A Treatise On The Principles And Practice Of Dock Engineering

HYDRAULIC ACCUMULATORS. 519 the former arrangement. Furthermore, the saving effected by omitting one cylinder and ram is largely discounted by the cost of making the other two cylinders double-acting, and almost necessarily of brass. The maintenance charges also are greater. Except, therefore, in the case of restricted space, the three-cylinder system with plain rams is generally adopted. We now turn our attention for a moment to the production of hydraulic power. Hydraulic Accumulators. —In the first instance the requisite pressure for driving hydraulic machinery was obtained by means of a natural head of water, but this system, in the majority of cases, the locality being flat, involved the erection of a lofty water tower and reservoir. The impossi- bility of economically erecting such a tower at New Holland on the Humber, where the foundation consists of silt to a considerable depth, led Armstrong, in 1850, to substitute an arrangement, since generally adopted and known as an “accumulator,” by which water was pumped into a large cylinder against the weight of a heavily loaded ram or plunger. As long as the ram is kept off its seat at the bottom of the cylinder the water is maintained at a high and constant pressure—at a much higher pressure, in fact, than could be obtained by natural means; for, whereas before the introduction of the accumulator, in no instance had a greater pressure than 90 Ibs. per square inch been used, at the present time pressures as great as 700 and 750 Ibs. per square inch are quite common, and 1,000 and 1,250 Ibs. pressures are also in use. The advantages arising from this incrément are apparent. The sizes of the distributing mains and of the pressure cylinders have been greatly reduced, while at the same time the capacity for work has been materially augmented. The accumulator has one drawback : it does not afford much storage room, consequently pumping is necessarily continuous, and the joints and pipes in the mains must be rendered pressure proof. These considerations, however, are of minor importance compared with the advantages accruing to the system as a whole. It is essential that the water used in connection with hydraulic apparatus should be both fresh and clean. Salt or acidulated water will corrode the mains and cylinders; grit and sediment will wear and choke the valves. Consequently, where the source is at all liable to contamina- tion there should be a settling tank, and supplies should be taken from the top in such a way as to ensure purity. There is no objection to the repeated use of the same water; in fact, this arrangement is generally adopted, the water being returned to the pumping well through an additional main, the diameter of which is rather greater than that of the pressure main. Slide valves are more liable to injury from grit than mitre valves, but if the settling tank be adopted and ordinary precautions observed, there is no reason why extensive repairs should be necessary in either case. Air vessels have been tried in place of weighted accumulators, but they