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Valve | Wikipedia audio article


A valve is a device that regulates, directs
or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening,
closing, or partially obstructing various passageways. Valves are technically fittings, but are usually
discussed as a separate category. In an open valve, fluid flows in a direction
from higher pressure to lower pressure. The word is derived from the Latin valva,
the moving part of a door, in turn from volvere, to turn, roll. The simplest, and very ancient, valve is simply
a freely hinged flap which drops to obstruct fluid (gas or liquid) flow in one direction,
but is pushed open by flow in the opposite direction. This is called a check valve, as it prevents
or “checks” the flow in one direction. Modern control valves may regulate pressure
or flow downstream and operate on sophisticated automation systems. Valves have many uses, including controlling
water for irrigation, industrial uses for controlling processes, residential uses such
as on/off and pressure control to dish and clothes washers and taps in the home. Even aerosols have a tiny valve built in. Valves are also used in the military and transport
sectors. In HVAC ductwork and other near-atmospheric
air flows, valves are instead called dampers. In compressed air systems, however, valves
are used with the most common type being ball valves.==Applications==
Valves are found in virtually every industrial process, including water and sewage processing,
mining, power generation, processing of oil, gas and petroleum, food manufacturing, chemical
and plastic manufacturing and many other fields. People in developed nations use valves in
their daily lives, including plumbing valves, such as taps for tap water, gas control valves
on cookers, small valves fitted to washing machines and dishwashers, safety devices fitted
to hot water systems, and poppet valves in car engines. In nature there are valves, for example one-way
valves in veins controlling the blood circulation, and heart valves controlling the flow of blood
in the chambers of the heart and maintaining the correct pumping action. Valves may be operated manually, either by
a handle, lever, pedal or wheel. Valves may also be automatic, driven by changes
in pressure, temperature, or flow. These changes may act upon a diaphragm or
a piston which in turn activates the valve, examples of this type of valve found commonly
are safety valves fitted to hot water systems or boilers. More complex control systems using valves
requiring automatic control based on an external input (i.e., regulating flow through a pipe
to a changing set point) require an actuator. An actuator will stroke the valve depending
on its input and set-up, allowing the valve to be positioned accurately, and allowing
control over a variety of requirements.==Variation==
Valves vary widely in form and application. Sizes typically range from 0.1 mm to 60 cm. Special valves can have a diameter exceeding
5 meters.Valve costs range from simple inexpensive disposable valves to specialized valves which
cost thousands of US dollars per inch of the diameter of the valve. Disposable valves may be found in common household
items including mini-pump dispensers and aerosol cans. A common use of the term valve refers to the
poppet valves found in the vast majority of modern internal combustion engines such as
those in most fossil fuel powered vehicles which are used to control the intake of the
fuel-air mixture and allow exhaust gas venting.==Types==Valves are quite diverse and may be classified
into a number of basic types. Valves may also be classified by how they
are actuated: Hydraulic
Pneumatic Manual
Solenoid valve Motor==Components==The main parts of the most usual type of valve
are the body and the bonnet. These two parts form the casing that holds
the fluid going through the valve.===Body===
The valve’s body is the outer casing of most or all of the valve that contains the internal
parts or trim. The bonnet is the part of the encasing through
which the stem (see below) passes and that forms a guide and seal for the stem. The bonnet typically screws into or is bolted
to the valve body. Valve bodies are usually metallic or plastic. Brass, bronze, gunmetal, cast iron, steel,
alloy steels and stainless steels are very common. Seawater applications, like desalination plants,
often use duplex valves, as well as super duplex valves, due to their corrosion resistant
properties, particularly against warm seawater. Alloy 20 valves are typically used in sulphuric
acid plants, whilst monel valves are used in hydrofluoric acid (HF Acid) plants. Hastelloy valves are often used in high temperature
applications, such as nuclear plants, whilst inconel valves are often used in hydrogen
applications. Plastic bodies are used for relatively low
pressures and temperatures. PVC, PP, PVDF and glass-reinforced nylon are
common plastics used for valve bodies.===Bonnet===
A bonnet acts as a cover on the valve body. It is commonly semi-permanently screwed into
the valve body or bolted onto it. During manufacture of the valve, the internal
parts are put into the body and then the bonnet is attached to hold everything together inside. To access internal parts of a valve, a user
would take off the bonnet, usually for maintenance. Many valves do not have bonnets; for example,
plug valves usually do not have bonnets. Many ball valves do not have bonnets since
the valve body is put together in a different style, such as being screwed together at the
middle of the valve body.===Ports===
Ports are passages that allow fluid to pass through the valve. Ports are obstructed by the valve member or
disc to control flow. Valves most commonly have 2 ports, but may
have as many as 20. The valve is almost always connected at its
ports to pipes or other components. Connection methods include threadings, compression
