Why Do Modal Testing During Operation?
By Maki Onari
One of MSI’s most essential troubleshooting methods for determining structural natural frequencies is known as Experimental Modal Analysis, or EMA for short. But is modal analysis the same for operating and non-operating equipment? Is one way better than another?
During a modal test, a modal impact hammer is used to excite the structure, while orthogonal triax accelerometers are placed in key stationary locations of interest around the machine, support, and foundation to identify the structural natural frequencies that could be in resonance with an internal excitation source. While modal analysis reveals important information, such as structural natural frequencies and their mode shapes, this is typically determined while the machine is at rest.
Modal Testing on Operating Equipment
MSI takes a very important step further to ensure that the structural resonance issues are representative of operation, and that rotor critical speeds can be determined while bearings, seals, and fluid forces are energized. This is accomplished by performing a specialized technique called Time Averaged Pulse (TAP™) while the equipment is operating in its actual problem condition. "Impossible," you say? MSI's Bill Marscher has been doing it since the 1980's. This method allows reliable identification of rotor lateral as well as structural natural frequencies on centrifugal pumps and other turbomachinery, without taking the machine offline.
MSI’s TAP™ method is more accurate than the traditional modal impact test for casing/ structural natural frequencies because it takes into account the internal water mass effects present in centrifugal pump applications (horizontally and vertically mounted pumps).
This technique is especially useful to identify the actual rotor lateral natural frequencies of horizontally mounted pump rotors. The rotordynamic characteristics of centrifugal pumps are strongly dependent on 1) rotor speed, 2) the resulting pressure across the sealing running clearances, and 3) the load-dependent bearing coefficients.
Specifically, this technique takes into account:
- the added mass effect of the pumped fluid entrapped within the impeller(s),
- the actual stiffness of the bearings,
- Lomakin effect at the wear rings (seal stiffness and damping), and
- the gyroscopic effect due to the rotational speed of the rotor.
For centrifugal compressors and steam turbines, the same rotordynamic characteristics apply, obviously except for the internal liquid mass effect within the impellers. In its place there is cross-coupled stiffness (which can act effectively as negative damping) provided by labyrinth seals. After all, natural frequencies are determined by the stiffness, mass, and damping. If the measured natural frequencies are not identified accurately by ensuring that stiffness, mass, and damping represent normal operating conditions, the End User may risk a skewed measurement, and the reliability of the unit.
As an added benefit, TAP™ identifies either rotor or structural natural frequencies without interfering with operation.
How TAP™ Works
MSI’s TAP™ technique incorporates time averaging statistics to rapidly improve the signal-to-noise ratio under operating conditions. Since the natural excitation from the pump is present, and its response competes with the response from the hammer impacts, this technique requires a large number of impacts in order to average-out and cancel the main and often strong excitation sources such as 1x rpm, 2x rpm, 1x VPF, 2x VPF, etc. Since the natural frequencies are usually in-line with the principal axes of the pump, this test is performed in three orthogonal directions (horizontal, vertical, and axial directions) by impacting the pump component, where the highest vibration was detected during observations while the machine operates without any impacting.
The statistics of time averaging reinforce any response signal coherent with the hammer impacts, and reduces the average amplitude of naturally excited vibration that occurs randomly relative to the data window triggering hammer impact. This greatly emphasizes the detected response to the “bump”, all while the machine continuously operates.
TAP™ in Action
The frequency response function (FRF) plots below depict modal analysis using the TAP™ method performed on an overhung pump rotor during operation, where the running speed was at 1770 rpm (29.5 Hz). The FRF plot recorded from one of the radial proximity probes shows noise and the main excitation at the 1x rpm frequency during the initial impacts on the coupling hub due to lack of averages from the time averaged analysis. The final FRF plot (after over 160\ impact averages) helped identify where the rotor lateral natural frequencies lie (in this case at 54 Hz, 102 Hz & 138 Hz), demonstrating the ability of a reasonable number of impacts over time to cancel out the main excitation source (1x rpm).
The multiple impacts are averaged to reveal clear natural frequencies.
Yes, we actually impact a rotating shaft! Note this machine is operating, so it is loud.
As an added benefit, TAP™ identifies either rotor or structural natural frequencies without interfering with operation. TAP™ can also be used to detect shifts in natural frequencies when some deterioration phenomenon is suspected, such as a cracked pedestal, or soft foot.
If you would like some examples of TAP™ in action, check out our Case Studies, and search for Experimental Modal Analysis. For more information about TAP™ or to request such services for your equipment, Contact Us today.