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.
The steady increase in the capacity of generating sets created a demand for the high specific-speed turbine which was met by several experimenters. “Specific speed” is the speed at which a turbine will run under unit head when developing unit power, and nowadays a “high specific-speed” water turbine denotes one having a runner of the propeller type and a specific speed between 100 and 230 r.p.m. Difficulties were encountered in the development of propeller turbines, especially in connexion with cavitation. Laboratory tests and the use of visual study methods have played an important part in the solution of these difficulties. The method of fixing suction head in conjunction with laboratory test results is explained, and a comparison is made between the various forms of suction tube in use. Much work has been done to overcome the effects of localized cavitation, and stainless steel has been found to be very effective, especially when runners are cast entirely of that material. A method of operation has been developed to dispense with the use of inlet sluice gates in large machines. Several methods in use for operating the movable runner blades are described. The introduction of the high specific-speed turbine has led to a large increase in the number of automatic stations. The great size attained by these turbines has entailed the construction of equally large generators, the development of which has had its own problems.
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