Evening Post 2 August 1924
THE FINAL FALL
PIPE LINE AT MANGAHAO
PROGRESS AT POWER-HOUSE
HOW ELECTRICITY IS GENERATED
(By -" The Post's" Special Reporter.)
ESSENCE OF THE SCHEME
The essence of the scheme has been explained. It is, briefly, to get the high waters of an inland mountain torrent cut to the edge of the hills, where a fall may be arranged through pipes to a power-house containing suitable machinery for the generation of electricity by hydraulic energy. The waters of the Mangahao are impounded by a hundred-foot high dam to form a lake holding approximately 525,000,000 gallons.
This lake is tapped by a tunnel a mile long and seven feet in diameter running into another reservoir created by the Arapeti dam, 430 feet long and about a hundred feet high. The second lake holds 290,000,000 gallons.
From it a second tunnel, a mile and a quarter long and eight feet in diameter, brings the water on to the edge of the hills overlooking the plain to a small but deep artificial pool, known as the surge tank or chamber.
From this the water runs down a spur of the hills in steel pipes to the power-house on the flat,- where it impinges from nozzles on the buckets of Pelton wheels on the same shafts as huge dynamos, which transform the rotary motion into electricity. By various electrical appliances the current is adapted for long-distance transmission, and by high tension trunk lines supplies - or will supply - places as wide apart as Wellington and Wanganui in one direction and out to the East Coast in the other.
THE SURGE CHAMBER
The surge channel, situated at a height of 1250 feet above the level of the sea, which may be seen from that conspicuous vantage point on the hills, fringing the further side of the plains for miles, is about 900, feet above the floor of the power-house. It represents the outlet from the mountains of the upland lake already described, part in the open and part underground. It is on exactly the same level normally as the water at Mangahao dam. It is 75 feet deep and 87 feet 5 inches in diameter at the top and 19 feet at the bottom. It is very like one of those straight-sided breakfast cups in shape, or the top part of a slender, funnel. Into the nineteen foot ring at the bottom runs in the tunnel from Arapeti at one side and out of it go two smaller tunnels on the opposite side leading to the two big six-foot steel penstocks of the upper stage of-the pipe line.
The surge chamber is an essential connecting link between the nigh level lake and the low level powerhouse. If the pipe line of steel is regarded as part of the mechanical side of the powerhouse, then the surge chamber is the end of the lake section and the beginning of the business department of the scheme.
PRESSURE ON PIPES
To understand the practical purpose served by the surge tank in the hydraulic system of Mangahao one must first describe the pipe line down to the powerhouse. In neither plane does the pipe line run straight. The pipes vary both in direction and in gradient of slope. They also vary in number from two at the top to four, and then finally to five at the power-house. The total length of the pipe line in a fall of 895 feet – by vertical measurement - is 3840 feet. As pressure increases, the lower the level, so the thickness of the steel is increased from 4 inch at the top, where the pressure is slight, to 11-16 inches at the bottom, where the pressure is something like 370lb to the square inch. The steel would need to be much thicker still if the single pair of pipes were kept right to the bottom. The diameter of the pipes also decreases from six feet to five feet in the upper pair of pipes and from 3ft 10in in the lower pipes to 3ft. It must be obvious that the flow of water down such pipe at such a pressure would tend to distort the pipe line at the several angles where the slope or direction, or both, change, and these angles are therefore embedded in immense, solid blocks of concrete to anchor them immovably in position.
HYDRAULIC SAFETY VALVE
Where the surge chamber comes into the scheme is when, for instance, the powerhouse should for some reason or other - possibly a breakdown in generators or a. sudden release of load - cut off the flow of water to the turbines. Anybody who has suddenly turned off a tap knows the effect in the pipe. There is a thudding, and thumping - which seems to put the pipe, under strain. Imagine the effect multiplied a million fold in the pipe line at Mangahao. It might very well split the seams in the steel pipe to strips, with all the weight of water in motion from the dams to the powerhouse.
