VALVES.
The valve is any device or appliance used to control the flow of a liquid, vapor or gas, through a pipe, outlet, or inlet in any form of vessel. In this sense the definition includes air, gas, steam, and water cocks of any kind.
The bellows was probably the first instrument of which they formed a part. No other machine equally ancient can be pointed out in which they were required.
By far the most important improvement on the primitive bellows or bag was the admission of air by a separate opening—a contrivance that led to the invention of the valve, one of the most essential elements of steam, of water, as well as pneumatic machinery.
Valves and Cocks.—Generally described, a valve is a lid or cover to an opening, so formed as to open a communication in one direction and close it in another by lifting, turning, or sliding—among the varieties may be classed as, the cock, the slide-valve, the poppet valve and the clack-valve. A common form of this valve is shown in [Fig. 139], page 261.
An every day example of a valve, and almost the simplest known, is that of an ordinary pump where the valve opens upward to admit the water and closes downward to prevent its return.
A valve has a seat, whether it be a gate or circular valve, and is generally turned by a circular handle fitted to the spindle.
Difference between a cock and valve.—The cock is a valve, but a valve is not a cock; the cock is a conical plug slotted and fitted with a handle for turning the cone-shaped valve, with its opening in line, or otherwise, with the opening of the pipe.
Globe Valve is a valve enclosed in a globular chamber, [Fig. 135]. This, like many other valves, takes its name from its shape.
Globe valves, whenever possible, should be placed so that the pressure comes under the valve, or at the side, for if the valve should become loose from the stem (which they often do) if the pressure is on top, there would be a total stoppage of the steam.
Relief Valve is a valve so arranged that it opens outward when a dangerous pressure or shock occurs; a valve belonging to the feeding apparatus of a marine engine, through which the water escapes into the hot well when it is shut off from the boiler.
Hinged Valves constitute a large class, as for example the butterfly-valve, clack-valves, and other forms in which the leaf or plate of the valve is fastened on one side of the valve seat or opening.
Valve-bracket is a bracket fitted with a valve.
The Valve-chamber is where a pump valve or steam valve operates.
Valve-cock.—A form of cock or faucet which is closed by dropping of a valve on its seat.
Valve-coupling is a pipe coupling containing a valve.
Valve-seat is the surface upon which a valve rests.
Back pressure valves are ball or clack valves in a pipe which instantly assume the seat when a back pressure occurs. They are illustrated in “6,” [Fig. 144]. Their name signifies their use—to maintain a constant back pressure in heating systems.
Ball-valve—a faucet which is opened or closed by means of a ball floating in the water. It constitutes an automatic arrangement for keeping the water at a certain level.
Bib-cock—a faucet having a bent-down nozzle.
Check-valve—a valve placed between the feed pipe and the boiler to prevent the return of the water, etc.
Brine-valve—a valve which is opened to allow water saturated with salt to escape. In marine service it is “a blow-off valve.”
Ball-valve—a valve occupying a hollow seat. These valves are raised by the passage of a fluid and descending are closed by gravity.
Angle-valve is one which forms part of an angle, see [Fig. 137].
The double-seat valve or double-beat valve presents two outlets for the water. In the Cornish steam engine this is called the equilibrium-valve, because the pressure on the two is very nearly equalized.
Three-way cock is one having three positions directing the fluid in either of three directions. This is illustrated in [Fig. 138]. The three-way valve is also illustrated on page 259, [Fig. 136].
Four-way cock is one having two separate passages in the plug and communicating with four pipes.
Gate-valve—a valve closed by a gate. This is illustrated in [Fig. 140].
Swing or straight-way valve—this is shown in [Fig. 141], page 261.
Throttle-valve.—This is the valve used to admit steam to the engine and so termed to distinguish it from the main stop-valve located near the boiler—to throttle means to choke—hence the throttling of the steam.
Rotary valves are those in which the disc, or plug, or other device used to close the passage, is made to revolve for opening or closing, the common stop cock being an illustration.
Lifting valves are those in which the full cone or stopper is lifted from the valve seat by pressure from below, the poppet, and safety valves being examples.
Pressure regulator valve—this is sometimes called a reducing valve and is illustrated in Figs. [142], [143], on page 262. It is designed to reduce the pressure from a high point in the boiler to a lower one in a system of piping, etc.
Usually the smaller valves, not exceeding 11⁄4 inch in diameter, are wholly of gun-metal; the larger are commonly made with cast-iron bodies and gun-metal fittings. The smallest valves, from 1⁄4 up to 1⁄2 inch inclusive, have the disk solid with the spindle, and have an ordinary stuffing-box with external gland. Valves of 3⁄4 inch and upwards have the disk loose from the spindle; up to 3 inch valves the spindles are screwed to work inside the casing; above that size the screwed portion is outside the casing. Above the 3-inch size the nozzles of the cast-iron bodies are generally flanged instead of tapped.