1260
THEORY AND PRACTICE OF ENGINEERING. Book II
Fig. 2003.
CROWN OR EQUILIBRIUM VALVE.
by the pressure in pounds on a circular inch ; it will then move as soon as its narrowersurface is exposed to a given pressure.
The Cup Valve is made with the seat a portion of a sphere, and the valve itself a sphereor portion of it to fit it: they are occasionally used as safety-valves; a weight is sus-pended below’ them to prevent sticking.
Sliding Valves are in general use, and consist of a box with a slider at right angles to thepassage, moved by a rod passing through a stuffing-box. The slider is accurately ground,and is held by a spring; for small apertures it is moved by a handle, and for larger ones bya rack and pinion.
Crown or Equilibrium Valve used in theCornish engines, and also in the rotative : itrequires little force to work it. The steam isintroduced through the pipe S, and theaperture A is surrounded by an uprightring or collar, rising a few inches into thechamber, which ring is perforated by slits ofconsiderable size, but closed at the top. Thecrow’n or cover of the valve is a ring at-tached to a steel rod, by which it is raisedor depressed, care being taken that the collar and crown valve are both accurately groundand made to fit each other.
When the valve is on its seat, it is closedon all sides, so that no steam can enter:when raised up from its seat, the steammay enter freely from each of its sides:this kind of valve is similarly arranged tothe conical valves, and work in the sameway, four of them being used in a singleengine. The conical valve was contrived byINIr. Watt, and is shown when open in onefigure, and shut in the other.
S is the entrance for the steam, A theport, V the conical valve, N the seat ornozzle, which it covers: when the conicalcover V of the aperture N is up, the steamhas free admission ; when it is closed, thesteam presses the valve down into its seat,without escaping from the nozzle.
Murray's Sliding Valve _All the apertures
terminate in a case that is steam-tight, and
within w’hich a box slides up and down, which opens and closes the passagesalternately. The rod O,which passes through a stuffing-box, moves the slidingpart. The steam entering from the boiler at S passes through a to the topof the cylinder when the slide is down, while the passage to the condenseris open through the interior of the slide; when the slider is up, the pas-sage b from the bottom of the cylinder is open, and the passage a from thetop to c, the condenser, is open: to drive this slide a small reciprocatingmotion is sufficient, and it has considerable friction from the pressure ofthe steam against the box; they are generally of gun metal where salt-wateris used, and they act and wear well.
Rotary Valves are formed by a plate of metal moving on an axis, whichcrosses the plate, and passes through an air-tight aperture to the outside:for throttle-valves, or where perfect tightness is not the object, they may Murray’s slide.be usefully employed.
Eour-tvay Cock is used to open a communication alternately from the boiler and conden-ser to the top and bottom of the cylinder: it is simple in its action, but steam is lost ateach stroke of the engine, and there is considerablefriction; it is of a cylindrical form, and when trulyground the steam tends to keep it tight, but it soonwears into an elliptical shape and requires re-grinding.
In a four-passaged cock made for the purpose of shut-ting off the steam at any period of the stroke, withoutclosing the passage to the condenser, T shows thepassage to the top, B that to the bottom of the cylinder,and C that to the condenser; the figures show itsposition when the steam enters, and when shut off.
To form the four-way cock, so that the steam can be cut off at any portion of the stroke,it must be enlarged sufficiently to allow the breadth of two apertures between the middle
Fig. 2004.
VALVE
SHUT.
VALVE Of EN.
~D-TT
Fig. 2006. watt’s
CONICAL VALVE, OPEN
Fig. 2007. DITTO, SHUT.
Fig. 2008.
Fig. 2010. open.
Fig. 2009. shut.
POUR-WAY COCK.