Maybe final update after some work on site and evaluation
After presentation of results to involved parties it seems to be mutually agreed that
existing vibration levels are acceptable without further hardware changes
I share hereby some excerpts of the report and some facts about the approach
But without detailed figures. If there is some more interest, I can share
Main concerns were stresses on shaft, bearing housing and bearing
-Analysis showed high vibrations and are mainly due to mechanical resonance of the bearing pedestal.
-The excitation force most likely comes from typical and inherent forces in the reciprocating compressor.
-These forces (coming from Compressor) are low and do not produce significant vibration on the compressor frame, cylinders, and piping.
-There is only a raised noise floor
-Reducing high frequency forces in the compressor may not be practical,
-The mechanical resonance of the bearing housing and pedestal is the root cause of high vibrations on the bearing housing on the non-drive end of the motor.
4.1.A stresses on the shaft,
-reconstructed overall orbit by combining 1X to 5X proximity probe readings
Figure 4.1.1 Overall shaft displacement (not included)
-vibration-induced stresses are not in the danger zone.
4.1.A.III Impact test interpretation
Figure 4.1.6 (not included)
-it is perceived that 25 Hz is a MNF
-There is a peak for all three planes at about 23.5 Hz
-160 Hz could be another MNF of the shaft for a higher mode.
-The mode at 220 Hz seems to be more dominant by the response from the pedestal, so 220 Hz is believed to be a MNF for the pedestal.
-very small response are observed at 125 Hz and 260 to 285 Hz on the shaft.
Figure 4.1.7 (rotating shaft barring device, with running lift oil pump, not included)
-peak observed at about 70 Hz bracket related to the bracket that these proximity probes are installed on.
-270 Hz related to the natural frequency of the pedestal,
4.1.b Stresses on pedestal and grout,
-field measurements show higher than guideline vibration for a bearing housing on a motor (as per Motor Vendor standards)
-unlikely that this level of displacement will lead to structural problems
-Figure 4.1.8 (not included)
4.1.C Stresses on the bearing (Babbitt)
-displacements are less than 1.5% of diametrical bearing clearance for frequencies higher than 3X run-speed).
-The Overall vibration is a reconstruction of the waveform by combining 1X to 5X vibrations.
displacement of the bearing is less than 30% of the diametrical clearance of the bearing. And based on the Hugh article found acceptable
Reference:
Estimating Allowable Shaft Vibration Limits for Fluid Film Journal Bearings, James D. McHugh,
Figure 4.1.9 –1X to 5X vibration of the shaft relative to the bearing housing (not included)
4.2.e Torsional Measurement
-The first TNF is measured to be at about 72 Hz. For the second mode, the measurements showed a wide peak between 180 Hz and 195 Hz.
-The torsional natural frequency (TNF) is predicated to be at 74.4 Hz and 192.6 Hz, as per Burckhardt
The Outcome:
Good: Measurements confirmed the predicted bending and torsional natural frequencies
Bad: Vibration is a complex thing
I collect here some general points
Lessons learned (technical)
-Pay more attention to design of motor bearing design (found resonance in bearing housing)
-Consider also higher order (3x, 5x) as possible relevant excitation of natural frequency of bending
-Consider the lowering effects on natural frequencies by lubrication
Lessons learned (contracting)
-Ask upfront or specify motor vendor vibration acceptance limits
-quantify and try to validate alarming messages coming form site as early as possible by qualified personnel
-consider limited understanding of vibration issues and possible misunderstandings of vibrations limits found in international standards like e.g. ISO 10816-1 on all involved parties
-stay focused also under pressure from management side
-try to reduce involved parties and personnel to the necessary technical experienced ones
