When hydro-electric power was installed to drive the machinery of two mills on the banks of the River Tweed in Peeblesshire, an ingenious system was devised to store up power while the mills were not working
REINFORCED CONCRETE RESERVOIR being built 1,000 feet above the power station to take water pumped up by the turbines when electric current is not required for driving the mill machinery. The reservoir is in the form of a huge box some 192 feet square and 15 ft 6 in deep. The side walls taper in thickness from 21 inches at the base to 8 inches at the top. The rate of taper is not uniform.
WHEN the engineer taps Nature’s resources he must find means to change variable forces into steady power, available where it is needed and when it is needed. Often the power available at one time must be stored for use later.
The difficulties confronting the engineer are generally exceedingly complex, but one feature of the hydro-electric scheme in operation at Walkerburn, Peeblesshire, is that its problems were clear-cut and recognizable. The solution to those problems was unusually ingenious, but the difficulties which faced the engineer can easily be stated and understood.
In brief, the difficulty was to obtain from the River Tweed sufficient power to operate two large textile mills which had been established on the bank of the river in 1854. The site of the mills had been well chosen, and the map below shows its natural advantages. A long millrace served both mills and diverted ample water power, easily obtainable at first by means of old-
fashioned waterwheels. From the intake a long headrace led to the first mill, where a drop of 5 ft. 3 in. was available to operate the waterwheels. Soon after the water had left the upper mill it reached the waterwheels of the lower mill, where a drop of 5 feet occurred before the water entered the tailrace.
The difference in the level of the river between the end of the tailrace and the intake upstream is some 15 feet. Of this 15-feet fall the mills absorbed a little over 10 feet, the remainder being lost in the headrace and tailrace.
A busy community grew quickly round the mills, and the demand for water power increased as the industry thrived. In course of time the power from the old waterwheels proved insufficient, and engines of various types were installed to supplement it. By then the mills had been enlarged and covered a considerable area, so that the transmission of power to outlying buildings involved considerable losses. The problems of further supply were becoming acute. In 1918 occurred a change which altered the situation completely. In that year the two mills came under one ownership. This enabled the many problems of power to be regarded as one, namely, the efficient development of all the water power at the site.
The new owners did not propose half measures. They wanted to do away with supplementary steam, gas and oil engines, and to rely on water power alone. They consulted hydraulic engineers, and the scheme worked out by Boving and Company, Ltd., London, was approved and adopted.
This scheme had the supreme merit of simplicity. It proposed the replacement of the two old water-power installations by a single hydro-electric power plant, using low-pressure turbines.
The old waterwheels drove the machinery of the mills directly, assisted by auxiliary engines, through shafting and belts. The new turbines drove electric generators from which current could be readily conveyed, as required, to the motors of the various units requiring power. This improvement alone would cause greater efficiency, but the scheme went much farther. It suggested that the water power that ran to waste when the mills were inactive should be stored, in the form of reserve water power, for use during working hours.
Thus at the beginning of every day the power withdrawn overnight from the millstream would be on tap. Throughout Sundays and holidays extra power would be continuously collected, ready for the resumption of work, when it would augment the main power supply from the turbines.
The merits of this proposal were immediately apparent. In a normal week the mills worked for about fifty hours; the potential power in the mill-stream was running to waste during the remaining 100-odd hours; thus two-thirds of the latent energy were not being used. How could this surplus power be stored? It was not practicable to convert it into electrical energy, and to store it in that form. On the other hand, to store large quantities of water at river level was out of the question, for this would have entailed the flooding of large areas of valuable land.
The ingenious alternative was to raise some of the water to a reservoir, placed high on a neighbouring hill, by means of pumps worked from the main turbines. Millions of gallons of water could be forced up a pipe line and accumulated in this lofty eyrie, forming a source of tremendous potential energy.
All the time that the mills were not working the power house could be used to work the pumps and to accumulate reserve power in the reservoir. On the resumption of work the pumps could easily be switched off, and the stored water allowed to return down the pipe line. With all the pent-up force of the reservoir behind it the stream emerging from the pipe line could be used to drive a Pelton wheel, thus generating extra power to augment the supply from the turbines.
When the scheme was worked out quantitatively its many advantages became clearer. The total output from the original wheelhouses of the two mills was about 110 horse-power. Instead of these a single modernized power house could be relied upon to produce 220 horse-power, exactly double the amount. There was the further advantage that all the generating machinery and staff would be concentrated in one building, thus avoiding unnecessary duplication and waste.
Even the 220 horse-power developed by the turbines, however, was less than half the power required by the mills, for the total demand on the water power supply, if all the other generators were abolished, was 450 horse-power. The deficiency, of 230 horse-power, was to be supplied by the water power stored in the reservoir.
Holding 3,500,000 Gallons
From the banks of the River Tweed the ground rises sharply, and a suitable site for a reservoir was found near the top of Kirnie Law, a hill 1,000 feet above the power station and almost 1,500 feet above sea level. The distance from the power station to the site of the reservoir was 2,300 yards.
It was calculated that the pumps, working at night and during the weekends, would be able to deliver nearly 3,000,000 gallons to the reservoir. Even this vast head of water, stored in the hills, would have been insufficient to drive the Pelton wheel at full capacity for the whole of a fifty-hours’ working week. Fortunately, however, full-capacity working was not required, for the load on the power house varied during working hours.
