Rotordynamic Analysis of Rotating Machinery

Summary

Rotating machinery (e.g. compressor, turbine, electric motor) is subject to cyclic vibrating force such as unbalance force. Some torque oscillation is also inevitable in the train with reciprocating compressor or electric motor. In case the lateral or torsional natural frequency (lateral critical speed / torsional critical speed) of rotor system coincides with the frequency of such excitation force, excessive lateral or torsional vibration will result due to resonance, which may severely disturb plant operation or may lead to machinery damage.
Machinery manufacturer is responsible to carry out lateral and torsional rotordynamic analyses in order to investigate and mitigate the risk of such resonance problems. However, under special circumstances, it is occasionally needed for TOYO to also carry out such analyses in order to ensure design soundness or in order to investigate counter-measures against vibration problems.

Application Example of Lateral Rotordynamic Analysis

During commissioning of a centrifugal compressor driven by gas turbine via speed reducing gear, unexpectedly high level of shaft vibration occurred, which prevented the plant start-up. This problem happened even though design integrity of each machine had been confirmed by the shop test.
From the field vibration measurement data, unbalance vibration was suspected as the most probable root cause. Field balancing (i.e. balance correction in the field by adding counter-weights) was therefore tried numerous times, but the vibration level did not improve. In order to investigate the reasons behind the failed field balancing, train lateral rotordynamic analysis consisting of gas turbine rotor / coupling / pinion was carried out. This analysis suggested that the effectiveness of the field balancing could be much improved by using different balancing plane. By carrying out another field balancing using the plane which was indicated as most effective from the rotordynamic analysis, vibration level dramatically improved, and it eventually enabled the plant’s rated operation.
As illustrated in this example, lateral rotordynamic analysis technology can be a very powerful tool in swiftly solving the field vibration problem.
TOYO utilizes such vibration analysis technologies to ensure flawless engineering executions and swift field trouble shootings.

Fig.1 Lateral Rotordynamic Analysis Example (Response vibration of the turbine / gear train when unbalance is assumed at the coupling adopter)

Fig.1 Lateral Rotordynamic Analysis Example (Response vibration of the turbine / gear train when unbalance is assumed at the coupling adopter)

Application Example of Torsional Rotordynamic Analysis

In a reciprocating compressor train, mechanical damage was detected during the plant’s commercial operation. From the damage, torsional vibration had been suspected as one of the possible root causes, which was eventually concluded true after extensive investigation in the field and thorough analyses. During the trouble shooting, as a part of the root cause investigation, equipment manufacturer carried out torsional rotordynamic analyses (both torsional modal analysis and torsional response analysis). In order to correctly understand the phenomena and to ensure effectiveness of the counter-measure, TOYO also carried out similar analyses and confirmed equipment manufacturer’s calculation results. Such trace calculations strengthened confidence in the root cause identification and counter-measure effectiveness.
TOYO utilizes such vibration analysis technologies to ensure flawless engineering executions and swift field trouble shootings.

Fig.2:  Torsional Rotordynamic Analysis Example (1st Torsional Natural Mode of the Reciprocating Compressor / Induction Motor Train)

Fig.2:  Torsional Rotordynamic Analysis Example (1st Torsional Natural Mode of the Reciprocating Compressor / Induction Motor Train)