270 HYDRAULICS AND ITS APPLICATIONS 



height to p H-mit the service mains to be supplied with water under a 

 pressure of from 100 to 150 feet. Where this is not possible it becomes 

 necessary to adopt some pumping system and to pump water under 

 pressure into the pipes as required. 



Where the difference of level between the supply and service reservoirs 

 is great, the whole of the supply pipe line may be put under considerable 

 statical pressure if a valve is fitted at its outlet, and the cost of a pipe 

 line designed to withstand such a pressure may be excessive. With a 

 view of reducing the maximum pressures, the regulating valve may be 

 fitted at the pipe entrance, or one or more small reservoirs known as 

 break -pressure reservoirs may be provided at suitable intervals in the 



FIG. 118. Break-Pressure Reservoirs. 



line, as shown in Fig. 118. The service level in each of these reservoirs 

 is in the gradient line for the system, and the hydrostatic pressure in 

 any length of the pipe cannot now exceed that due to the next higher 

 reservoir. 



In order to reduce the risk of a total stoppage of the supply in case of 

 accident to the supply main, the latter may be laid in duplicate, two 

 pipes of smaller diameter taking the place of the single larger pipe. 

 Where the lines are long, there should be cross connections with valves, 

 so that in case of a break in either line a short section only need be cut 

 out. This gives a greater discharge than would be possible with the line 

 single throughout. 



For pipes up to about 24 inches in diameter the cost of the pipe itself, 

 the jointing, excavating, and laying, is roughly proportional to the pipe 

 diameter, and the cost of a twin pipe with cross connections and valves 

 is about 50 per cent, greater than that of a single pipe line. For larger 



