Introduction. Power may be developed from water by three fundamental processes: by action of its weight, of its pressure, or of its velocity; or by a combination of any or all three. In modern practice the Pelton or impulse wheel¹ is the only type which obtains power by a single process, the action of one or more high-velocity jets². This type of wheel is usually found in high-head developments³.

Types of Hydraulic Turbines. Hydraulic turbines may be grouped in two general classes: the impulse type which utilizes the kinetic energy of a highvelocity jet which acts upon only a small part of the circumference at any instant, and the reaction type which develops power from the combined action of pressure and velocity of the water that completely fills the runner and water passages. The reaction group is divided into two general types: the Francis, sometimes called the reaction type, and the propeller type. The propeller class is also further subdivided into the fixedblade or propeller type, and the adjustable-blade type of which the Kaplan is representative⁴.

Impulse Wheels. With the impulse wheel the potential energy of the water in the penstock is transformed into kinetic energy in a jet issuing from the orifice of a nozzle. This jet discharges freely into the atmosphere inside the wheel housing and strikes against the bowl-shaped buckets of the runner.

Impulse wheels are used at heads of 800 ft. or more, although they may be used at lower heads, depending on the horsepower capacity involved. Usually not more than one or two jets are applied to the circumference of the runner or bucket wheel. The specific speed suitable for a given head and capacity of unit is much lower than that for the Francis or for the propeller type.

One of the largest impulse wheels, from the standpoint of physical size, operates at the San Francisquito No. 1 Plant in California. It is of the double-runner overhung single-jet type developing 32,200 hp under 800-ft. head at 143 rpm (50 cycles) and is now operated without change alternately at that speed and at 60 cycles (171 rpm), as demand may require. There is practically no loss in output or efficiency at the higher cycles. A high-head plant, the Bucks Creek Plant on the Feather River, California, with a maximum head of 2,575 ft., uses impulse wheels. In Switzerland a few units operate under 5,000 ft. or more.

At each revolution the bucket enters, passes through, and passes out of the jet⁵, during which time it receives the full impact force of the jet. This produces a rapid hammer blow upon the bucket. At the same time the bucket is subjected to the centrifugal force tending to separate the bucket from its disk. On account of the stresses so produced⁶ and also the scouring effect of the water flowing over the working surface of the bowl, material of high quality of resistance against hydraulic wear and fatigue is required. Only for very low heads can cast iron be employed⁷ Bronze and annealed cast steel are normally used.

To permit ready replacement of damaged buckets, they are usually individually bolted to disks, either singly or in segments of two or more, if the pitch has become so limited that not enough bolts could be provided to fasten a single bucket safely. Single buckets permit ready and high finishing of the working surface of the bowls, thereby assuring highest efficiency.

Francis Runners. With the Francis types the water enters from a casing or flume with a relatively low velocity, passes through guide vanes or gates located around the circumference, and flows through the runner, from which it discharges into a draft tube sealed below the tailwater level. All the water passages are completely filled with water, which acts upon the whole circumference of the runnner. Only a portion of the power is derived from the dynamic action due to the velocity of the water, a large part of the power being obtained from the difference in pressure acting on the front and back of the runner buckets. The draft tube allows maximum utilization of the available head, both because of the suction created below the runner by the vertical column of water and because the outlet of the draft tube is larger than the throat just be low the runner, thus utilizing a part of the kinetic energy of the water leaving the runner blades.

A comparison of various types of reaction runners of the same power, but of different specific speeds, is shown in fig. 3. The top three sections show Francistype runners and the bottom section a propeller-type runner. As the specific speed is increased, the figure indicates that the flow through the runner changes from radially inward to more nearly axial.

Roughly the propeller runner may be considered as a development of the Francis type in which the number of blades is greatly reduced and the lower band omitted. Strictly, however, the process of flow of water from the guide case to and through a propeller is quite different because of the large space, called the whirl chamber, between the stationary guide case and the entrance into the propeller runner.

Francis runners having a specific speed of 20 to 100 may be constructed of cast iron, cast steel or bronze, or with cast-iron or cast-steel hubs and rims and plate-steel buckets. For heads above 250 ft., either bronze or cast-steel runners are recommended, as the tensile strength of ordinary cast iron is low and too great a thickness of section would be required. For low-head conditions the plate-steel buckets have been used successfully up to 75 ft. This construction greatly reduces the cost of building the runner, and it is entirely suitable for these conditions as the plate steel is an excellent metal for this purpose and by proper treatment a practically perfect bond can be made between the end of the blades and the cast-iron or caststeel hub. Plate-steel buckets, being thinner than cast buckets, allow more water to pass through the runner and, consequently, more power to be obtained for the same size of wheel.

