Water Supply

The tunnel and cut and cover portions are made capable of taking the ultimate yield of the source : for this quantity two or more pipe lines, side by side, may be required, but initially only one line would be laid, the others being added wholly or in part and cross-connected to the original line and to each other as the growth of demand necessitates. These portions act also as break pressure tanks, so that the maximum pressure to which any par ticular length of pipes is subjected is that corresponding to the top water level in the preceding tunnel or cut and cover.

When of appreciable length, the tunnel and cut and cover por tions should be provided with automatic self-closing outlet valves designed to close when the flow exceeds the normal by more than a predetermined amount, to prevent undue loss of water in the event of a burst in the ensuing pipes, and each pipe should have a reflux valve at its termination to prevent loss of water in the other direction. Where an aqueduct consists wholly of pipes, break-pressure tanks of relatively small capacity are located at suitable high points, and the inlet to these tanks is controlled by some form of float regulator to prevent loss by overflow, and the outlet by a self closing valve. To facilitate testing and repairs, long mains are provided with stop-valves at intervals of about 13-2 miles with scour valves at depressions, and air valves at all summits. Where the lining is with either cement, mortar or bitumen, it is well to combine the air-valve and an access man hole to facilitate the making of joints. Where unlined pipes, liable to encrustation, are used, hatch pipes should be provided for the passage of the "scraper." Additional air valves are provided in any long lengths of main which are laid to flat gradients, or at any sudden change of gradient.

Pumping Machinery.

Where coal is available of suitable cost and quality, the triple expansion steam engine using super heated steam is still the most efficient pump against a constant head. Where centrifugal pumps are used they may be driven by steam turbines. The Diesel Engine has proved so reliable in ship propulsion, that it is being widely used. It is equally efficient in large and small units. Where electric current is cheap, motor driven pumps are frequently adopted : better terms can be ob tained if the pumping avoids the peak hours of the generating station, and automatic controls can be installed to effect this.

For intermittent work or small quantities, electric motors, oil engines, or gas engines, may be used.

Where the suction lift does not exceed about 25 feet either direct acting or centrifugal pumps may be used : where the suction lift exceeds this limit, three-throw plunger pumps may be used in conjunction with a suction well of ample size, or where space is a consideration, as in a borehole, vertical spindle turbine cen trifugal pumps may be employed.

Where water has to be raised from great depths by means of boreholes, the air-lift pump is frequently the only arrangement available : it has the advantage of having no moving parts, and although its efficiency is low it is considered to have advantages in certain cases, which compensate for the lack of efficiency.

Rising Main.

In contradistinction to a gravitation supply, where the general hydraulic gradient and therefore the dimensions of component parts of the aqueduct are, for a given capacity, fixed within close limits by the levels of the impounding and main service reservoirs, the hydraulic gradient of the rising main is determined by purely economic considerations. The terminal level, i.e., the top water level of the service reservoir, is fixed by the level of the area to be supplied from that reservoir, but the head against which the pumps are designed to work, and which determines the hydraulic gradient under which the rising main will operate, must be above that due to the service reservoir.

The carrying capacity of the main is a function of its size and hydraulic gradient : the flatter the gradient the larger the pipe and the smaller the head against which the pumps will operate, for, in addition to the purely static lift given by the difference between the level in the suction well and the level in the service reservoir, there is the friction head which is the product of the hydraulic gradient and the length of the main. Conversely, a smaller and less costly pipe may be used at the expense of increas ing the pumping head, the size of the pumping units, and the cost of pumping. In its simplest terms, the economic size of the rising main is that diameter for which the sum of the interest and sink ing fund charges on the cost of the pipe line, the pumping units and the building, the service reservoir and filters (if required) and of the annual working expenses, is a minimum.