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Appendix C • Vibration Measurements - Single Screw Compressor
Evaluation Zone limits
The values for the zone boundaries given below are
based on the maximum broad-band values of velocity
and displacement when measurements are taken from
two orthogonally oriented radial transducers. Therefore
when using these tables, the higher of each of the values
measured from the two transducers in each measure-
ment plane should be used. When the maximum mea-
sured values of velocity and displacement are compared
to the corresponding values in the table, the severity
zone which is most restrictive shall apply.
Operational limits
For long-term operation, it is common practice to es-
tablish operational vibration limits. These limits take the
form of ALARM and TRIP set points.
Support
*RMS= 0.707 X peak (sine wave only)
Setting of ALARMS
The ALARM values may vary considerably, up or down,
for different machines. The values chosen will normally
be set relative to a baseline value determined from expe-
rience for the measurement position or direction for that
particular machine.
It is recommended that the ALARM value should be set
higher than the baseline by an amount equal to 25%
of the upper limit for zone B. If the baseline is low, the
ALARM may be below zone C.
Where there is no established baseline (for example
with a new machine) the initial ALARM setting should be
based either on experience with other similar machines
or relative to agreed acceptance values. After a period of
time, the steady-state baseline value will be established
and the ALARM setting should be adjusted accordingly.
It is recommended that the ALARM value should not
C − 4
VIBRATION MEASUREMENTS – SINGLE SCREW COMPRESSOR*
RMS Displacement
ZONE
μmm
A
0-30
Class
B
30-57
C
57-90
D
Above 90 Above 3.55 Above 7.1 Above .28
Table C-1. Vibration Zone Values
VSS/VSM/VSH/VSSH • Installation, Operation and Maintenance Manual • Emerson • 35391SD
ALARM: To provide a warning that a defined value of
vibration has been reached or a significant change has
occurred, at which remedial action may be necessary.
In general, if an ALARM situation occurs, operation can
continue for a period while investigations are carried out
to identify the reason for the change in vibration and de-
fine any remedial action.
TRIP: To specify the magnitude of vibration beyond
which further operation of the machine may cause dam-
age. If the TRIP value is exceeded, immediate action
should be taken to reduce the vibration or the machine
should be shut down.
Different operational limits, reflecting differences in dy-
namic loading and support stiffness, may be specified
for different measurement positions and directions.
RMS Velocity
mils
mm/s
0-1.15
0-2.3
1.15-2.25
2.3-4.5
2.25-3.55
4.5-7.1
normally exceed 1.25 times the upper limit of zone B.
If the steady-state baseline changes (for example after a
machine overhaul), the ALARM setting should be revised
accordingly.
Setting of TRIPS
The TRIP values will generally relate to the mechanical in-
tegrity of the machine and be dependent on any specific
design features which have been introduced to enable
the machine to withstand abnormal dynamic forces. The
values used will, therefore, generally be the same for all
machines of similar design and would not normally be
related to the steady-state baseline value used for set-
ting ALARMS.
There may, however, be differences for machines of dif-
ferent designs and it is not possible to have clear guide-
lines for absolute TRIP values. In general, the TRIP value
will be within zone C or D, but it is recommended that
In/sec
0-.09
.09-.18
.18-.28
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