HYDRAULICS AND ITS APPLICATIONS 



usually introduced at the inlet to a reservoir or near the outlet from such 

 an inverted syphon. This consists of an enlargement in the pipe fitted 

 with a diaphragm pierced with a series of valve openings. These are 

 furnished with flap valves opening in the direction of normal flow, and 

 automatically close if the direction of the current is reversed. 



Where serious damage would follow fracture of a main it is usual to 

 fit an automatic stop valve. Such a valve is placed, if possible, under 

 comparatively light pressure, near a deep depression in the pipe line. In 



this type of valve a disc which is 

 held by a lever, and projects into 

 the water way, is thrown back 

 when the velocity of flow exceeds 

 a predetermined limit. This 

 releases a trigger and an attached 

 weight which closes the valve. 

 The speed of closure may be 

 regulated by means of a dashpot. 

 Such an arrangement is shown 

 diagrammatically in Fig. 120. In 

 other valves of the same class the 

 main valve is of the sluice type 

 with parallel faces and is opened 

 or closed by means of a loaded 

 piston in a hydraulic cylinder 

 surmounting the valve. Water 

 under pressure is admitted to 

 the under or upper side of this 

 piston by a four way valve 

 actuated either by the release o: 

 a trigger as in the former type 

 or by the motion of a loadec 

 piston in an auxiliary cylinder which is coupled to the main. 



Where different parts of a town are at widely different elevations, the 

 head necessary to supply the more elevated sections may lead to an 

 excessive pressure in the more low lying portions of the supply system 

 and in such a case it is usual to divide the area into zones at respectively 

 lower levels, each zone carrying an independent distribution system fee 

 from the main. The lower zones are then supplied through a series of 

 reducing valves, each set to maintain the required maximum pressure in 

 its respective zone. In case of fire it is advisable that the full head be 



FIG. 120. Automatic Throttle Valve. 



