A valve is a device that regulates, directs or controls the flow of a fluid by opening,
closing, or partially obstructing various passageways. Valves are technically valves
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 simplest, and very ancient, valve is simply a freely hinged flap which drops to obstruct
fluid 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.
Valves have many uses, including controlling water for Irrigation, industrial uses for
controlling processes, residential uses such as on / off & pressure control to dish and
clothes washers & taps in the home. Even aerosols have a tiny valve built in. Valves are also
used in the military & transport sectors. Valves are found in virtually every industrial
process, including water & sewage processing, mining, power generation, processing of oil,
gas & petroleum, food manufacturing, chemical & 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, & 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 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.
Applications 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.
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 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 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 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, or rotate on a hinge or trunnion. 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.
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 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 or normally open 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 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 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.... 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 97EC. 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
References
External links ISO-15926-4 - Nearly 500 valve base classifications
and definitions from the ISO 15926 standard. Animations showing Internal Function of Various
Types of Valve, tlv.com Flow in known Design Types of Shut-off Valves,
home.arcor.de Valves: Piping and Instrumentation Diagram
Standard Notation, controls.engin.umich.edu