Tribology Online (Jan 2013)
Oil-Free Bearings and Seals for Centrifugal Hydrogen Compressor
Abstract
Deployment of a safe, efficient hydrogen production and delivery infrastructure on a scale that can compete economically with current fuels is needed in order to realize the hydrogen economy. While hydrogen compression technology is crucial to pipeline delivery, positive displacement compressors are costly, have poor reliability and use oil, which contaminates the hydrogen. A completely oil-free, high-speed, efficient centrifugal compressor using 4th generation compliant foil bearings and seals has been designed for hydrogen pipeline delivery. Using 6-12 MW drives operating at speeds to 56,000 rpm, a modular, double entry compressor was configured to deliver 500,000 kg/day at pressures greater than 8 MPa. Each of the two or three multi-stage compressor frames operate above the bending critical speed of the rotating group since speeds are 5 to 7 times faster than conventional compressors. To assure a structurally and economically feasible design, the rotor of each compressor frame spins at the same speed with blade tip velocities near 600 m/s. An iterative aerodynamic/structural/rotordynamic design process was used, including both quasi-three dimensional inviscid internal flow and Computational Fluid Dynamic (CFD) analyses. The flow field was carefully analyzed for areas of excessive diffusion, sudden velocity gradients and flow separation. Excellent correlation between the preliminary design and CFD analysis was obtained. Structural and rotor-bearing system dynamic analyses were also completed to finalize the compressor system configuration. Finite element analysis of the compressor impeller was used to verify structural integrity and fatigue limits for selected materials. Rotor-bearing system analysis was used to define acceptable bearing locations and dynamic coefficients, system critical speeds and dynamic stability. Given the high speeds, supercritical operation, and required reliability, efficiency and freedom from contaminants, compliant foil gas bearings and seals were designed and evaluated. Since hydrogen will be used as the lubricant for the foil bearings, substantially lower power loss than oil lubricated bearings will be experienced and the auxiliary supply or scavenge system is eliminated.
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