Dresser Roots DVJ WHISPAIR, Installation Operation & Maintenance

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DVJ WHISPAIR
™
Dry Exhauster | 15
Rotary Lobe Blower Vibrations
The general vibration severity charts derived from Rathbone
vibration severity charts provide guidelines for machines
basically having mass unbalance-turbomachinery, electric
motors, etc. The German specification VDI 2056 - Criteria
for Assessing Mechanical Vibrations of Machines - provides
vibration guide lines for machines with rotating masses
(turbomachinery) and machines having mass effects which
cannot be balanced (reciprocating machines), but does
not specifically address rotary lobe blowers (also known as
ROOTS Blowers) with inherent fluctuating dynamic bearing
loads and torques.
API Standard 619, Rotary Type Positive Displacement
Compressors for General Refinery Services, limits the
vibration level to 0.1 in/sec peak, which is quite ambitious.
Based on experience, practical acceptable vibration levels
lie somewhere between API 619 requirement and VDI 2056
allowance for group D reciprocating machines.
Elements Generating Vibrations in Rotary Lobe Blower:
1. Blower inherent characteristic -
a. I mpacting bearing loads excite component/system
natural frequencies.
b. Pressure pulsations set up vibrations at four times the
running speed.
2. Rotary lobe blowers use very close clearances between
the impellers and the housing. The impeller contact will
setup vibrations as follows:
a. Impeller to impeller frontal lobe contact - if contact is
between only one set of lobes, the vibration frequency
will be 1XRPM, if both sets of lobes contact, the
vibration frequency will be 2X RPM.
b. Impeller to cylinder contact - the vibration frequency
will depend on the number of impeller tips contacting
the cylinder which could range from one to four times
the RPM.
c. Impeller to head plate contact - the vibration
frequency will be erratic and unsteady.
3. Damaged gears will generate vibrations at mesh
frequency, number of teeth times RPM.
4. Damaged bearings will generate vibrations at ball pass
frequency, fundamental train frequency and ball spin
frequency.
5. Rotor unbalance and bent shaft will generate vibrations
at 1XRPM.
6. Blower/driver coupling misalignment will generate
vibrations at 1XRPM and 2XRPM.
7. Acoustic resonance in the blower inlet/discharge piping
will generate vibrations at 4XRPM.
8. Operation of rotary lobe blower at or near system tor-
sionals may cause impeller lobe contact and increases
vibrations.
9. External piping if not properly isolated will transmit
vibrations into the blower.
10. Foundation design and method of mounting has
considerable effect on blower vibrations.
Vibration Criteria:
1. Units of measurement: Rotary lobe blower vibrations
are measured in inches/sec. Measurements of spike
energy is not recommended for judging blower condition
because the rotary lobe blower has inherent impacting
bearing loads.
2. Measurement location: Vibrations should be measured
at the bearing locations on the housing.
The following table provides an appropriate assessment
guideline for rotary lobe blowers rigidly mounted on the
stiff foundations.
Unfiltered Vibrations Assessment
(in/sec peak)
>0.62 thru 1.0 Satisfactory
>1.0 Review Required
If the blower is operating at “review required” levels then the
installation must be fully evaluated to determine the source or
cause of vibration and the cause shall be corrected.
In general, blower vibration levels should be monitored on a
regular basis and the vibration trend observed for progressive
or sudden change in level. If such a change occurs, the
cause should be determined through spectral analysis.
The blower vibrations will be transmitted into the motor,
speed reducer etc. and more so if they are mounted on the
common blower baseplate. Allowable vibration levels into
these accessories should be obtained from the vendors.