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A Globe valve is a type of valve used for
regulating flow in a pipeline, consisting of a
movable disk-type element and a stationary ring seat
in a generally spherical body.
Globe Valves are named for their spherical body
shape with the two halves of the body being
separated by an internal baffle. This has an opening
that forms a seat onto which a movable plug[2] can
be screwed in to close (or shut) the valve. The plug
is also called a disc or disk.[3][4] In globe
valves, the plug is connected to a stem which is
operated by screw action in manual valves.
Typically, automated valves use sliding stems.
Automated globe valves have a smooth stem rather
than threaded and are opened and closed by an
actuator assembly. When a globe valve is manually
operated, the stem is turned by a handwheel.
Although globe valves in the past had the spherical
bodies which gave them their name, many modern globe
valves do not have much of a spherical shape.
However, the term globe valve is still often used
for valves that have such an internal mechanism. In
plumbing, valves with such a mechanism are also
often called stop valves since they don't have the
global appearance, but the term stop valve may refer
to valves which are used to stop flow even when they
have other mechanisms or designs.
Globe valves are used for applications requiring
throttling and frequent operation. For example,
globe valves or valves with a similar mechanism may
be used as sampling valves, which are normally shut
except when liquid samples are being taken. Since
the baffle restricts flow, they're not recommended
where full, unobstructed flow is required.
Parts of a typical globe valve
Body
The main pressure containing structure of the
valve and the most easily identified as it forms the
mass of the valve. It contains all of the valve's
internal parts that will come in contact with the
substance being controlled by the valve. The bonnet
is connected to the body and provides the
containment of the fluid, gas, or slurry that is
being controlled.
Globe valves are typically two-port valves, although
three port valves are also produced. Ports are
openings in the body for fluid flowing in or out.
The two ports may be oriented straight across from
each other on the body,[5] or oriented at an angle
such as a 90° angle.[6] Globe valves with ports at
such an angle are called angle globe valves. A globe
valve can also have a body in the shape of a y.
Bonnet
Provides leakproof closure for the valve body.
The threaded section of stem goes through a hole
with matching threads in the bonnet. Globe valves
may have a screw-in, union, or bolted[7] bonnet.
Screw-in bonnet is the simplest bonnet, offering a
durable, pressure-tight seal. Union bonnet is
suitable for applications requiring frequent
inspection or cleaning. It also gives the body added
strength. A bonnet attached with bolts is used for
larger or higher pressure applications. Bonnets also
contain the packing, which is a wearable material
that maintains the seal between the bonnet and the
stem during valve cycling operations
Plug or disc (disk)
The closure member of the valve. Plugs are
connected to the stem which is slid or screwed up or
down to throttle the flow. Plugs are typically of
the balance or unbalanced type. Unbalanced plugs are
solid and are used with smaller valves or with low
pressure drops across the valve. The advantages are
simpler design, with one possible leak path at the
seat and usually lower cost. The disadvantages are
the limited size; with a large unbalanced plug the
forces needed to seat and hold the flow off become
impractical. Balanced plugs have holes through the
plug. Advantages include easier shut off as the plug
does not have to overcome static forces. However, a
second leak path is created between the plug and the
cage, cost is generally higher.
Stem
The stem serves as a connector from the actuator
to the inside of the valve and transmits this
actuation force. Stems are either smooth for
actuator controlled valves or threaded for manual
valves. The smooth stems are surrounded by packing
material to prevent leaking material from the valve.
This packing is a wear material and will have to be
replaced during maintenance. With a smooth stem the
ends are threaded to allow connection to the plug
and the actuator. The stem must not only withstand a
large amount of compression force during valve
closure, but also have high tensile strength during
valve opening. In addition, the stem must be very
straight, or have low runout, in order to ensure
good valve closure. This minimum runout also
minimizes wear of the packing contained in the
bonnet, which provides the seal against leakage.
Cage
The cage is part of the valve that surrounds the
plug and is located inside the body of the valve.
Typically, the cage is one of the greatest
determiners of flow within the valve. As the plug is
moved more of the openings in the cage are exposed
and flow is increased and vise versa. The design and
layout of the openings can have a large effect on
flow of material (the flow characteristics of
different materials at temperatures, pressures that
are in a range). Cages are also used to guide the
plug to the seat of the valve for a good shutoff,
substituting the guiding from the bonnet.
Seat ring
The seat ring provides a stable, uniform and
replaceable shut off surface. Seat rings are usually
held in place by pressure from the fastening of the
bonnet to the top of the body. This pushes the cage
down on the lip of the seat ring and holds it firmly
to the body of the valve. Seat rings may also be
threaded and screwed into a thread cut in the same
area of the body. However this method makes removal
of the seat ring during maintenance difficult if not
impossible. Seat rings are also typically beveled at
the seating surface to allow for some guiding during
the final stages of closing the valve.
Economical globe valves or stop valves with a
similar mechanism used in plumbing often have a
rubber washer at the bottom of the disc for the
seating surface, so that rubber can be compressed
against the seat to form a leak-tight seal when
shut.
Many globe valves have a class rating that
corresponds to the pressure specifications of ANSI
16.34. Bibcocks and sillcocks are variations of
globe or stop valves used in plumbing. Needle valves
are variations of globe valves where instead of a
separate attached disc piece, the internal end of
the stem is conically tapered to act as the disc to
fit into a matching seat for fine flow adjustment.
Other different types of valve usually are called
globe style valves because of the shape of the body
or the way of closure of the disk. As an example
typical swing check valves could be called globe
type.
Materials
Typically globe valves are made of metallic
alloys, although some synthetic materials are
available. These materials are chosen based on
pressure, temperature, controlled media properties.
Corrosive and/or erosive process streams may require
a compromise in material selection or exotic alloys
or body coatings to minimize these material
interactions and extend the life of the valve or
valve trim components. Typically, carbon steel
alloys are specified for noncorrosive applications.
Other alloys such as Hastelloy, Monel, Inconel and
others are available.
Packing material must also be considered during
valve selection. Typically the requirement for a low
friction packing conflict with a durable material
that will provide low maintenance requirements
during service life. Corrosive applications can
further complicate packing material selection as the
typical packing materials may or may not be
compatible with the processed materials. Typically
graphite or PTFE is used due to its low friction
coefficient. Enviro-seal applications also have the
availablilty of constant applied force (live-load)
packing. While more complex, it allows for constant
packing force load throughout the life of the
packing material. This packing helps meet
contemporary environmental laws. |
