level drops at a steady rate. Otherwise:
If the vibration levels start rising at any time while the equipment is being shut down, the speed at which the amplitudes increase is a possible natural frequency.
A more sophisticated version of this test, coast down peak phase, monitors both vibration level and its phase shift while the equipment shuts down. This helps find the natural frequency, which is in the middle of the 180-degree phase shift.
So what do you do if your equipment has a resonant frequency somewhere in its operating range? Alain explains that natural frequency is a function of the equipment’s stiffness and mass. To modify the natural frequency:
…either change the stiffness or the mass. Increasing the mass or lowering the stiffness will lower the natural frequency while reducing mass or increasing stiffness will increase natural frequency.
I know of some welders from my days offshore in the Gulf of Mexico who would be excited at the opportunity to weld on some additional mass or stiffener brackets, but these solutions are not always possible or practical. If you can do it, the easiest way is to change the operating speed 20-30% from the natural frequency.
Another possible solution Alain describes:
…install a dynamic absorber on the equipment to significantly reduce the vibration levels of the equipment. The dynamic absorber is a spring-mass system that is installed in series with the resonant system to create an out-of-phase exciting force to effectively counteract the initial exciting force.
Alain sums up his thoughts by stressing the need to use at least two of the tests to identify and confirm the resonant frequencies before taking action to impact the mass, stiffness, operating speed, or vibration absorption.