Less Costly Turboequipment Uprates Through Optimized Coupling Selection
Compressor and driver shafts often prove to be overstressed in equipment uprate situations. This paper shows how a change from conventional gear-type couplings to the more recent diaphragm coupling design can lower the shaft stresses sufficient to avoid shaft replacement during power uprates of centrifugal compressors or compressor drivers. Equipment uprate projects involving turbomachinery often appear to require time consuming and costly shaft replacements. However, closer examination of how the equipment vendor arrived at his maximum allowable stress levels may frequently show that such shaft replacements can be avoided without undue risk if the coupling selection is optimized. This conclusion is based on the fact that gear-type coupling have the potential of inducing in a shaft both torional stresses and bending stresses, whereas diaphragm couplings tend to primarily induce torisonal stresses and insignificant bending stresses at best. The economic incentives of finding ways of salvaging major rotating equipment shafts illustrated on a steam turbine shaft originally rated to transmit 17,600 hp maximum at 6,400 rpm. If uprating the turbine to 19,600hp were being considered, the required changeout of stationary steam path components would cost around $60,000, but a combined replacement cost of about $500,000 would probably be quoted for the main and spare rotor shafts. A rigorous calculation of shaft stresses shows the shaft factor of safety to be greater at 19,600 hp using a diaphragm coupling than at 17,600 hp using a conventional gear coupling.
Bloch, Heinz P. (1975). Less Costly Turboequipment Uprates Through Optimized Coupling Selection. Texas A&M University. Gas Turbine Laboratories. Available electronically from