VALVE. A moveable partition introduced in machinery for the purpose of alternately opening and closing a passage through which steam or water may be intended to peas, is called a valve ; and this generic name receives many specific designations according to the position, or the function, the valve may be required to occupy, or to perform. Thus there are head, or feet valves; suction valves; delivery, dia. charge, air-pump, steam, safety, blow-off valves ; spindle, clack, flap, slide, cup, crown valves ; stop, expansion, distribution, equilibrium, and countless other varieties of valves, which can only be described in the detailed notices of the machinery of which they form part. It may suffice, then, to state hero briefly, that the conditions a valve is required to fulfil are, that it shall open freely in the required direction; that it offer no obstruction to the passage of the fluid it is designed to peso when it is open ; that when it is closed it shall not allow the fluid to return. Valves must, therefore, fit closely on their scats; be sufficiently strong to resist the pressure to which they are to le exposed ; they must be composed of materials which should not be likely to suffer deterioration or wear ; they must be accessible at any time for examination and repair. In the exceptional forme of throttle valves, the majority of the lastsnamed conditions apply ; though, of course, as those valves are not intended effectually to close the passage of the fluids they iutt•reept, they do not require the same amount of strength, or the same perfection in the ecatiog, as the ordinary valves do.
It follows, from the nature of the work ordinary valves, that great mechanical perfection is necessary in their execution.
The bearing surfaces are, therefore, carefully planed, turned, and fitted; and the face of the valve Reel/ is frequently covered with a semi elaatio material, for the purpose of more effectually excluding the teenage of the fluids; or it may be kept close against the seating by means of springs. These contrivances, however, necessarily give rise
to considerable friction on the opening and shutting of the valves. and It thence becomes necessary, in designing machinery, to diminish the weight and the number of the valves as much as possible. Isloreover, as valves only open when the pressure on one side °secede that upon the other, the effort required to open them may become a question of serious importance, and the reaction which may ensue upon their being suddenly closed may frequently exercise a powerful effect upon the solidity of the machinery ; as for instance, in the cases of a valve upon the rising main of a large pump, or of the expansion gear of a steam-engine. It Is for the purpose of obviating these sources of inconvenience and danger, that the cup-valves and the valves, before alluded to, under ruse and Sexam-Exotsx, are intro duced ; for ordinary purposes it may be considered that lifting-salt-en aro for rough work and for dense fluids; that hinged-valves present sonic mechanical advantages over the lifting-valves; and that wba n there is a necessity for gradually c'oslmm n puaaaago, or for pre venting ',the escape of :a rare fluid, the sliding-valves should be re sorted to.
Flap-valves, and screw-valves, are occasionally used in hydraulic engineering ; the former, for the purpose of securing no autouoinic discharge of drainage waters whenever the internal pressure on the valve shall exceed the external pressure ; and the latter, for the purpose of Intercepting the flow of water, or even of gas, in pipes. Solf-actiug balaneo-valves are often used for ventilation, and occasionally for the purpose of regulating the draught in furnaces, chunnies, &a. A very Ingenious system of self-acting balance valves was Introduced by Mr. Thom in the Greenock Water Works, to ensure the uniformity of flow of the trill streams ho had there to deal with : a full account of these contrivances is to be found in the' Amnion des Ponta of Chausatfiea ' for 1331.