Propeller Runners. Inherently suitable for lowhead developments, the propeller-type unit has effected marked economies within the range of head to which it is adapted. The higher speed of this type of turbine results in a lower-cost generator and somewhat smaller powerhouse substructure and superstructure.

Fixed-blade Type. The fixed-blade propeller-type unit has a high efficiency (88 % in this case) at a point near full load, but the efficiency drops off rapidly as the load decreases, until, at 40 % of full load, the efficiency is only about 50 %. The installation of the fixed-blade propeller-type runner should therefore be limited to developments where the units may be operated at a point near maximum efficiency or not at all, and where, under variable head, the load of the unit could be shifted to an output resulting in best efficiency.

Materials. Propeller-type runners for low heads and small outputs are sometimes constructed of cast iron. For heads above 20 ft., they are made of cast steel, a much more reliable material. Large-diameter propellers may have individual blades fastened to the hub.

Adjustable-blade Runners. The adjustable-blade propeller type is a development from the fixed-blade propeller wheel. One of the best-known units of this type is the Kaplan unit, in which the blades may be rotated to the most efficient angle by a hydraulic servomotor. A cam on the governor is used to cause the blade angle to change with the gate position so that high efficiency is always obtained at almost any percentage of full load. Earlier developments in the United States provided for a maximum adjustment of the blades with turbine at rest to compensate for changes in the head or limitations of discharge.

As a step between the two, the so-called motormatic type, which permits a manual or electric adjustment while the unit remains in operation, is used in the United States.

Selection of Type of Turbine. The problem confronting the engineer is to determine the type and setting that will result in the most economical development under the given existing conditions. Each type of turbine has its limitations in application. The impulse wheel is, in general, used for very high heads, above 800 ft., although, in special cases where it is used to drive plant service equipment requiring small horsepower capacity, this type of wheel is used at heads much lower than 800 ft. By reason of its lower costs of parts exposed to natural wear and tear it may be preferred to other types in cases where there is an excess of water.

The Francis type can be efficiently and economically employed at heads ranging from 50 to 800 ft. One installation has been in operation for several years under a head of 960 ft. Under such high heads the unit should be operated near best point of efficiency (head) so as to avoid excessive hydraulic wear, especially at low part load. Operating water free of sand and chemicals is indispensable. There are still many plants installed long before the development of the propeller type for heads down to 8 ft. for direct coupling to generators, or for even lower heads, employing bevel gears or belt, or rope drives, to higher-speed generators or transmission-line shafts.

The propeller type is generally used for low-head developments of heads from 10 to 80 ft., but units of this type have been used under heads somewhat exceeding 100 ft.

Other factors affecting selection of a propeller type as against the Francis type are:

The propeller has a higher runaway speed, thus involving a correspondingly more expensive design of generator.

The hydraulic thrust is higher, requiring a more costly thrust bearing.

With increasing head the suitable specific speed becomes lower. For instance, for heads greater than 100 ft. it is below 90. This brings it nearer to a Francis turbine speed so that the over-all cost of a propeller turbine and generator combined as compared with that of a Francis turbine and generator combined may not offset the increased cost of excavation.

On account of the long passageway from the end of guide vanes to runner vanes of a propeller, it cannot respond as promptly to a load change as can the Francis type, especially one of moderate specific speed. Therefore, a propeller unit requires a relatively larger WR2 of revolving masses to assure the same stability.

General rules may be stated representing presentday good practice. Such rules serve as a guide for preliminary design. Each individual plant, however, has particular characteristics that make it differ from some similar plant previously built. The final decision, therefore, should be the result of considerable study as well as of consultation with experienced manufacturers and turbine designers.

In selecting a turbine an effort is made to use a type of runner which will give the required power at the greatest practicable speed, for this reduces the size and cost of hydraulic and electrical equipment, and powerhouse. There are practical limits, however, for any given set of conditions which restrict the speed that can be used and therefore more or less determine the type of wheel to be installed. The type of wheel is usually determined by the specific speed.

Specific Speed. The specific speed provides a means of comparing the speed of all types of hydraulic turbines on the same basis of head and horsepower capacity. A single runner having a higher specific speed than another therefore runs at a higher number of revolutions per minute to deliver the same horsepower under the same head. By definition, specific speed is the number of revolutions per minute at which a given runner would revolve if it were so reduced in proportions that it would develop 1 hp under 1 ft head. All homologous wheels of the same type but of different size, have the same specific speed.

The equation for calculating the specific speed for a given wheel from data obtained by test is as follows:

 ,

where          Ns — specific speed;

N — revolutions per minute at full-gate and head H;

P — horsepower per runner;

H — head in feet.

(Encyclopedia Britanica, Vol. 15, 1946)