REMAINING LIFE ASSESSMENT AND PERFORMANCE CHARACTERIZATION OF CENTRIFUGAL COMPRESSORS IN LNG TRAINS

Abstract

RasGas LNG Trains-1 and 2, comprising of three refrigeration centrifugal compressors each, were commissioned in 1999 and 2000, respectively. Three compressors have achieved 15 years of trouble free operation through risk based Predictive and Preventive Maintenance Strategies and surveillance program. None of these compressors’ major components such as the rotor, dry gas seals or bearings have been replaced so far except three cases due to design upgrades (Rotor Re-rate, Thrust Disk fretting issue). Industry and technical literature survey indicates that several hundred compressors have achieved service life of over 20 years. However, the frequencies of maintenance interventions are not common/standardized due to site specific variations, shutdown 3rd Middle East Turbomachinery Symposium (METS III) 15-18 February 2015 | Doha, Qatar | mets.tamu.edu Copyright© 2015 by RasGas Co. Ltd, Qatar, and Turbomachinery Laboratory, Texas A and M Engineering Experiment Station 2 windows and differences in applications. In the LNG business, major maintenance activities of these compressors are required to be aligned with Gas Turbine Driver’s 8 yearly Major-Inspection Cycles to avoid extended outages and very high adverse economic consequences. Train-1/2 compressors’ running life would exceed 23 years, well beyond API recommended service life of 20 years, when the next Gas Turbine Major-Inspection (MI) is scheduled. This Paper presents Engineering Studies on these six Compressors for the assessment of all potential failure modes, estimating remaining operational life and identifying requisite life-extension recommendations for meeting the challenge of achieving failure free operation for the extended period of 8 years. The compressor components are susceptible to low and high cycle fatigue, erosion, corrosion, creep, wear, and accumulated damages resulting in performance degradation and may eventually lead to failures. Review of rotor-dynamic design and performance, head/end wall O-ring life estimation, balancing drum condition review, and acoustic mapping were performed to evaluate the compressor train reliability and to benchmark the current system performance. To predict the static and dynamic stress distribution on compressor impellers (14 in total), detailed finite element (FE) models of the impellers are developed and validated using results from modal testing of spare impellers. The stress predictions using conservative estimates of alternating fluid pressure loads and rotor spin motion as forcing functions allow computing the impeller endurance limit, i.e., the stress limit corresponding to its infinite life. A stress based life prediction method, using Goodman diagram, was used to determine the impeller margin of safety. Aerodynamic excitations and structural resonant vibrations mostly contribute to the acoustic emissions. Non-intrusive acoustic and vibration measurements were performed near the compressors and piping to benchmark the current operating condition of the system, which will serve for future Condition monitoring and periodic system evaluation. Rotordynamic response measurements from bode plot, waterfall plots, and orbit plots was reviewed to identify the rotor critical speeds, sub-synchronous whirl, rotor misalignment, and rubbing. The compressor efficiency and pressure ratios are characterized and trends developed to evaluate historic performance. This study for the assessment of the remaining life of centrifugal compressors provides a reference for development of long term maintenance philosophy for all RasGas LNG trains and gas processing plants.