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Operational modal analysis to identify modal parameters in reciprocating compressors
Technical paper – GMRC Gas Machinery Conference 2019
Abstract
A classical experimental modal analysis (EMA) is typically used to identify the modal parameters (natural frequency, mode shape, damping ratio)of a mechanical system. An EMA applies a known and measurable excitation force to calculate a set of frequency response functions (FRF) and estimate the modal parameters.
For massive structures, such as foundations, large cylinders or piping, it is impractical to use EMA. The artificial forces generated in a typical EMA are insignificant compared to the mass and size of the mechanical system under study.
For large structures, an operational modal analysis (OMA) can be performed to extract the modal parameters. The main difference to an EMA is that the excitation forces are unknown in the OMA. This means the theoretical framework in the OMA needs to be different from a known (deterministic) input/output relation or frequency response function (FRF). In an OMA, the input is assumed to be a so-called stochastic process. There are different algorithms to determine modal properties from an OMA, ie, data captured during random excitation. When dealing with rotating machinery, the harmonic excitation of the structure caused by the equipment needs to be removed.
This paper discusses the theory and application of OMA on reciprocating machinery and piping modal analysis, including case studies in which OMA was used to identify modal properties of a compressor package foundation, cylinders and piping modes. In these cases, EMA could not be used to calculate modal properties due to the large mass of the structures.
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