Compression Hydrants are of three kinds: Those with the gate opening against the pres sure; with the gate opening with the pressure; This hydrant is particularly adapted for high pressures, on account of the ease with which it can be operated, and as the main valve closes both with and against the pressure no water ram or hammer can take place. In operation, the upper valve opens with the pressure and the and the double or balance gate. The com pression hydrant with gate opening against the pressure is shown in Fig. 19. It will be noticed in this hydrant that a frost case is necessary, and also that the drip being above the hydrant bottom it drains the barrel only, the water re maining in the bottom when the gate is closed. The thread for opening and closing the gate is in the top of the hydrant; the gate is moved downward by turning the nut or sleeve at the top of the hydrant, through which the threaded stem works. The drip is closed during the opening of the hydrant and opened during its closing, being completely opened or closed when the gate is closed or open. This hydrant's working parts are bronze mounted, and it has either a leather or rubber faced gate. Fig. 20 represents the compression hydrant with a double balanced, or compensating rubber valve.
It is constructed with bronze working parts and like the gate hydrant has a positive drip directly in the bottom of the hydrant, permit ting no water to remain in the hydrant to freeze. All its working parts can be removed without digging, and also like the gate hy drant the so-called frost case is unnecessary.
lower valve against the pressure; in closing, the conditions are reversed. The valve or double header is thus in equilibrium and can be opened or closed without effort. As the hydrant is opened, the drip or waste outlet is at once closed, the drip valve being drawn up into the bronze drip cylinder. In closing the hydrant, the drip valve is pushed out of the bronze cylinder in the flared lower end of the same. The water in the standpipe then passes out through the corrugations of the bronze guide on the lower end of stein or valve rod. Nozzles, on all hydrants, vary in number and size, accord ing to the purposes for which they are intended to be used. See VALVE TERMS; and for other f Orals of valves, see PNEUMATICS; PUMP ; STEAM-HAMMER and STEAM-ENGINE. For valves of circulation, see HEART.