Page:EB1911 - Volume 22.djvu/245

HYDRAULIC] distance from the central station. They act in the same way as air-vessels. The mains should be laid in circuit, and valves placed at intervals, so that any section can be isolated for repairs or for making Connexions without affecting the supply at other points. The main valves adopted in London are shown in fig. 4. Valves are also fixed to control all branch pipes, while relief valves, washouts and air valves are fixed as required.

The largest pipes used in London are 10 in. internal diameter, and the smallest laid in the streets are 2 in. The pipes from 8 in. and below are usually made in cast iron, flanged and provided with spigots and faucets. The joint (fig. 5) is made with a gutta-percha ring, though sometimes asbestos and leather packing rings are used. Cast iron pipes or hydraulic power transmission have been standardized by the Engineering Standards Committee. Fig. 6 shows the 10 in. steel main as used in London. The main was laid in 1903 from the Rotherhithe Pumping Station of the London Hydraulic Power Company through the Tower Subway, and is used as a feeder main for supply to the City. It is the first instance of the use of feeder mains in hydraulic transmission. The velocity of flow is 6 ft. per second, and is automatically disconnected from the general system should the pressure in this main fall below that of accumulator pressure. Other mains, similarly controlled, are now in use. Ellington’s system of hydraulic feeder mains has been developed by the laying of a 6-in. steel main from the Falcon Wharf Station at Blackfriars to the Strand, over Waterloo Bridge. . 7.

The Falcon Wharf Pumping Station at Blackfriars was the original central station in London, and the accumulators there are loaded to per square inch. The other pumping stations are situated about 3 m. from Falcon Wharf and about the same distance from each other. The accumulator pressure at the outlying stations is during the busy time of the day maintained at about per square inch. Consequently the smaller variations in demand for power throughout the system caused very intermittent running) of the plant at Falcon Wharf, and the load-factor there is very low. The pumping plant has now been considerably increased, and part of the plant is constructed to pump into the feeder main at pressures of 890, 900, or per square inch according to the demand existing from hour to hour in the Strand district. By this means the output from Falcon Wharf has been doubled with a much improved load-factor. The accumulator in this system is of special construction (fig. 7). The pressure per square inch is maintained in the cylinder A from the ordinary hydraulic supply main. The working ram B forms the cylinder for the fixed hollow ram C which is connected to the 6 in. bore feeder main D. The balancing rams E, E attached to the fixed head F serve the purpose of adjusting the pressure in the feeder main from 800 to per square inch according to the quantity of pressure water require to be transmitted through it. The higher pressure is required when the velocity in this main is 10 ft. per second. There is an automatic control valve at the junction of the feeder main with the service mains in the Strand, adjusted so that the same effect is produced as if a pumping station were in operation at that point of equal capacity to the maximum flow through the 6 in. main. The length of the feeder main in this case is 2000 yds., and at 10 ft. per second there is a loss of pressure of  per square inch, but the effect on the coal consumption is almost negligible, as the maximum flow is seldom needed. The engines are specially constructed to take the pressure overload. The feeder main is made of steel. The economical limit of the use of feeder mains is reached when the additional running expenses involved equal the annual value of the saving effected in the capital expenditure.

In public supplies the power used is in all cases registered by meters, and since 1887 automatic instruments have been used at the central stations to record the amount supplied at each instant during the day and night. The ratio between the power registered at the consumers’ machines and the power sent into the mains is the commercial efficiency of the whole

system. The loss may be due to leakage from the mains or to defects in the meters; or if, as is often the case, the exhaust from the machines is registered, to waste on the consumers’ premises. The automatic recorders give the maximum and minimum supplies during 24 hours every day, the maximum record showing the power required for a given number and capacity of machines, and the minimum giving an indication of the leakage. It has been found practicable to obtain an efficiency of 95% in most public power transmission plants over a series of years, but great care is required to produce so good a result. In some years 98% has been registered. Until 1888 no meters were available for registering a pressure of per square inch, and all that could be done was to register the water after it had passed through the machines and lost its pressure. This method is still largely adopted; but now high-pressure meters give excellent results, exhaust registration is being superseded to a considerable extent by the more satisfactory arrangement of registering the power on its entry into the consumers’ premises. In Manchester Kent’s high-pressure meters are now used exclusively. Venturi meters have also been used with success for registering automatically the velocity of flow, and, by integration, the quantity in hydraulic power mains, and form a most useful check on the automatic recorders. The water after the pressure has been eliminated by passage through the machines, may run to a drain or be led back to the central station in return mains; the method adopted is a question of relative cost and convenience.

We proceed to the machines actuated by hydraulic power, and by a comparison of the useful work done by them with the work done by the engines and boilers at the central station the mechanical efficiency of the system as a whole can be gauged. At the central station and in the distribution there is no great difficulty in determining the efficiency within narrow limits; it should be 80% at the point of entry to the machine in which the pressure is used.

Where feeder mains are in use the efficiency of the system is necessarily reduced, owing to the higher velocities allowable in the feeder mains. Mechanical efficiency is then sacrificed for the sake of economy. The mechanical efficiency of the machines is a very uncertain quantity; the character of the machines and the nature of the conditions are so variable that a really accurate general statement is impossible. In most cases the losses in the machine are practically constant for a given size and speed of working; consequently the efficiency of a given machine may vary within very wide limits according to the work it has to do. For instance, a hydraulic pump of a given capacity, delivering the water to an elevation of 100 ft., will have an efficiency of 80%; but if the elevation of discharge is reduced to 15 ft., even though the hydraulic pressure rams may be proportioned to the reduced head, the efficiency falls below 50%. The ultimate efficiency of the system, or $pump h.p⁄i.h.p.$ in the one case is 64%, and in the other under 40%. In crane or lift work the efficiency varies with the size of the apparatus, with the load and with the speed. Efficiency in this sense is a most uncertain guide. Some of the most useful and successful applications of hydraulic power—as, for instance, hydraulic capstans for hauling wagons in railway goods yards-have a very low efficiency expressed on the ratio of work done to power expended. Hydraulic cranes for coal or grain hoisting have a high efficiency when well designed, but it is now very usual to employ grabs to save the labour of filling the buckets, and their use lowers the efficiency, expressed in tons of coal or grain raised, by 33% or even 50%. When hydraulic machines are fully loaded, 50% to 60% of the indicated power of the central station engine is often utilized in useful work done with a radius of 2 or 3 m. from the station. In very favourable circumstances the efficiency may rise to over 70% and in a great many cases in practice it no doubt falls below 25%. If, however, energy in any form can be obtained ready for use at a moderate rate, the actual efficiency of the machines (i.e. the ratio of the useful work done to the energy absorbed in the process) is not of very great importance where the use is intermittent.

Hydraulic pressure is more particularly advantageous in cases where the in compressibility of the fluid employed can be utilized, as in hydraulic lifts, cranes and presses. Hydraulic machines for these purposes have the peculiar and distinct advantage of direct action of the pressure on the moving rams, resulting in simplicity of construction, slow and steady movement of the working parts, absence of mechanical brakes and greatest safety in action. When the valve regulating the admission of the pressure to the hydraulic cylinder is closed, the water is shut in, and, as it is