A Treatise On The Principles And Practice Of Dock Engineering

PUMPS. 113 which find their way into a pumping well in excavations carried on under circumstances, perhaps more peculiarly characteristic of dock work than of any other branch of engineering, pumps of the strongest and simplest construction are advisable. Such, for instance, are the chain pump and the centrifugal pump. The first of these which has demonstrated its utility from remote ages, being originally an invention of the Chinese, consists of a series of flat blades, strnng at regular intervals upon two parallel endless chains. These chains hang vertically, being suspended from a revolving reel or barrel at the summit, over which they travel continuously. The descent is in the open, but on reaching the bottom the blades enter the splayed orifice of a rectangular funnel extending upwards to the point of discharge. The blades fit the interior of the funnel sufficiently closely to take the bulk of the enclosed water with them without incurring excessive friction against the sides. The pump acts admirably in lifting with absolute impartiality water, mud, pieces of brick, wood, stone, and concrète ; any substance, in short, which can enter the funnel. The only thing to check its action is the intrusion of a chance wedge or plank end, transverselv, between the buckets and the orifice The blades, which are of wood, are, of course, subject to a considerable amount of abrasion and have to be replaced from time to time, but repairs of this kind are easily effected. A stock of fresh blades is kept at hand, and the operation of removing a damaged blade is simply that of taking out the split keys which hold it in position on the chain. Chain pumps with rectangular blades, 2 feet long and 6 inches wide, 14|-inch centres, running at a speed of 500 feet per minute have proved capable of discharging regularly 600 tons of water per hour, which represents an efficiency of slightly less than 70 per cent. The speed may be increased to 600 or 700 feet per minute, with a corresponding greater discharge, but such speeds throw an undue strain upon the apparatus. The action of a centrifugal pump is the revolution of a series of blades radiating from a common axis, by means of which the water is whirled round in a confined space until it acquires sufficient velocity to be projected up the discharge pipe. The blades are short, thick, and curved in form. This class of pump will “ throw ” a good deal of extraneous material, but there is always the possibility of a fairly large object being drawn through the suction pipe and getting jambed in the blades, which are less accessible for repairs than those in a chain pump. The usual sizes of such pumps for temporary duties varies between 6 and 18 inches diameter. Before leaving the subject, it will be well to observe that the provision of a duplicate pumping system is a commendable arrangement. One set of pumps might easily break down at a critical moment, and even if the amount of pumping is sufficiently small to allow adequate intervals for cleaning and repairs, yet an auxiliary pump is an advisable precaution for unforeseen coutingencies. 8