Page 205 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Best Practice 3 .19

Vibration is the result of a system being acted on by an
excitation.
This excitation produces a dynamic force by the relationship:
F DYNAMIC =Ma
Where: M = Mass (Weight/g)
2
g = Acceleration due to gravity (386 IN/SEC )
2
a = Acceleration of mass M (IN/SEC )
Vibration can be:
Lateral _________ ↕ __________
Axial → _________________ ←
Torsional _____________________

Fig 3.19.8 Vibration Fig 3.19.10 Non contact displacement measuring system

and assumes the bearing bracket is fixed. Some systems in-
corporate an additional bearing bracket vibration monitor and - Application characteristics (prone to fouling, liquid,
thus record vibration relative to the earth or ‘seismic vibration’. unbalance, etc.)
As previously discussed, vibration limits are usually defined by: - Bearing clearance
- Speed
r ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
12000 - Rotor actual response (Bode Plot)
Vibration ðmils pepÞ¼
RPM - Rotor mode shapes (at critical and operating speeds)
This value represents the allowable shop acceptance level. Figure 3.19.11 presents a vibration severity chart with
API recommends alarm and trip shaft vibration levels should be recommended action, and a schematic of a shaft vibration and
set as follows: shaft displacement monitor are shown in Figure 3.19.12.
As mentioned above, vibration is measured unfiltered or
r ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
24000
V ALARM ¼ presents ‘overall vibration’. Figure 3.19.13 shows a vibration
RPM
r ffiffiffiffiffiffiffiffiffiffiffiffiffiffi signal in the unfiltered and filtered conditions. All vibration
36000 diagnostic work (troubleshooting) relies heavily on filtered
V TRIP ¼
RPM vibration to supply valuable information to determine its root
In my opinion, shaft vibration alarm and trip levels should be cause.
based on the following parameters as a minimum and should be Figure 3.19.14 presents an example of a radio tuner as an
discussed with the machinery vendor prior to establishing analogy to a filtered vibration signal. By observing the pre-
levels: dominant filtered frequencies in any overall (unfiltered) vibra-
tion signal, valuable information can be gained to add in the
- Application (critical or general purpose) troubleshooting procedure and thus define the root cause of the
- Potential loss of revenue problem.

Fig 3.19.9 Excitation forces with
examples

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