The Value of Experimental Modal Analysis
By Maki Onari
Back in the day (before my time), using a shaker to perform modal analysis was a common tool to excite large structures in the field and determine their structural natural frequencies. However, the weight and size of these shakers are an issue for transportation, and a challenge to simply accommodate /mount it next to the asset to perform the job of exciting the subject unit. This extra effort is even more extensive to excite the equipment in three orthogonal directions (through a stinger) in order to identify all potential offending structural natural frequencies. The shaker is more convenient for a laboratory-controlled application if the equipment of concern is not operating.
In the last two decades, a modal impact hammer has become the more popular excitation tool, striking the rotating equipment casing in three orthogonal directions using different hammer tips to cover various frequency ranges. The hammer has been used to excite a rotating shaft as well as an entire building structure, such as a 1200 MW turbine deck floor in a power plant. The modal hammer can have an upper frequency range of around 6 to 8 kHz (depending on the hardness of the tip). While a piezoelectric shaker could excite frequencies as high as 80 kHz, most vibration issues on turbomachinery do not require such a high frequency excitation response. Note that at a high frequency the amplitude of vibration (in terms of displacement) becomes very low, while at lower frequencies the amplitude is much larger, and it is this larger motion that typically causes higher stresses (roughly directly proportional to displacement, resulting in potential fatigue) or tribological distress, in each case reducing the life span of the equipment.
At MSI, EMA (Experimental Modal Analysis) testing has been an essential troubleshooting tool that has helped engineers and technicians to identify the natural frequencies and the mode shapes of structural components of rotating machinery, as well as the rotor assembly. For example, MSI’s Time Average Pulse or TAP™ method is used to identify the rotor lateral modes on centrifugal pumps and other turbomachinery while they are in operation (provides sufficient signal-to-noise). This is subject for another Blog.
With the EMA test, when it is conducted in detail (i.e. with sufficient measurement locations), and with enough number of impacts per direction (averaging), it should be able to identify whether a structural natural frequency might be in resonance with an existing internal or external excitation source, while the equipment is at rest conditions or (if signal-to-noise can be made acceptable) while it is operating. In addition to identifying the frequency, the mode shape for each natural frequency can be clearly identified. Note that not every mode is expected to be offending, since this depends on the mode shape. Judgment from the troubleshooter should be used to determine whether one particular mode can be strongly excited or not. It depends on the location where the excitation source is taking place and the location where most of the activity occurs.
The first video below depicts the 1st cantilever mode of a vertical turbine pump (VTP) at 7.75 Hz, where most of the motion is occurring at the top of the motor. The second video shows the same pump indicating the 2nd bending mode of the above-ground structure at 47.8 Hz (“C” shape mode), where the main activity is taking place at the base of the motor or top of the discharge head. Based on their respective mode shape, it is expected that the first mode would be excited by the residual imbalance load of the motor rotor, and the second mode is more susceptible to being excited by misalignment between the motor and the pump upper shaft.
Some more examples of additional mode shapes at different frequencies.