The analysis identifies natural frequencies of the torsional dynamic system and compares them with excitations from the motor (or engine) and compressor. This is frequency domain analysis.
The only way to account for transients (misfire, start-up, VFD operation) or response to other forces is by using a ‘time domain’ type analysis. More often called time transient analysis it takes into account the timing of the torque bursts produced by the driver, and their relative timing with respect to the compressor torque demand cycles. It also analyzes the behavior of the compressor/driver train under acceleration and deceleration.
In performing the time domain analysis, the operating characteristics of the driver and of the compressor are used as sources of torque excitation and as contributors to the balance of energy in the system. This level of detail is only possible with the time domain analysis. This form of force response analysis determines whether there is enough damping in the system and whether allowable stress levels are exceeded during operation.
Re-staging with new cylinders results in changes to the torque levels imposed on the shaft by the cylinders as well as changes to the response of the shaft itself. A system with different torsional characteristics is therefore created. It is recommended that an analysis be carried out on the new system to ensure the long-term reliability of the compressor train.
•Complete report with natural frequencies and modal shapes. Verification of stress levels on the shaft to comply with acceptable limits. Check of adequacy of couplings, dampers and flywheels.
•Sizing recommendations are provided for the flywheel and/or the coupling in case of misfits. Verification that peak vibrations are within manufacturer’s specified limits.