Tribology and Rotordynamics of Small High-Speed Cryogenic Turboexpander

Abstract

Turboexpander is considered as the heart of present-day cryogenic process plants such as helium, hydrogen and nitrogen liquefiers, low-temperature refrigerators and air separation units, . The operational objective of a turboexpander is to refrigerate a gas stream, by removing work from the gas, and expanding the gas nearly isentropically. The turbine based cryogenic process plants in recent years are low-pressure system and have the advantage of high thermodynamic efficiency and high reliability. The high efficiency is possible at highspeed of the turboexpander, and these turboexpanders in a typical cryogenic refrigerator or liquefier run at high-speed greater than 50,000 rpm without contaminating the process gas. Such operating condition imposes rigorous constraints on tribo-pair design. Oil-free gas bearings have advanced as the most acceptable solution for supporting small and high-speed cryogenic turboexpander rotors. An inherent issue with classic gas bearing is its lower dynamic properties such as stiffness and damping because of its low viscosity. Low stiffness and damping are prone to instability at high rotational speed. So gas foil bearings (GFBs) have received much attention for research, development, and experiment over past three decades for its ability to tailor the stiffness and damping with the use of compliant foils. Bump type compliant foil gas being is quite popular among researchers for various turbomachines for its high load carrying capacity, simplified numerical analysis, and easy fabrication methodology compared to other types. In the present work, a modest attempt is made to understand, standardize and document the numerical analysis, design methodology and fabrication methodology. It evaluates the rotor bearing performance to determine the feasibility of bump type gas foil bearings for axial and radial support of cryogenic turboexpanders.

The work presented in current dissertation classified into five parts. The first part includes the status of research and development in the field of gas bearings in turboexpanders and a broad literature review of gas foil bearings. The outcome of the literature review directs that extensive research is essential for designing and development of gas bearing for a more advanced cryogenic system which is technically and economically better than present gas bearings.

The second part deals with the design and numerical analysis of gas foil journal and thrust bearings and its feasibility to apply in a small and high-speed cryogenic turboexpander. The numerical analysis helps to fix the dimensions of foils such as its thickness, bump length, and pitch. for a previously designed rotor and its load carrying capacity. The dynamic properties of the bearings are determined to be used in the rotordynamics analysis. Finally, a step by step detailed design procedure itemized for both the gas foil bearings. Transverse vibration being a major issue for high speed rotating machinery such as a cryogenic turboexpander, a detailed vibration analysis completed in part three. The vibration analysis includes determination of critical speeds, mode shapes and unbalanced response for the desired configuration of the rotor-bearing system with determined stiffness and damping from previous part.

A small clearance between the gas bearings and the rotor is maintained in order of 10 to 40 m; This makes a cryogenic turboexpander with gas foil bearing a precision equipment. All precision equipment demands micron scale manufacturing tolerance, so fourth part of the dissertation explains the details design methodology for gas foil bearings, the rotor and other associated parts of turboexpander. A broad analysis is done on bump forming methodology for fabrication of bump foil of the desired dimension. A Finite Element Method (FEM) simulation of forming process carried to simplifies the die design process. Special attention is given to the material selection of bearing components, balancing of the rotor, tolerance analysis, fabrication, coating of solid lubricant and assembly of the turboexpander. The last part includes performance study of the fabricated turboexpander with gas foil journal and thrust bearing. Several issues are encountered during this phase, and most of them are rectified either by modification of design process or rectification in fabrication methodology. A vibration study is done using accelerometers on the bearing housing close to the journal bearings. The vibration analysis reveals gas foil bearings can be an alternative rotor bearing system for a high-speed small sized cryogenic turboexpander. A satisfactory operation is carried out for the duration of 30 hrs with an achievable speed of 81,000 rpm with multiple starts and stops.