The scheme evolved, which met all requirements, was as follows. The reservoir at the top of the hill was to have a capacity of 3,500,000 gallons. At all times when the mills were not working water would be forced up the pipe line to the reservoir by pumps driven by the low-pressure turbines in the power house.
At the beginning of the working day pumping would cease, leaving the low-pressure turbines free to drive a 145-kilowatts generator on constant load. Supplementing this generator, and providing the fluctuating remainder of the power required, would be a 155-kilowatts generator driven by the Pelton wheel. The Pelton wheel would be driven by the stored water returning to the river through the reservoir pipe line. All the electrical power generated by these means could easily be transmitted by cables to independent motors throughout the mills.
The main features of the scheme having been settled, its details were worked out with equal ingenuity and efficiency. The two original falls of water were united by deepening the channel between them, and the lower wheelhouse with its pair of wheels was demolished. At the upper wheelhouse the old pair of wheels was removed to make room for the pair of low-pressure turbines.
The old wheels had been in service for about sixty-five years and their reliability was almost legendary, for during that time there had been virtually no breakdowns. Their efficiency, however, was low, for they could develop only about 40 per cent of the power theoretically available in the water, as compared with the 82 per cent given by the turbines.
BY THE RIVER TWEED at Walkerburn, Peeblesshire, two textile mills derive their power from hydro-electric turbines. When the mills are inactive, the turbines drive pumps which force the water up to a reinforced concrete reservoir standing 1,000 feet above the mills. When electric power is required again in the mil! the gravity flow back through the pipe line is used to supply auxiliary power.
It was decided to place the turbines at the upper rather than at the lower wheelhouse because the danger of flooding the intermediate buildings could thus be avoided. Considerable excavation was necessary to concentrate all the fall at the upper mill, but eventually the hydraulic conditions were so improved that the operating head of water is now a little more than 11 feet, instead of the 10 ft. 6 in. Estimated.
While the mills are working, the low-pressure turbines are connected to the generator by belts. As soon as work ceases these belts are moved across to pump pulleys, and the pumps begin to force water up the pipe line. There are two pumps, each capable of delivering 220 gallons a minute. Working together, they can fill the 3,500,000-gallons’ reservoir in about 132 hours. The pumps are of the high-pressure centrifugal type, running at 3,000 revolutions a minute. The turbines run at 200 revolutions a minute.
The pipe line, the vital artery through which the water is always running in one direction or the other, is divided into two sections which meet at a surge tank on the hillside. Running straight down the hill as steeply as possible from this tank is the 9-in. high-pressure section of the pipe line. The other section is a 12-in. cast-iron pipe which runs round the contour of the hill almost on the level, to join the surge tank to the reservoir.
Because of its great size the reservoir had to be placed on a saddle some distance away from the surge tank. Measuring some 192 feet square by 15 ft. 6 in. deep, the reservoir is of reinforced concrete. Its construction required 400 tons of cement, about 100 tons of reinforcing bars, and over 3,000 tons of aggregate. All this material was carried to the hilltop by a cableway.
Over the greater part of its area the floor slab of the reservoir is 9 in. thick, and contains a light reinforcement near its upper surface. Six feet away from the inner face of the wall, however, the thickness of the floor slab begins to increase, reaching a maximum thickness of 18 in. under the wall.
The side walls are only 8 in. thick at the top, but in the upper 10 ft. 6 in. they taper to a thickness of 14 in., after which the taper increases more rapidly to a thickness of 21 in. Round the outer face of the wall the floor slab is produced to form an outside toe, which extends for a distance of 2 feet, and varies uniformly in thickness from 9 in. at its edge to 18 in. under the wall. No other foundation is provided for the side walls, which are heavily reinforced and are tied well into the floor slab.
From this reservoir the buried 12-in. cast-iron pipe. leads round the hill to the surge tank, the capacity of which is sufficient to compensate for sudden variations in the water demand. As the 9-in. high pressure pipe leads steeply downhill from the surge tank the tendency is for the high-pressure pipe to draw off more water than the low-pressure pipe can immediately supply; the surge tank overcomes the difficulty, and supplies (or absorbs) water until the flow has adjusted itself to the change of load.
Exceptional care had to be taken in the burial of the high-pressure pipe line, as any leakage at the joint flanges would have seriously impaired the efficiency of the high-pressure plant. When the trench was being refilled the workmen had to be careful that boulders were not accidentally dropped on to the pipe.
In the power house, about 1,000 feet below the surge tank, the high-pressure water issues from a jet of only 1⅞ in. diameter. The velocity is 244 feet a second, and the water strikes with terrific impact the buckets mounted on the circumference of the Pelton wheel. The machine runs at 1,000 revolutions a minute and develops 230 horse power.
Its ease of regulation makes the Pelton wheel particularly suitable for varying loads. When the load is suddenly reduced, a deflector diverts the jet of water issuing from the nozzle, thus throwing the power off the wheel. The adjusting needle is then fed forward into the hole until the jet is reduced to a size adapted to the new load. Then the deflector is returned to its normal position, and the reduced power is applied to the wheel after the lapse of only a few seconds.
Near the Pelton wheel is a switchboard, from which the electricity developed in the power house is distributed to the mill in the form of direct current, at 250 volts. This power supply is always ample, accessible and reliable.
A 9-INCH PIPE LINE was laid be-tween the power station and the surge tank on Kirnie Law, 1,450 feet above sea level. The rocky ground made the cutting of the trench a long and difficult task, and special care had to be taken in the packing of the joint flanges to prevent leakage.
