128

MODERN STEAM PRACTICE.

Fig. 71.—Equilibrium Valve.

three ports in the cylinder. Some of these valves for marine enginessimply consist of a frame, having metallic packing rings bearing onthe back of the valve casing or cover. In thevalve delineated the middle recess is simplyformed to lighten the casting as the exhauststeam passes through the valve itself in itspassage to the condenser. The packing ringfits into a recess on the back of the valve, aplaited gasket is interposed between the pack-ing ring and a thin metallic plate with springsfor pressing the valve and ring to their respec-tive faces. It will be seen that this valve verynearly approaches to what we, may term anequilibrium valve. The only objection to thisdass is that there are two faces to keep tight,and that the ring depends on its accurate fit,along with the gasket packing, to keep it steam-tight. To obviate this difficulty a variety ofpacking rings have been devised, depending ontheir metallic contact alone so as to make themsteam tight; and as it is an object in some engines of the high-pressure type, having great piston speed, to reduce the weight ofthe reciprocating parts, the rings have beenmade very light. An improvement upon thepreceding example is that the valve is fittedwith a metallic piston, having a spring ringfitted to the piston, the piston and slide valvebeing pressed to the faces with springs insertedat the bottom of the cylinder, which is cast onthe slide-valve. The exhaust, as in the pre-vious example, passes through the valve intothe condenser; in such cases it is advisable tofit a brass face on the condenser casting, thesteam ehest for the valve, as it were, formingpart of the condenser, that is, the valve ehestand the condenser are cast all in one piece.Another form has the piston and face for press-ing to the back of the valve-casing of a lightersection, and the piston made steam-tight with steel-spring ringsrecessed in the piston. The piston in this arrangement is simply

Fig. 72.—Equilibrium Valve.