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Plug valves are valves with cylindrical or
conically-tapered "plugs" which can be rotated
inside the valve body to control flow through the
valve. The plugs in plug valves have one or more
hollow passageways going sideways through the plug,
so that fluid can flow through the plug when the
valve is open. Plug valves are simple and often
economical.
When the plug is conically-tapered, the stem/handle
is typically attached to the larger diameter end of
the plug. Plug valves usually do not have bonnets
but often have the end of the plug with the handle
exposed or mostly exposed to the outside. In cases
like that, there is usually not much of a stem. The
stem and handle often come in one piece, often a
simple, approximately L-shaped handle attached to
the end of the plug. The other end of the plug is
often exposed to the outside of the valve too, but
with a mechanism which retains the plug in the body.
The simplest and most common general type of plug
valve is a 2-port valve, which has two positions,
open to allow flow, and shut (closed) to stop flow.
Ports are openings in the valve body through which
fluid can enter or leave. The plug in this kind of
valve has one passageway going through it. The ports
are typically at opposite ends of the body;
therefore, the plug is rotated a fourth of a full
turn to change from open to shut positions. This
makes this kind of plug valve a quarter-turn valve.
There is often a mechanism limiting motion of the
handle to a quarter turn, but not in glass
stopcocks.
Slightly conically-tapered metal (often brass) plug
valves are often used as simple shut-off valves in
household natural gas lines.
It is also possible for a plug valve to have more
than two ports. In a 3-way plug valve, flow from one
port could be directed to either the second or third
port. A 3-way plug valve could also be designed to
shift flow between ports 1 and 2, 2 and 3, or 1 and
3, and possibly even connect all three ports
together. The flow-directing possibilities in
multi-port plug valves are similar to the
possibilities in corresponding multi-port ball
valves or corresponding multi-port valves with a
rotor. An additional possibility in plug valves is
the have one port on one side of the plug valve and
two ports on the other side, with two diagonal and
parallel fluid pathways inside the plug. In this
case the plug can be rotated 180° to connect the
port on the one side to either of the two ports on
the other side.
Stopcocks used in laboratory glassware are typically
forms of conically-tapered plug valves. When fused
with the glassware, the valve bodies are made of
glass. Otherwise, they can be made of an inert
plastic such as Teflon. The plugs can be made of a
similar plastic or glass. When the plug is made of
glass, the handle and plug are fused together in one
piece out of glass. When glass is used for both the
stopcock body and the plug, the contacting surfaces
between them are special ground glass surfaces (see
Laboratory glassware) often with stopcock grease in
between. Special glass stopcocks are made for vacuum
applications, such as in use with vacuum manifolds.
Stopcock grease is always used in high vacuum
applications to make the stopcock air-tight.
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