ArticleHow to Fix Vibration Problems
by Tom Spettel
This paper is a series of six case histories that share a common theme of trying to identify and eliminate the cause of a machine’s persistent vibration problem(s). The case histories presented are both well-written and concise – a good combination. The problems identified range from foundation and base issues, coupling and alignment problems, fan blade resonance problems and compressor surge. In every case, other information such as machine design information, oil analysis results, finite element analysis results, alignment results or simply careful field inspections are used in conjunction with vibration data to finally identify and fix the vibration problem(s). This paper provides a first-hand look inside the logical thinking process necessary to identify and solve vibration problems.
“Case History 1: Boiler Primary Air Fan
A fan manufacturer reported a vibration problem on the non-drive end of a newly installed 4000 HP direct drive fan (Figure 1). Turning speed frequency (1x) vibration amplitudes of 25 mils (p-p) or more had been measured by the start-up technician. The vibration amplitudes were lower in the vertical and axial directions on the fan, and lower in all directions on the motor points. Numerous balance weights had been placed on the fan with no reported decrease in vibration. P-M-S-I was contracted to assist in balancing the fan.
- Prior to driving to the fan location, a telephone conversation with the on-site start-up technician provided the following information.
While this was a new installation, an identical fan was installed in the same plant and was running without excessive vibration for 6 months.
- No vibration plots were available, but the excessive lx turning speed frequency horizontal vibration, with low vertical and axial amplitudes, was confirmed.
- Installation of balance weights of similar magnitude as those used on the first fan did not have much effect on the vibration amplitudes.
- The field technician’s balancing procedures appeared to be technically correct.
On arrival at the job site, the following additional information was obtained.
- The identical fan was installed in the same building, but the new fan was in an annex to the building.
- After the lack of balancing success, several attempts to stiffen the fan bearing pedestal were attempted. Fabricated steel braces were bolted to the sides of the concrete pedestals, and “outrigger” braces between the top of the pedestal and the floor were added. Neither attempt was effective.
The fan supplier required limiting the fan operation to a maximum of 30 seconds!
A test plan was constructed that would meet the 30 second limit. This included:
- Full speed FFT measurements on the non-drive end fan pedestal in both the vertical and horizontal directions, using a 2-channel analyzer to acquire simultaneous data.
- Capture of coast down Bodé (1x amplitude and 1x phase versus turning speed) information.
The FFT data is shown in Figures 2 & 3. The magnitude of the horizontal vibration amplitude of 22 mils (p-p) suggests a resonance condition. Otherwise, the amount of force required to produce this level of vibration would be considerable. The low vertical amplitudes (2 mils p-p) and the lack of response to the balance weights reinforce the suspicion of a resonance condition.”
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