The information value of the animation was extraordinary.  It showed clearly that the exciter was acting as a vibration shaker, drumming the floor at 30 Hz, with the turbine deck responding with a “rug-flapping” mode at 30 Hz as well.  The undulating motion of the turbine deck drove an acoustic mode of the major portion of the switchgear floor-level, the air “glowing” red and blue (for clarity, not plotted in the figure above left) at acoustic antinode points in concert with the motion of the floor wave-shape and motion.  In turn, the switchgear cabinet wall panels pulsed in and out in concert with the acoustic pressure pulsation next to the cabinet, the acoustic pulsation and cabinet response all occurring at 30 Hz. 

Once this combined structural/ acoustic “Operating Deflection Shape” (ODS) was discerned from the test results, impact modal testing was performed on the various machines and floor of the turbine deck, as well as on the switchgear cabinet panels.  The impact testing was all performed with low enough energy to not harm or trip the equipment, and was performed while the equipment remained operating, using MSI’s Time-Averaged Pulse (TAP™) technique.  The modal testing confirmed that the switchgear high-vibration panels, the turbine deck, and the generator exciter all had natural frequencies close to 30 Hz, and pulses from a loud speaker confirmed the acoustic natural frequency of the switchgear room/ zone at 30 Hz.

An in-detail ODS of the exciter as well as the modal testing showed that its motion was a back-and-forth rocking motion.  A structural natural frequency at approximately 30 Hz was identified as having tuned into resonance as one of the feet of the exciter had become “soft” with time.  The resulting exciter motion, driven by typically acceptable levels of residual imbalance, excited the turbine deck resonance, which excited the switchgear floor-level acoustic resonance, which excited the switchgear panel resonances, which were damaging the switchgear bearings.  Only detailed, careful combined acoustic and vibration ODS and modal testing could have unraveled this complex mystery.  Once the root cause was known, a practical solution could be identified.

The combined acoustic – structural resonant vibration in the 4 kV switchgear was addressed by reducing the excitation at its source, the exciter.  In-service structural bypass of the loose foot de-tuned the natural frequency from the 30 Hz excitation, dramatically reduced switchgear vibration, and eliminated the switchgear reliability issues without the need to pre-maturely shut the plant down, saving millions of dollars in revenue.

Back to Top

Urea Screener Failure Problems

MSI was called in by an end-user to help troubleshoot a newly commissioned critical compressor train experiencing unexplained vibration trips. These sudden vibration trips threatened to jeopardize delivery of product and thereby result in a substantial loss of revenue. The combined service six-stage (three plus three) integrally geared compressor was installed in an industrial gas facility supplying a major petrochemical plant. The compressor was electric motor driven with three stages for high-pressure nitrogen compression and three stages for high-pressure dry air compression. The compressor was commissioned with bearing pad temperatures that were operating continuously over 200°F. Subsequent bearing inspections showed excessive oil coking and wear.

The OEM attempted to modify the bearings to reduce the pad temperatures but this was unsuccessful due to unacceptably high vibration amplitudes. Following the vibration trips MSI traveled to the facility on very short notice and began testing.  The data collection involved a series of process and bearing oil tests to recreate the trip phenomena. MSI was able to conclusively identify the problem as a rotordynamic instability caused by a strong subsynchronous excitation of a cantilevered rotor natural frequency located at 47 percent of running speed (refer to figure). Rotordynamic analysis suggested that the subsynchronous excitation was due to labyrinth seal cross coupling effects. Based on the recommendation of MSI, the manufacturer installed swirl breaks to decrease the subsynchronous vibration excitation. This eliminated the rotordynamic instability and allowed the bearings to be modified in order to reduce bearing pad temperatures. Following the installation of swirl breaks the machine has not experienced a vibration related trip.

Because of the successful field testing and redesign work, the end customer has used MSI for subsequent upfront design audit work on other integrally geared compressor trains.   MSI utilizes its state-of-the-art rotordynamic and FEA codes along with years of design audit experience to help identify possible problems.  This experience base includes the use of empirical stability criteria such as the Fulton/Sood Stability Map. Use of this screening criteria would have identified the instability seen in this particular compressor (refer to figure).  MSI’s audit and field testing expertise has made it a leader in successful commissioning of critical compressor trains.  

Note: The results of this successful testing were summarized during a lecture at the 30^th Turbomachinery Symposium of Texas A&M (2001).

Back to Top