fittings, glue, cement, flanges, or welding.===Handle or actuator===
A handle is used to manually control a valve from outside the valve body. Automatically controlled valves often do not
have handles, but some may have a handle (or something similar) anyway to manually override
automatic control, such as a stop-check valve. An actuator is a mechanism or device to automatically
or remotely control a valve from outside the body. Some valves have neither handle nor actuator
because they automatically control themselves from inside; for example, check valves and
relief valves may have neither.===Disc===A disc or valve member is a movable obstruction
inside the stationary body that adjustably restricts flow through the valve. Although traditionally disc-shaped, discs
come in various shapes. Depending on the type of valve, a disc can
move linearly inside a valve, or rotate on the stem (as in a butterfly valve), or rotate
on a hinge or trunnion (as in a check valve). A ball is a round valve member with one or
more paths between ports passing through it. By rotating the ball, flow can be directed
between different ports. Ball valves use spherical rotors with a cylindrical
hole drilled as a fluid passage. Plug valves use cylindrical or conically tapered
rotors called plugs. Other round shapes for rotors are possible
as well in rotor valves, as long as the rotor can be turned inside the valve body. However, not all round or spherical discs
are rotors; for example, a ball check valve uses the ball to block reverse flow, but is
not a rotor because operating the valve does not involve rotation of the ball.===Seat===The seat is the interior surface of the body
which contacts the disc to form a leak-tight seal. In discs that move linearly or swing on a
hinge or trunnion, the disc comes into contact with the seat only when the valve is shut. In disks that rotate, the seat is always in
contact with the disk, but the area of contact changes as the disc is turned. The seat always remains stationary relative
to the body. Seats are classified by whether they are cut
directly into the body, or if they are made of a different material: Hard seats are integral to the valve body. Nearly all hard seated metal valves have a
small amount of leakage. Soft seats are fitted to the valve body and
made of softer materials such as PTFE or various elastomers such as NBR, EPDM, or FKM depending
on the maximum operating temperature.A closed soft seated valve is much less liable to leak
when shut while hard seated valves are more durable. Gate, globe, and check valves are usually
hard seated while butterfly, ball, plug, and diaphragm valves are usually soft seated.===Stem===The stem transmits motion from the handle
or controlling device to the disc. The stem typically passes through the bonnet
when present. In some cases, the stem and the disc can be
combined in one piece, or the stem and the handle are combined in one piece. The motion transmitted by the stem may be
a linear force, a rotational torque, or some combination of these (Angle valve using torque
reactor pin and Hub Assembly). The valve and stem can be threaded such that
the stem can be screwed into or out of the valve by turning it in one direction or the
other, thus moving the disc back or forth inside the body. Packing is often used between the stem and
the bonnet to maintain a seal. Some valves have no external control and do
not need a stem as in most check valves. Valves whose disc is between the seat and
the stem and where the stem moves in a direction into the valve to shut it are normally-seated
or front seated. Valves whose seat is between the disc and
the stem and where the stem moves in a direction out of the valve to shut it are reverse-seated
or back seated. These terms don’t apply to valves with no
stem or valves using rotors.===Gaskets===
Gaskets are the mechanical seals, or packings, used to prevent the leakage of a gas or fluids
from valves.===Valve balls===
A valve ball is also used for severe duty, high-pressure, high-tolerance applications. They are typically made of stainless steel,
titanium, Stellite, Hastelloy, brass, or nickel. They can also be made of different types of
plastic, such as ABS, PVC, PP or PVDF.===Spring===
Many valves have a spring for spring-loading, to normally shift the disc into some position
by default but allow control to reposition the disc. Relief valves commonly use a spring to keep
the valve shut, but allow excessive pressure to force the valve open against the spring-loading. Coil springs are normally used. Typical spring materials include zinc plated
steel, stainless steel, and for high temperature applications Inconel X750.===Trim===
The internal elements of a valve are collectively referred to as a valve’s trim. According to API Standards 600, “Steel Gate
Valve-Flanged and Butt-welding Ends, Bolted Bonnets”, the trim consists of stem, seating
surface in the body, gate seating surface, bushing or a deposited weld for the backseat
and stem hole guide, and small internal parts that normally contact the service fluid, excluding
the pin that is used to make a stem-to-gate connection (this pin shall be made of an austenitic
stainless steel material).==Valve operating positions==Valve positions are operating conditions determined
by the position of the disc or rotor in the valve. Some valves are made to be operated in a gradual
change between two or more positions. Return valves and non-return valves allow
fluid to move in 2 or 1 directions respectively.===Two-port valves===
Operating positions for 2-port valves can be either shut (closed) so that no flow
at all goes through, fully open for maximum flow, or sometimes partially open to any degree
in between. Many valves are not designed to precisely
control intermediate degree of flow; such valves are considered to be either open or
shut. Some valves are specially designed to regulate
varying amounts of flow. Such valves have been called by various names
