Solid Couplings With Flexible Intermediate Shafts For High Speed Turbocompressor Trains.

LecturePg. 101-110The design and application of solid couplings combined with flexible quill shafts for high speed turbocompressors are presented. Solid couplings are rigid in the axial direction. They transmit any residual thrust from one compressor casing to another, and even into the gear. A single thrust bearing then positions the complete rotor train. Elimination of individual thrust bearings results in a smaller lube-oil system and a reduced power loss. Any additional thrust components from a prestressed flexible-disc coupling or a gear-type coupling (gear lock) are completely avoided. The use of a flexible intermediate shaft (quill shaft) gives the solid coupling the same lateral flexibility as for a geartype or flexible-disc coupling. Criteria for allowable misalignment are given in this paper. A solid coupling is neither exposed to any kind of wear nor endangered by corrosion or stress corrosion so that no maintenance is necessary. Couplings have a decisive influence on rotordynamics. The conventional arrangement of a multicasing train with gear-type or flexible-disc couplings incorporates individual thrust bearings located outboard of the journal bearings (long shaft over hangs). Introducing solid couplings leads to a stiffer rotor by eliminating individual thrust bearings. A comparison of critical speeds of a high-speed/high-power compressor train is made alternatively equipped with solid couplings, flexible-disc or gear-type couplings. The response of the different rotors and couplings to specified unbalance weights is calculated and compared throughout the complete speed range. Torsional and lateral critical frequencies can easily be influenced by modification of intermediate shafts, even after installation of a compressor train. Operating experience on nearly 1500 compressors equipped with solid couplings have proven their excellent reliability especially on high speed, high power turbocompressor applications