TURBINE : WATER. Modern hydraulic turbines may be divided into two classes, impulse, and pressure or reaction tur bines. Of the former the Pelton wheel, and of the latter the Francis turbine or one of its modifications, are the only types used in recent important installations.
In an impulse turbine, the whole head of the supply water is converted into kinetic energy before the wheel is reached. The water is supplied to the wheel through a nozzle which delivers a high velocity jet at atmospheric pressure on to the vanes or buckets mounted on the periphery of the wheel.
In the pressure or reaction turbine, the wheel or runner is pro vided with vanes into which water is directed by a series of guide vanes extending around the whole periphery. The water on leaving these guide vanes is under pressure, and supplies energy partly in the kinetic and partly in the pressure form. In its passage through the runner the pressure energy is utilized in increasing the rela tive velocity of flow between the vanes, and the water finally leaves the runner at the pressure obtaining in the discharge pipe or draft tube.
In the earliest of these turbines, the Fourneyron, the guide vanes were inside the runner, and the water flowed outward. This was followed by the Jonval turbine, in which the guide vanes are above the runner and the water flows axially into and through the wheel. Both types are now obsolete, and one or other modification of the Francis or inward-flow turbine, in which the guide vanes surround the outer periphery of the runner, is now in general use.
The supply of water to the runner depends upon the opening between the guide vanes. These are pivoted on stems which pro ject through stuffing boxes in the turbine casing. Each stem car ries a lever. These are all coupled to a regulating ring whose position is regulated by the governing mechanism, so that all the guide vanes are opened or closed simultaneously.
In a low-head installation the turbine may be erected in the open forebay or supply canal. This method has the disadvantage that the guide-vane mechanism is submerged and cannot be in spected or repaired without draining the wheel pit, and in most recent installations, even of the low-head type, the guide-vane ring is surrounded by a spiral volute chamber, from which the pressure water is delivered with uniform velocity around the entire peri phery of the guide ring. For heads not exceeding about r oo ft.,
modern practice favours the moulding of the volute chamber in the concrete of the substructure (fig. I). For higher heads, con siderations of strength necessitate a metal casing, to which the water is supplied through a pressure pipe line (fig. 2). The tur bine runner is usually of cast iron, although where corrosion or erosion is to be anticipated, it may be made of cast steel if large, and of phosphor bronze if small.
The general changes in the shape and proportions of the run ner which have accompanied re cent development of the low head high-speed turbine are indicated in fig. 3. The change has been generally in the direction of in creasing the axial depth of the buckets, and at the same time of maintaining or increasing the ratio of the discharge area at exit to that at entrance. At the same time the width of the buckets in the direction of flow has been reduced. An extreme example is shown in fig. 3D, in which flow through the wheel it self is almost axial. In many recent low-head turbines the run ners are of the propeller type. They are characterized by the fewness of the vanes, as few as three or four in many cases, and are in appearance very like the ordinary ship's propeller.