563

MODERN STEAM PRACTICE.

Fig. 421.—Safety Valve with pointof contact rounded.— a, Valve seat.b, Valve. C c, Part rounded.

valves is obtained to resist the internal steam pressure. The othersafety valves are fitted with a lever and spring, which can beregulated with a thumb screw to any pressure within ränge of theSalter balance fitted with a helical spring, graduated to indicate1 lb., 5 lbs., &c., intervals of pressure; the spring is so adjustedthat a weight of 1 lb. attached to the balance would move the indexto 1 lb. as marked by the scale, the lever being so proportioned to suitthe area of the valve that a pressure of 1 lb. indicated by the scalegives 1 lb. per square inch on the valve. The valves formerly muchused were conical spindle ones, with the bearing surface bevelled to an

angle of 30°. Some makers round the pointof contact on the valve, and although theyare not so liable to gag in locomotive prac-tice owing to their being frequently opened,still they are objectionable, as they do notgive so free a passage for the steam as theflat-disc valve or the spherical one. Theformer has been arranged in a variety ofways; one good form being an inverted cone with a Hange atthe base which bears on the seating, the two surfaces being accur-ately turned, and then ground together to make them perfectlysteam tight, the thrust pin on the lever bearing on the internal

apex of the cone, the valvebeing cast hollow for that pur-pose. The annular flat-sur-face valve (Fig. 422) differsmaterially from the ordinaryones and from the foregoing,inasmuch as the steam, be-sides escaping round thevalvewhen raised, escapes alsothrough an annular aperturecast in the valve, thus posses-sing the advantage of two edges, instead of one edge as in the ordinarydisc valves, for the steam to escape by, and consequently with a givendiameter, and the lift of the valves of both kinds being equal, theannular valve will pass about double the quantity of steam, whilethe area exposed to the steam pressure in the boiler being muchless, the holding-down load is reduced in proportion. The spindleis hollowed out for the reception of the thrust pin, as in the previous

Fig. 422.—Annular Safety Valve .a, Valve . b, Seat for valve. c, Spindle. d. Lever.e, Stud for lever.