A hydro-electric station in the centre of the jungle supplies power to several factories near the Pangani River Falls in Tanganyika. The unusual difficulties which have been overcome have made this one of the most interesting water-power schemes in the world
PIPE LINE AND HAULAGE TRACK for the Pangani Falls power scheme during early stages of the work. The pipe line is 1,230 feet long; the first 418 feet have a diameter of 6 ft. 6 in; the rest of the line is 6 feet in diameter. There are four main concrete anchorages, two of which appear in this illustration. They are reinforced with steel, their foundations being carried down to solid rock.
SOME of Africa’s rivers, notably the Congo, have enormous hydro-electric possibilities which exceed even those of the largest rivers of the United States and Canada. Until industrial plants having large current demands are built near these power sites, however, it is impracticable and uneconomical to attempt to recover the enormous amount of energy now going to waste.
The building of a complete hydro-electric station with dams, canal, headworks, pipe line and tailrace, and the installation of the generating plant and transmission lines are extremely expensive matters. It often happens that the annual interest on the large capital sum involved exceeds the total cost of producing electricity from coal.
In certain circumstances, however, it is found to be an economically sound proposition to build a hydro-electric station in what may appear to be an unpromising area. Such a scheme is in operation on the Pangani River, in Tanganyika. It is the most recently completed hydroelectric scheme in Africa and the only one of its kind in Tanganyika.
The hydro-electric station is designed to supply power to a rapidly growing number of factories within a radius of sixty miles of the Pangani River Falls. The work of these factories is decortication — the removal and cleaning of the fibre from the leaves of the sisal plant. Sisal fibre is extensively, used for making twine, and has other uses in the manul'acture of string and cordage for marine and other purposes.
The existence of these factories provided a market for the electric power which could be produced in greater quantity and more cheaply from the Pangani River Falls than from oil or steam engines in the industrial factories of the area. Nearly all the local factories were thus converted to use electrical power and the scheme has proved highly successful. The work was designed and carried out by Balfour, Beatty and Co. Ltd., who were also the contractors for the Lochaber Power Scheme.
The principal source of the Pangani River is on Mount Kilimanjaro (19,320 feet), the highest mountain in Africa. In its flow through a tract of barren country the Pangani loses much of its volume without any appreciable compensating gain from tributaries. The catchment area above the Great Pangani Falls is approximately 12,000 square miles.
The river flows into the Indian Ocean at the village of Pangani, between Tanga and Dar-es-Salaam, and in its last hundred are several rapids. The suitable for are the Great Pangani Falls, which have been surveyed and studied by various engineers since 1904. The river suddenly breaks up into a number of small channels which wind their way among rocks and through jungle and continue by devious routes down a steep slope to the edge of a cliff or escarpment, down which they fall before combining again.
The river then drops another 50 feet in a series of rapids, and in all the total head of water is some 400 feet. Because of the nature of the site, it was not practicable to use the entire head of water, and the plant was designed for a head of 313 feet.
Access to the site was exceedingly difficult and involved the solution of many problems. A road had to be cut through ten miles of dense jungle. This road joined the site to the nearest point on the Government railway, running inland from Tanga to Moshi and Arusha, at the foot of Mount Kilimanjaro.
A ferry barge, operated by winches, was built near the site. This barge could carry a loaded lorry and trailer, as well as heavy machinery and materials, across the river. The barge was less costly to build than a bridge; it served its purpose during the construction period (1934—36), and was still in use for various purposes in 1937.
Every hydro-electric development scheme has its own particular problems. At Pangani the chief problems were due to the remoteness of the site and to complete absence of knowledge about the state of the river during the wet and dry seasons. Because of the lack of data about flood water and the height to which the river might rise in the rainy season, it was necessary to provide adequate escapes for flood water. Such passages had to be so designed that they would prevent damage to the intake structures, and at the same time provide means for diverting the water required for the turbines during a drought. The most easterly channel of the river was selected to carry the water to the hydro-electric plant, and the channel was deepened from two feet to six feet to give the necessary capacity. During the excavation of this channel the whole of the flow was diverted by building a temporary dam at the upstream end of the channel. The sides of the channel had to be raised with a few embankments to keep out unwanted flood water. Water was diverted to the channel by a weir, sixty feet long, across another channel at the point where the river breaks up. The weir was a simple concrete structure with a maximum height above the river bed of five feet.
