Emerson VILTER VSS Installation, Operation And Service Manual page 173

Appendix C • Vibration Measurements - Single Screw Compressor
the TRIP value should not exceed 1.25 times the upper
limit of zone C.
Vibration limits For piping and Tubing
The piping and tubing on the compressor units must be
supported with the appropriate brackets and supports
to minimize the vibration levels. These brackets and sup-
ports should also be strategically placed to prevent the
natural frequency from matching the normal operating
speed. The typical goal is to have the natural frequency
of an assembly to be at least 10% above or below the
operating speed. In the case when a compressor is oper-
ated by a VFD (Variable Frequency Drive), there is a high
probability that there will be an opportunity to match ei-
ther the natural frequency of the assembly or it's 2nd or
3rd order since the compressor's speed will vary within a
large range of RPM's.
The compressor unit should first be operated at ei-
ther the normal operating speed or if it utilizes a VFD,
through the operating range of speed. Visual observa-
tions of the vibration levels of all the piping and tubing
should first be observed. After this initial survey, the vi-
bration measurement equipment should be individually
mounted or attached to each specific piece of piping or
tubing as required in the location of what is perceived as
the maximum amplitude or worse vibration.
While there are references which specifically allow high-
er levels of vibration for piping, the goals for the vibra-
tion levels of the piping and tubing on the compressor
unit should still utilize the same criteria outlined in this
standard. If possible, the vibration levels should be with-
in the zone boundaries of Zone B or better. These values
will ensure that the stress levels in the piping and tubing
are acceptable for continuous operation. These values
provide guidelines for ensuring that gross deficiencies
or unrealistic requirements are avoided. In certain cases,
there may be specific features associated with a particu-
lar compressor unit which would require different zone
boundary values.
For compressors that are operated at a fixed speed,
brackets and supports may be added or moved to re-
duce the vibration levels of the specific component. If
the compressor is operated utilizing a VFD, a skip fre-
quency should be inputted into the controls to ensure
the compressor passes through the harmonic and that it
operates either below or above the operating speed that
matches the natural frequency of the specific compo-
nent. It is not unusual to have three or four skip frequen-
cies within the normal operating ranges of a compressor
utilizing a VFD.
C − 5
VSS/VSR/VSM/VSH/VSSH Compressor • Installation, Operation and Service Manual • Emerson • 35391S