Page 82 - Compression Machinery for Oil and Gas

72 SECTION II Types of Equipment

Other Related Phenomena
While unbalance response and rotordynamic stability are commonly covered in a
lateralanalysis,otherphenomenacancauselateralvibrationproblems.Theseaddi-
tional topics are listed here for reference and are not discussed in detail. Lateral
response or vibration can be influenced by other bearing-related issues, including
oilwhirlwithinfixedarcjournalbearings,andwiped/damagedjournalbearingsur-
faces.Off-designcompressoroperationcanalsoimpactlateralvibration,including
rotating stall and surge. In addition, turbulence in the flow field can cause a lateral
vibration response through an unsteady aerodynamic excitation. Mechanical
issues, such as looseness on the rotor, looseness on stator components, misalign-
ment, rubs, and shaft cracks can also cause lateral vibration issues.
In summary, lateral rotordynamics must be considered for all centrifugal
compressors operating in the oil and gas industry, where modern analysis tools
and techniques minimize the risk of encountering a critical speed or stability
problem on new equipment. It is emphasized that a proper lateral rotordynamic
analysis is performed during the design phase where modifications can be easily
implemented, compared to costly trouble-shooting and retrofit solutions in
the field.

Torsional Rotordynamics
The avoidance of torsional dynamic problems can be difficult since nearly all
machinery trains lack a torsional measurement system. Proper torsional analysis
is therefore critically important.
Torsional evaluations generally involve a steady-state analysis, preparation
of interference diagram(s), forced response analyses, and transient analyses (if
necessary and applicable). These analyses are often conducted for trains involv-
ing compressors, pumps, motors, turbines, or engines, and any associated rotat-
ing components such as gearbox shafts and couplings.
The steady-state torsional analysis involves preparation of a torsional mass-
elastic model derived from manufacturer provided drawings and mass elastic
information for the equipment involved. The steady-state torsional natural fre-
quencies (critical speeds) and mode shapes are then calculated. The mode
shapes are plotted to graphically as shown in Fig. 3.37 to show the deflection
associated with the torsional natural frequencies. This allows for investigation
of controlling stiffness, and relative modal participation of the major inertias in
the system.
An interference diagram (Campbell diagram) similar to that shown in
Fig. 3.38 is prepared to graphically display prevalent excitation energy orders
versus speed and frequency. The specified operating speed range(s) are super-
imposed on the interference diagram, and any coincidences between calculated
critical speeds and prevalent excitation energy in the system are identified.
The likelihood of exciting the critical speeds involved in any such coincidences