Contraction Joints
The intake dam is located at the foot of the Upper Rapids at a point not far above the main fall. It is 290 feet long and is designed to serve as a spillway for passing excess water. The spillway section of the dam is 250 feet long, and the dam, which is a gravity structure, contains six sluice gates, each six feet wide and four feet high. These gates are provided with hand-operated headstocks which are controlled from a deck carried above the spillway. The floor of this deck is six feet above the crest level and is carried on rolled steel channels.
In the intake dam three contraction joints have been provided, at intervals of approximately eighty feet. At the west end the body of the dam is keyed into the rock; on the east it joins the intake works to the pipe line. The intake works, running at an angle of approximately 118° to the dam, consist of a mass concrete structure in which are set two sluice gates and the transition piece from square section to circular section for the pipe line.
On the upstream face of this structure are several sloping screens of bars, ⅜ in. thick, 3 in. deep, and spaced ¾ in. apart. These prevent the passage down the pipe line and through the machines of floating debris, stones or anything that might damage the turbines. Above the screens there is a platform from which the removal of debris may be carried out periodically.
THE GENERATING PLANT consists of two horizontal single-runner turbines, each rated at 3,600 horse-power. The turbines drive alternators at 750 revolutions a minute, and power is generated at 6,600 volts. Provision has been made for the addition of a third turbine, and the capacity of the pipe line is 10,800 horse-power.
The transition piece changes the section from 7 ft. 6 in. square to the 6 ft. 6 in. diameter of the pipe line; an air inlet pipe prevents the vacuum in the main pipe from falling when the sluice gate is closed. This main gate is normally operated by hand, but there is also an automatic closing device which comes into action when the velocity of the water in the pipe line exceeds a predetermined figure. Thus an accidental break in the line or in the turbine casing may be prevented from causing serious trouble.
The pipe line is 1,230 feet long. The first 418 feet have a diameter of 6 ft. 6 in; for the rest of its length the pipe is 6 feet in diameter. The steel plates of which it is made vary in thickness from ⅜ in. to ½ in. The larger size of the top section is intended to prevent excessive loss of head due to hydraulic friction.
Four main anchorages are provided, each at a change in the natural gradient of the slope. They are large concrete structures reinforced with steel and with their foundations carried down to solid rock. During their construction the pipe line was held rigidly in position at each point. Provision has been made for the installation of a second pipe line, should power requirements necessitate it, and the excavation of rock for the anchorages was carried out for the full width required for the two lines, thus obviating the risks that would be incurred by further blasting after the first line had been placed in position.
In the lowest anchorage the pipe line divides into two smaller pipes, each of 3 ft. 6 in. diameter. These pipes feed the two turbines in the power station, and provision for the supply to a third turbine has been made by a third branch taken from the main pipe upstream from the bifurcation. The capacity of the single pipe line is roughly 10,800 horse-power (7,500 kilowatts) and each of the two turbines of the installation is of 3,600 horse-power.
On the concrete pedestals the weight of the water-filled pipe is carried by saddles which have bronze rubbing plates. These rubbing plates are lubricated by a grease gun.
Because of the difficulties of transport and the fact that the pipe line had to be transported by sea, rail and jungle road to the site of the development, it was sent out in the condition known as “plate small”, having been previously assembled in sections in the manufacturers’ yard. All the rivet holes were matched, and all the plates were marked for assembly in the correct positions. The erection and the riveting of the pipe were carried out entirely by native labour.
The rise-and-fall of the river level at the exit from the power station is a matter which has an important bearing-on the design of a power house building and on the level at which the turbines are to be placed. At Pangani, to obviate all risks of flooding during an abnormally rainy season, it was considered necessary to keep the floor ten feet above the recorded high water level. This, however, gave too high a suction head on the discharge of the turbines, and weirs were built across the tail race. These weirs, though depriving the plant of a few feet of available head, ensure that the bottom ends of the draught tubes from the turbines would be always submerged.
