A Treatise On The Principles And Practice Of Dock Engineering

WATER UNDER PRESSURE. JH advantage be derived if the point of application be situated more than 350 to 400 yards from the point of generation. Even at less distances, a machine will be but indifferently served. These considérations all point to the conclusion that steam is an admir- able motive agency for locomotive cranes and other appliances in which, in addition to local action, movement through an extensive range of position is essential, but that in order to be economical such machines must be at work continuously for long periods. It has advantages, also, for small detached installations, where the cost of a centralised generating plant, with extensive ramifications, would be out of proportion to the duty required. In all other cases, a system of hydraulic or electrical energy will be found préférable. Water under Pressure.—In contradistinction to the previous elements, water is an incompressible medium; but its very inelasticity, while freeing it from loss of power in one direction, only exposes it to loss in another, and not improbably to an equal extent. The motive effort of water-power is obviously invariable, whatever resistance may be opposed to it, and, consequently, the same expenditure of energy is necessary whether the work done be considerable or insignificant. On the other hand, hydraulic machinery, when working at full power, is characterised by a high efficiency ; the loss due to the friction of the working parts then rarely exceeds 8 or 10 per cent.* Furthermore, there is great smoothness and regularity of movement, and the appliances are capable of being manipulated with extreme precision, while they do not call for specially trained or skilled operators. As against this, must be set the trouble and inconvenience caused by frost. Apart from the freezing of water in the conduits, which in many cases are unavoidably exposed to atmospheric influence, there is the con- sidération that the neighbourhood of hydraulic machinery is invariably wet and sloppy, and this leads to the formation of ice there, which is manifestly dangerous to those working at a quay side. The evils have to a certain extent been mitigated by the provision of gas jets in machinery pits, or by bringing all the service pipes and valves into a closed cabin which can be artificially warmed when necessary. But such arrangements whilst more or less effective in themselves, are undoubted evidence of the difficulties attending the use of water-pressure machinery in the winter * Mr. Robinson gives the following coefficients for hydraulic rams with ordinary hemp packing:— Direct-acting,.....................................-93 efficiency. 2 to 1, . ........................-8 )( 4„1...................................76 6 „ 1, ............................-72 8 „ 1, ............................-67 „ 10 „ 1..............................63 „ “Transmission of Power,” Min. Proc. Inst. C. P., vol. xlix.