A Treatise On The Principles And Practice Of Dock Engineering

PUMPS. III cation it may be added that the nett costs of the two dams were about £15 and £17 respectively. For tidal work a dam may be conveniently contrived by sinking iron pontoons and banking them up and between with clay. The height of such a dam is necessarily small, but it materially increases the period of working within the enclosed area. Pumps.—The subject of pumping demands the most careful and earnest attention of the dock engineer, seeing that the practicability and success of his undertakings depend largely upon the efficiency of his pumping arrange- ments. Some evidence of this will be afforded in subséquent chapters, but the fact is almost sufficiently obvious in itself. There are many varieties of pumps on the market, each with its own special features and capabilities. A study of the catalogues of well-known manufacturers will generally enable a satisfactory selection to be made for the particular purpose required, and the following remarks are simply appended by way of indicating such practical points as seem worthy of consideration in exercising a choice. Valve Pumps—thai is to say, lift pumps and force pumps, or any com- bination of these in which the action dépends upon the alternate opening and closing of small valves—are only suitable for comparatively clear water. Water which is highly charged with solid matter in suspension and with floating objects is very likely to derange these delicately adjusted parts and to put the pump out of action. The gritty nature of sand causes excessive wear of the leather washers and packings, necessitating frequent renewals. Chips and gravel lodge in the valves and prevent them from closing. The jambing of the bucket packings may cause serious trouble owing to the great force frequently required to release the bucket. For drainage purposes in trench excavations, a lift pump has this advantage over a force pump, in that, if the working should by any accident become suddenly fiooded, the lift pump can still discharge its function, being actuated from the summit level, whereas the machinery of a force pump is in the bottom and, consequently, would be submerged. A very handy drainage pump for use in confined situations is the Pulsometer. It represents a rather unusual principle in pumping. The action consists in the alternate admission and exclusion of steam to and from adjoining chambers. The water is forced out of one of the two chambers by steady pressure until it sinks to the level of the discharge orifice, at which point the steam obtains a free vent, and being in contact with a large surface is so rapidly condensed as to cause a vacuum in the chamber and draw over the steam ball at the top which closes the aperture and transfers the supply to the next compartment. Meanwhile, continued condensation in the empty chamber increases the vacuum, which is filled by a fresh supply of drainage water through the lower valve leading from the suction pipe. The apparatus is compact and easily suspended by a rope or chain in any desired position.