Operational Modal Analysis of a Centrifugal Compressor in Field Operation

Challenging applications, such as re-injection of natural gas and carbon dioxide for the oil and gas upstream market, continue to push centrifugal compressor original equipment manufacturers (OEM) to the edge of their experience working with ultra-high density fluids. Given the uncertainties associated with the machinery design with regards to rotor stability, an increasing number of end-users are requiring stability verification tests to validate the rotordynamic models to reduce the risk of rotor instability issues during operation, as well as the risk of critical speeds with low damping inside the operational speed range at full or partial load conditions. The method of choice for stability tests has been the experimental modal analysis (EMA) techniques. These techniques generally require the temporary installation of an electromagnetic shaker on the compressor’s non-drive end (NDE) to non-synchronously excite the mode(s) of interest. While such EMA techniques have advantages for verification testing, the need for an external exciter does create additional costs and complications that must be managed. A possible alternative to these traditional EMA techniques is to adopt operational modal analysis (OMA) methods, where no external device is needed and the excitation comes solely from the ambient. The use of a OMA technique in a very low density gas (hydrogen) is the focus of this present work, in order to demonstrate the limitation, advantages and applicability of the proposed method.