HYDRAULIC POWER TRANSMISSION. In many instances the use of water under considerable pressure as a medium for the transmission of energy offers advantages over other methods of power transmission, and this is particularly the case where the power is required to operate machinery in which the action is either (a) comparatively slow, but in which a con siderable force is required, and particularly where the motion re quires to be regulated with great precision ; or (b) very inter mittent, a large force being required at intervals and for a corn paratively short time. It is thus well adapted for the operation of presses, flanging and riveting machinery, lifts, hoists, cranes, capstans and testing machines.
In many large towns, of which London, Manchester and Glas gow are examples, customers are supplied from a central pumping station through a system of hydraulic mains. The pressure adopted varies from 700 to 1,600 lb. per square inch, being 75o lb. per square inch in the City of London, and 1,12o lb. per square inch at Manches ter and Glasgow. In the older installa tions the pressure water was obtained from reciprocating pumps driven by steam en gines, but in the most recent extensions, multi-stage high lift centrifugal pumps are used. The pumps feed accumulators, which in turn feed the supply mains. These are usually of cast iron. Flanged spigot and faucet joints are used with $ inch gutta-percha packing rings as shown in fig.
I, which illustrates a joint as used for a 6 inch pipe. Owing to the very high pressures used, even a small leakage is serious, and to guard against such a leakage from the mains or valves, a daily record of the minimum flow during the time the demand is at its lowest (between II P.M. and 4 A.M.) is kept by means of an auto matic electrical recorder. Should this show an abnormal increase in the output for several consecutive nights, a detailed examina tion of the mains is made.
In the London installation the water is taken from the river or from wells, and as it is essential that all deposit should be removed before use, it is allowed to stand for some time in stor age tanks. The greater part of the solid matter thus becomes deposited. The water on its way to the pumps is then passed through the surface condensers of the engines to a series of filters, in which it is passed first through a layer of broken sponge, and afterwards through a bed of charcoal.
Transmission Losses.—The transmission losses are due to fluid friction in the pipe line. With water at a given pressure, the energy transmitted varies directly as the volume flowing per second, so that since the loss due to friction increases as the square of the velocity, the proportional effect of this diminishes as the working pressure increases, and for high efficiency of trans mission the working pressure must be high.
It may readily be shown that the friction loss is equal to where H is the horsepower entering the pipe ; 1 is the length and d the diameter of the pipe in feet; p is the pressure at the pipe inlet in lb. per sq. in. ; and f is a coefficient of friction whose value, for a new cast iron pipe, is approximately •oo6. It follows that the efficiency of transmission equals and that this efficiency increases as p and d are increased. An increase in p however involves an increase in the thickness of the pipe walls and in the difficulty of preventing leakage at the joints, so that in practice it has not been found advisable to use pressures much greater than 1,5oo lb. per square inch.
The point at which it ceases to pay to increase the diameter of the pipe line for a given horsepower, depends on the relative cost of the pipe line, including excavation, jointing and laying, and of the power production per horsepower. In general a size of pipe which allows of a pressure drop of about Io lb. per square inch per mile is found to give most economical results in practice. With this drop and with a pumping pressure of 1,120 lb. per sq.in. the following are the efficiencies of transmission : It will be seen that for distances not exceeding 1 o miles the efficiency is high. In modern practice the largest pipes are about 6 inches diameter, the pipe lines being duplicated for large pow ers. Such a pipe will transmit about 14o h.p. with a drop of pressure of i o lb. per sq.in. per mile. The velocity of flow usually ranges from 2.5 to 4.o ft. per second.
It may be shown that the maximum amount of power is trans mitted through a given pipe line when the velocity of flow is such as to make the outlet pressure equal to two-thirds of the pressure at inlet, in which case the efficiency of transmission is only 66.6%. Under these conditions a 6 inch pipe line 1 mile long, having a supply pressure of 1,120 lb. per sq.in., would deliver energy at the rate of 57o h.p., the velocity of flow being 14.8 ft. per sec.
Accumulators.—Since the delivery from a reciprocating pump is not uniform and since it is necessary to have some reserve of energy to meet a sudden or abnormal demand, some means of storing pressure energy is a necessary adjunct to the hydraulic power station. With the high pressures in common use an ele vated gravity storage tank is impracticable and the accumulator, devised by Sir W. G. Armstrong, takes its place. This is fitted between the pumps and the pressure main. The accumulator con sists of a vertical cylinder fitted with a weighted ram, whose weight is adjusted so as to give the required pressure in the mains. In its most common form the ram (fig. 2) carries a platform which is loaded with some heavy material, usually pig iron or iron slag. If the output from the pumps exceeds the demand, the ram rises, and on reaching the upper limit of its travel moves a stop which, by suitable link connections, causes steam to be shut off from the pumping engine.
The energy-storage capacity is equal to the potential energy of the lifted ram and weight, and, if L is the length of its travel in feet, and W its weight in lb., is given by LW foot lb., or by pAL foot lb., where p is the working pressure in lb. per square inch and A is the cross sectional area of the ram in square inches. Thus if the diameter of the ram is 18 in. and if p= i,i 20 lb. per sq.in. and L=20 ft., the storage capacity is 5,700,000 ft.lb. or 2.9 h.p. hours. Such an accumulator could not give out energy in excess of the rate at which energy is being supplied to it by the pumps, at a greater rate than 2.9 h.p. for I hour; 17.4 h.p. for 1 o min utes; or 174 h.p. for I minute.
From this example it is evident that the storage capacity of such an accumulator is not large and that its main function is not so much to store energy in the sense that an electric accumulator stores it, as to permit of momentary fluctuations in the rates of supply and demand, or in other words to act as a flywheel does in the case of a steam or gas engine. It also serves to regulate the delivery pressure. Its efficiency is high, up to 98 per cent of the energy expended in charging being re turned during delivery.
Where the hydraulic power is to be used for operating such machines as riveters or presses, a small accumulator is often installed at the consumer's end of the pipe line. A modification of the simple type, known as the differential accumulator is shown in fig. 3. This consists of a fixed ram of area A, surrounded over the lower portion of its length by a closely fitting bush of area a. This bush terminates be low the inlet and outlet holes. The ram passes through both ends of the storage cylinder, through glands of area (A-1--a) and A, so that the effective cylinder area exposed to upward pressure is a.
Thus pa =W, and by making the bush of small thickness, a very large pressure may be maintained by a comparatively small weight.
The system, which was devised by G. Constantinesco, has been applied to the operation of rock drills, etc. It has the merit of safety and flexibility, but has not yet been adopted sufficiently extensively for an opinion to be expressed as to its possibilities. Little information is available as to its efficiency under normal operating conditions.