SYPHONS 



273 



available, and for this purpose the type of reducing valve shown 



diagrammatically in Fig. 121 has been devised. 1 This consists essentially 



of a double-beat equilibrium valve 



whose opening is normally regulated by 



the pressure on its discharge side. In 



case of an abnormal draught on the 

 main, such as might occur in case of 

 fire, the pressure falls sufficiently low 

 for the weight W to open the auxiliary 

 valve V, and the under side of the 

 plunger P is put into free communica- 

 tion with the atmosphere. This causes 

 the weight, and the valve, to fall to 

 its lowest limit, giving an uninterrupted 

 flow past the main valve. 



Two methods of lay-out are common 

 in the demand area of supply systems. 

 In the first of these a large main is 

 laid through the centre of the district 

 with branch pipes extending from it 

 to form a gridiron covering all parts 

 of the district, the outer ends of these 

 branches being connected by a small 

 main so as to eliminate dead ends. 



In the second system, commonly 

 termed the " ring " system, a large 

 main is laid to enclose the district, and from this main branch pipes are 

 taken to form a grid covering the whole area. 



If both systems are designed to give the same discharge at the same 

 pressure at all points of the district, the former system usually costs 

 considerably less than the latter, and is to be preferred. 



FIG. 121. Reducing Valve. 



ART. 82. SYPHONS. 



Where a pipe line is to be laid to connect two reservoirs at different 

 levels, over ground which is higher than either water level, the cost of 

 excavation is often so great as to preclude the use of a pipe line which 



1 By Mr. G-. F. Deacon. This device is used on the distribution system of the Liverpool 

 Water Works. 



H.A. fi 



