Present Tendencies in Water Turbine Machinery

Abstract

As the high specific-speed water turbine has been fully described elsewhere, the paper concentrates on the present state of knowledge and details of design of the remaining types of water turbines, namely, Francis turbines and Pelton water wheels. In the absence of any spectacular developments in performance, efficiency, or speed regulation during the last twenty years, notable progress has been made in several other directions, such as the increased size of units, the more advanced state of knowledge regarding the incidence and avoidance of cavitation, and the use of special materials and devices to combat particular problems. On the subject of cavitation, the paper stresses the importance of the influence which the discharge velocity from the runner has on the allowable static suction head, and so directly on the occurrence of cavitation. The author goes on to describe how the effects of localized cavitation can be eliminated both in Francis and Pelton turbines, and thereafter reviews the settings and arrangements of Francis and Pelton turbines which are mostly favoured to-day, including brief references to arrangements of special interest. The latter part of the paper discusses the new ideas which are being tried out in the design and manufacture of the various portions of the two types of turbines dealt with in the paper. Among such features are included the use of welding in the repair of runners and in the manufacture of casings, and other portions of water turbines; the use of rubber in runner clearances and water-lubricated bearings; the researches carried out to determine the best form and shape of suction tubes for double-runner turbines; the difficulties encountered in the cracking and fixing of Pelton buckets; and finally, a review of the several types of main inlet valves at the disposal of the turbine designer.