OF the several types of turbine now converting the energy of falling water into electrical energy all over the world, the Pelton wheel, so called after its inventor, most nearly resembles the old-fashioned water wheel in that it consists of a number of buckets arranged in a circle round the shaft which is to be driven. Here, however, the resemblance ends. In the commonest type of waterwheel, the overshot wheel, the shaft was really turned by the weight of the water as bucket after bucket at the top of one side of the wheel was filled, to be emptied again as it neared the bottom.
The Pelton wheel, on the other hand, is driven round by the impact of a powerful jet of water striking the buckets which come successively into line with it as the wheel turns. The illustration shows four Pelton wheels supplied by Boving and Co, Ltd, for a generating station in the Punjab, India. The old type of waterwheel developed perhaps 7 or 8 horse-power. Each of the turbines here shown can develop 17,000 horse-power at a speed of 428·5 revolutions a minute. A steel pipe line conveys the water from the intake canal, 1,668 feet above the turbine, to the turbine nozzle, from which the jet issues. The pressure in the turbine inlet is, roughly, 720 lb per square inch.
The photograph was taken in the erecting shop and does not show the buckets, as they are cased in, but they resemble deep oval basins (see colour plate below), with ridges across the centre. The jet of water is directed to the inside of the buckets and, striking the ridge, is deflected to both sides, thus obtaining a greater area on which to exert its force. The jet is directed through a circular nozzle by what is termed a needle valve. This is a conical streamlined valve tapering to a fine point, and projecting through the nozzle. In front of the nozzle is a hinged flap called the deflector, and this, as well as the valve-moving gear, is controlled by a governor. The valve is connected to a piston on one side of which is a spring which tends to close the valve, and on the other side there is oil pressure tending to open it.
When the load on the turbine is reduced, that is, when less current is required from the generators, the governor affects a mechanism which turns down the deflector in front of the jet and bends it away from the buckets to a greater or less extent as required. At the same time the needle begins to close slowly under the action of the springs and the size of the jet is decreased, the deflector being then moved back sufficiently to clear the smaller jet. Increase of load is followed by the simultaneous opening of the needle and the raising of the deflector, a regulating device preventing shock in the pipe line.
The curved pipes in the photograph are the water inlet pipes, and the needle-actuating oil cylinder is seen at the bend. The vertical cylinders seen near two of the turbines contain the governor and control gear. There are various safety devices fitted which act automatically in such circumstances as failure of the oil supply, overspeed should the governor gear fail, and so forth.
THE PELTON WHEEL is one of the several types of turbine that convert the energy of falling water into electrical energy. A powerful jet of water is directed through a circular nozzle by a needle valve into the buckets of the wheel. The wheel is shown here without its casing and the nozzle casing is cut away to show its construction. This turbine, designed for a power station in India, works at a speed of 375 revolutions a minute with a head of 1,630 feet of water.