The surge chamber interposes a sort of safety valve or governor in the hydraulic system, and takes up the recoil, as it were, in the water by allowing it to spend itself by rising and falling in the open-mouthed crater of concrete. The surge chamber is for the water to surge in freely without putting ay excessive strain on the steel part of the watercourse.
It should be added that the Mangahao scheme offers many safeguards against breakdown in any part involving a complete shut down on power. The Mangahao dam can be isolated by a gate at the mouth of the tunnel leading to Arapeti, and Arapeti can be cut off from the surge chamber by gates at the mouth of its outlet tunnel, and the pipe line itself can be disconnected from the surge chamber by similar gates.
At a pinch the water in the surge chamber would supply power for quite a while.
THE PELTON WHEELS
The water enters the huge powerhouse—2l5 feet long, 84 feet wide, and 70 feet high—underneath to one side, by way of the basement; There are now five pipes—three a yard in diameter and two two-feet in diameter — for the five sets of Pelton wheels and generators, three of 6000 h.p. and two of 3000 h.p. A common or bus pipe joins together the separate pipes before they reach the nozzles.
There are two nozzles to the three big sets, and one each to the smaller sets, so that the water issuing, with its 900 feet fall behind it, from each nozzle, represents 3000 h.p. Each nozzle is 6¾ inches in diameter, and the jet may be controlled by the movement of a sort of bronze spear valve in the centre of the nozzle. The Pelton wheels themselves are about the simplest form of water-wheel for generating power. They consist, each of twenty-two buckets - twin buckets - bolted in sets of two on the rim of a solid boss about seven feet in diameter. They resemble nothing so much as two giant tablespoons joined together about one of their edges and presenting two hollow surfaces to the nozzles with a knife-edge partition between them. The jet of water strikes this knife-edge, and parts each way on a curve to leave the buckets in the opposite direction to which if struck them. The ideal is for the peripheral speed of the buckets to be just half that of the velocity of the jet from the nozzles The water would then drop lifeless into the tail-race below, with its, work done and its energy exhausted.
CONTROLLING SPEED
The water-wheels, running at 375 revolutions per minute have at the other end of their shaft a dynamo much bigger than themselves, the total weight of wheel, shaft, and rotor being about 28 tons. The speed is governed automatically by a special type of hydraulic governor actuating the bud-shaped spear valves inside the nozzles, and a deflector which masks the whole jet when required. Should the load be suddenly thrown off the deflector immediately comes into action, and turns the jet off the buckets, while the spear valve advances and closes up the opening in the nozzles, so that the movement of the two opening and closing controls the speed of the water-wheel, and consequently of the generator.
The generators are of the alternating current three-phase type, and require exciters for the filed magnets in starting. These exciters are two sets of miniature Pelton wheels and generators of 250k.w. each. The smaller 3000 h. p. sets have their own exciters on the same shaft.
ELECTRICAL ENERGY
The water has now done its work, and may be left to pursue its placid, harmless course, down the Mangaore Stream, under the railway at Shannon Railway Station, and across the flat, to join the Manawatu River a mile away, cutting out sixty odd miles of its former course via the Manawatu Gorge. Power is now in the form of electrical energy of 11,000 volts pressure coming from the big generators in the hydro station.
For transmission to .Wellington, Bunnythorpe, and elsewhere over long distance lines the voltage has to be stepped up tenfold to 110,000 volts. This is done in two huge transformer sets of three units each, like nothing more than six high steel tanks with insulators on top of their lids, each box being 14 feet 2 inches high, 5 feet 6 inches wide, and 7 feet 6 inches deep. The boxes are filled with special oil for the insulation of the extremely high tension current used. From the transformers the current is taken through special oil switches of huge size to the roof, where it goes off from ponderous porcelain insulators along its mission of usefulness in the distant city or farm. But, before it can he used, it has to be stepped down again twice, first to 11,000 volts in receiving stations, and then to 230 volts and 400 volts for domestic and industrial purposes. This phase of the hydroelectric system does not lend itself to description, as its working is beyond the understanding of the average citizen. It is a matter still for the expert and may, for the present, be left with him.