such as regulating, throttling, metering, or needle valves. For example, needle valves have elongated
conically-tapered discs and matching seats for fine flow control. For some valves, there may be a mechanism
to indicate by how much the valve is open, but in many cases other indications of flow
rate are used, such as separate flow meters. In plants with remote-controlled process operation,
such as oil refineries and petrochemical plants, some 2-way valves can be designated as normally
closed (NC) or normally open (NO) during regular operation. Examples of normally-closed valves are sampling
valves, which are only opened while a sample is taken. Other examples of normally-closed valves are
emergency shut-down valves, which are kept open when the system is in operation and will
automatically shut by taking away the power supply. This happens when there is a problem with
a unit or a section of a fluid system such as a leak in order to isolate the problem
from the rest of the system. Examples of normally-open valves are purge-gas
supply valves or emergency-relief valves. When there is a problem these valves open
(by switching them ‘off’) causing the unit to be flushed and emptied. Although many 2-way valves are made in which
the flow can go in either direction between the two ports, when a valve is placed into
a certain application, flow is often expected to go from one certain port on the upstream
side of the valve, to the other port on the downstream side. Pressure regulators are variations of valves
in which flow is controlled to produce a certain downstream pressure, if possible. They are often used to control flow of gas
from a gas cylinder. A back-pressure regulator is a variation of
a valve in which flow is controlled to maintain a certain upstream pressure, if possible.===Three-port valves===Valves with three ports serve many different
functions. A few of the possibilities are listed here. Three-way ball valves come with a T- or L-
shaped fluid passageways inside the rotor. The T valve might be used to permit connection
of one inlet to either or both outlets or connection of the two outlets. The L valve could be used to permit disconnection
of both or connection of either but not both of two inlets to one outlet. Shuttle valves automatically connect the higher
pressure inlet to the outlet while (in some configurations) preventing flow from one inlet
to the other. Single handle mixer valves produce a variable
mixture of hot and cold water at a variable flow rate under control of a single handle. Thermostatic mixing valves mix hot and cold
water to produce a constant temperature in the presence of variable pressures and temperatures
on the two input ports.===Four-port valves===A 4-port valve is a valve whose body has four
ports equally spaced round the body and the disc has two passages to connect adjacent
ports. It is operated with two positions. It can be used to isolate and to simultaneously
bypass a sampling cylinder installed on a pressurized water line. It is useful to take a fluid sample without
affecting the pressure of a hydraulic system and to avoid degassing (no leak, no gas loss
or air entry, no external contamination)….==Control==Many valves are controlled manually with a
handle attached to the stem. If the handle is turned ninety degrees between
operating positions, the valve is called a quarter-turn valve. Butterfly, ball valves, and plug valves are
often quarter-turn valves. If the handle is circular with the stem as
the axis of rotation in the center of the circle, then the handle is called a handwheel. Valves can also be controlled by actuators
attached to the stem. They can be electromechanical actuators such
as an electric motor or solenoid, pneumatic actuators which are controlled by air pressure,
or hydraulic actuators which are controlled by the pressure of a liquid such as oil or
water. Actuators can be used for the purposes of
automatic control such as in washing machine cycles, remote control such as the use of
a centralised control room, or because manual control is too difficult such as when the
valve is very large. Pneumatic actuators and hydraulic actuators
need pressurised air or liquid lines to supply the actuator: an inlet line and an outlet
line. Pilot valves are valves which are used to
control other valves. Pilot valves in the actuator lines control
the supply of air or liquid going to the actuators. The fill valve in a toilet water tank is a
liquid level-actuated valve. When a high water level is reached, a mechanism
shuts the valve which fills the tank. In some valve designs, the pressure of the
flow fluid itself or pressure difference of the flow fluid between the ports automatically
controls flow through the valve.==Other considerations==
Valves are typically rated for maximum temperature and pressure by the manufacturer. The wetted materials in a valve are usually
identified also. Some valves rated at very high pressures are
available. When a designer, engineer, or user decides
to use a valve for an application, he/she should ensure the rated maximum temperature
and pressure are never exceeded and that the wetted materials are compatible with the fluid
the valve interior is exposed to. In Europe, valve design and pressure ratings
are subject to statutory regulation under the Pressure Equipment Directive 97/23/EC
(PED).Some fluid system designs, especially in chemical or power plants, are schematically
represented in piping and instrumentation diagrams. In such diagrams, different types of valves
are represented by certain symbols. Valves in good condition should be leak-free. However, valves may eventually wear out from
use and develop a leak, either between the inside and outside of the valve or, when the
valve is shut to stop flow, between the disc and the seat. A particle trapped between the seat and disc
could also cause such leakage.==Images====See also

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