GATES AND VALVES 157 



the valve. The waterways throughout the valve offer no obstruc- 

 tion to the flow and consequently there is no appreciable loss of 

 head. 



No external source of power is required for operation. When 

 the plunger is withdrawn into the internal or operating chamber, 

 the valve is open and presents an unobstructed passage for the 

 water. When the plunger protrudes from the operating chamber, 

 it seats against a ground ring in the neck of the valve body, form- 

 ing a water-tight joint. The standard control mechanism pro- 

 vides for only the open and closed positions of the plunger, but it 

 may be specially arranged to hold the plunger at intermediate 

 positions if desired. 



The valve plunger is of the differential type, forming an annular 

 chamber A within the operating cylinder, in addition to the cen- 

 tral chamber B. By means of a suitable external control valve 

 and piping, either pipe-line pressure or atmospheric pressure may 

 be alternately applied to the chambers A and B. Admitting pipe- 

 line pressure to A and exhausting it from B opens the valve; 

 reversing the operation closes it. 



The external control valve may be operated by hand or 

 electricity and may, therefore, be located remotely from the 

 valve as from the switchboard, if desired. 



Another application of this valve is for automatic pressure 

 relief. The valve plunger is held closed by air pressure so arranged 

 that it is automatically released, permitting the valve plunger to 

 open when the pipe-line pressure exceeds normal by some pre- 

 determined margin. The advantage of using air, rather than 

 water, lies in the rapidity with which air may be discharged, and 

 the consequent rapid opening of the relief valve. 



Air Valves. In addition to sluice gates and gate valves pre- 

 viously described, air valves are often required in connection with 

 the pipe lines of hydro-electric developments. These may be of 

 two kinds: the automatic lever and float valve and the automatic 

 poppet valve. 



The former is for use on pipe lines which follow the contour 

 of hilly country and where air may accumulate at nigh summits 

 and obstruct the flow of water. The valve is connected to the 

 outside of the pipe at its highest point or points, and when air 

 takes the place of water about the float in the valve chamber, the 

 float which is attached to a lever drops, thus opening a small valve, 



