. Electric railway review . ter on its face. Ifvery hot water is used the piston can be made of composi-tion metal fitted with packing rings. The valve is guided bymeans of the hub, f, and can be readily reground by re-moving the upper cylinder and clamping the hub piece withthe screws which attach the cylinder. The pilot relief valve, a, should be of a sensitive andhigh-grade type and if different sizes of main relief valvesare used in the same plant they should all be supplied withpilot valves of the same size. A spare pilot valve should beprovided for substitution in case it is desired to r
. Electric railway review . ter on its face. Ifvery hot water is used the piston can be made of composi-tion metal fitted with packing rings. The valve is guided bymeans of the hub, f, and can be readily reground by re-moving the upper cylinder and clamping the hub piece withthe screws which attach the cylinder. The pilot relief valve, a, should be of a sensitive andhigh-grade type and if different sizes of main relief valvesare used in the same plant they should all be supplied withpilot valves of the same size. A spare pilot valve should beprovided for substitution in case it is desired to repair one inuse. The valve, a, has but little service to perform and ifit is merely off its seat, the pressure will enter the chamber,d, allow the valve, c, to open and remain open during thetime the valve, a, is discharging. When the valve, a, closes,the pressure on the under side of the piston is zero. Thenif it is assumed that the pressure on the upper side of thepiston and on the under side of the valve c, is 120 pounds. Figure 182—(D1-1). with the pressure above the valve 20 pounds, and if thepiston has twice the area of the valve, c, or the area of thevalve is 25 square inches and that of the piston 50 squareinches, there would then be a pressure of 6,000 pounds tend-ing to close the valve, and a pressure of 2,500 pounds tend-ing to open it. This is a net pressure of 3,500 pounds tendingto close the valve. When, owing to the opening of the valve,a, the pressure in the chamber, d, is the same as that on theupper side of the piston, there will be a pressui-e of 3,000pounds exerted in opening the valve, c. When the valve, a,is closed and there is no pressure in d, there will be a pres-sure of 3,000 pounds exerted toward closing the valve c. Theobjectionable feature in the type of valve shown in Fig. 182is that there is pressure upon the piston at all times , a tight fit must be maintained in order to avoid excessive leakage. (To be Continued.) July, 1906. ELEC
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