Emerson VILTER VSS Installation, Operation And Service Manual page 170

Appendix C • Vibration Measurements - Single Screw Compressor
Figure C-2. Main Rotor Cross-Section VSS/VSMC Compressors
Continuous and Non-continuous Monitoring
While it is common practice on large or critical machin-
ery to have installed instrumentation for continuous on-
line monitoring of vibration values at key measurement
points, this is not necessarily carried out in industrial
applications.
Changes in unbalance, bearing performance, alignment,
etc. can be detected with sufficient reliability from peri-
odic measurements with permanently installed or hand-
held instruments. The use of computers for trend analy-
sis and warning against malfunctions is also becoming
more common.
C − 2
VSS/VSR/VSM/VSH/VSSH Compressor • Installation, Operation and Service Manual • Emerson • 35391S
Operational Conditions
Measurements shall be carried out when the compres-
sor has reached normal steady-state operating tem-
peratures and with the machine running under specified
conditions. If the measured vibration is greater than the
acceptance criteria allowed and an excessive background
vibration is suspected, measurements should be made
with the machine shut down to determine the degree
of external influence. If the vibration with the machine
stationary exceeds 25% of the value measured when the
machine is running, corrective action may be necessary
to reduce the effect of the background vibration.