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 1-1/4 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.