Automatic Relief Valves
A massive stone building for the power house was considered unnecessary and the most economical type of structure was built. It was a steel frame covered with asbestos cement and galvanized iron sheeting. To keep the interior of the building reasonably cool a double roof was provided, having flat asbestos cement sheets. Extractor fans were provided in each gable end.
For handling the machinery during construction and, later, during overhauls, a travelling hand-operated 25-tons crane was provided. The generating plant consists of two 3,600 horse-power single-runner turbines of the horizontal type, each of them driving a 2,500-kilowatts alternator operating at 6,600 volts, three-phase 50-cycles alternating current. The speed at which these alternators are driven is 750 revolutions a minute.
LOWERING A TRANSFORMER down the haulage track by an electric winch. The cable is visible above the rails. The pipe line and power equipment had to be transported by sea, rail and jungle road. A ferry barge, big enough to carry a heavily loaded lorry and trailer across the Pangani River, was built near the site of the hydro-electric works. A 25-tons travelling crane was used also.
To limit the pressure rise in the pipe line, when full load is suddenly thrown off, automatic pressure relief valves, of 20 in. diameter, governed and operated by means of a dashpot, are connected to the spiral case of each turbine. By this means the pressure rise is restricted to a maximum of 10 per cent.
An air inlet valve is provided for admitting air to each draught tube when the unit is running on light load. The turbines have manganese bronze runners, cast in one piece and accurately balanced, and the spiral casings are built up from steel sheets and welded. A brake to bring each unit to rest in a reasonable time has also been provided. Satisfactory balance was ensured by forwarding the turbine runners to the manufacturers of the alternators, to be assembled on the alternator shafts so that the combined balance of turbine and rotor could be tested. The whole of the hydraulic plant was tested at a pressure equivalent to that of a head of 629 feet of water.
Each of the alternators is provided with a main exciter and a pilot exciter. Because of the high speed at which they operate, the eight alternator poles are dovetailed into the rotor forging and forced into position under hydraulic pressure. The rotors are built of a number of steel disks shrunk on to the shafts. Each alternator has two water-cooled pedestal bearings, and the turbine runners are mounted on overhung portions of the alternator shafts.
The pilot exciters, which supply the excitation current to the main exciter field windings, are mounted on shaft extensions overhung from the main exciters.
System of Interlocks
The alternators are cooled by air taken from the outside of the power house and discharged-through ducts in the concrete below the machines. Propeller fans are used to give axial ventilation to the alternators. A 6,600-volts cubicle-type switchboard on a gallery controls the two alternators and the two outgoing feeders to transformers which step up the voltage to 33,000 for the main system of transmission lines.
As the gear is operated solely by native attendants, a system of interlocks has been provided. This system not only prevents access to the interior of the cubicles until the apparatus is “dead”, but also prevents incorrect switching during synchronizing and changing over from one main busbar to another. Duplicate busbars with special switching arrangements enable changeovers to be carried out “on load”.
The main electrical transmission system operates at 33,000 volts with 11,000-volts distributors as branch feeders or ring-mains. It is carried on concrete poles 36 feet long which were manufactured at a factory set up near the centre of the system. Step-down transformers at many points on the 33,000 and 11,000 volts systems supply sisal factories and small townships with a three-phase 400/230-volts four-wire system.
The density of the jungle and the absence of all-weather roads caused many transport problems, particularly during abnormally heavy rains. These conditions, combined with the unhealthy climate, made the building of the transmission and distribution lines an unusually arduous task. Not only had clearings to be cut through the tropical forest for the right-of-way of transmission and distribution lines, but also temporary roads had to be made to provide access for the poles, which weighed a ton each, and for other construction materials.
GATE SECTION OF THE INTAKE DAM, located at the foot of the Upper Rapids, not far above the main fall on the Pangani River. The spillway section of the dam is 250 feet long; the intake works consist of the mass concrete section illustrated above. On the right is the connexion to the pipe line; on the left is another connexion which may be used later with a second pipe line. The sluice gates are operated by hand, but an automatic closing device has also been installed.
