valve

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These water valves are operated by handles.

A valve is a device that regulates the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure.

Valves are also found in the human body. For example, there are several heart valves which control the flow of blood in the chambers of the heart and maintain the correct pumping action.

Valves are used in a variety of contexts, including industrial, military, commercial, residential, and transportation.

The industries in which the majority of valves are used are oil and gas, power generation, mining, water reticulation, sewerage and chemical manufacturing.^{[citation needed]}

Plumbing valves, such as taps for hot and cold tap water are the most noticeable types of valves. Other valves encountered on a daily basis include gas control valves on cookers, small valves fitted to washing machines and dishwashers, and safety devices fitted to hot water systems.

Valves may be operated manually, either by a hand wheel, lever or pedal. 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.

Valves are also found in the Otto cycle (internal combustion) engines driven by a camshaft, tappets or push rods where they play a major role in engine cycle control.

Applications

Valves vary widely in form and application. Sizes^[ambiguous] typically range from 0.1 mm 60 cm (2 ft). Special valves can have a diameter exceeding 5 meters.^[which?]

Valve cost ranges from simple inexpensive disposable valves to specialised valves cost thousands of US dollars per inch of diameter.

Disposable valves may be found inside common household items including mini-pump dispensers and aerosol cans.

Types

The inside of an extremely large butterfly valve

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
• Motor

Basic types

All valves can be categorized into the following basic types:

• Ball valve, for on/off control without pressure drop.
• Butterfly valve, for flow regulation in large pipe diameters.
• Choke valve, a valve that lifts up and down a solid cylinder which is placed around or inside another cylinder which has holes or slots. Used for high pressure drops found in oil and gas wellheads.
• Check valve or non-return valve, allows the fluid to pass in one direction only.
• Diaphragm valve, some are sanitary predominantly used in the pharmaceutical and foodstuff industry.
• Gate valve, mainly for on/off control, with low pressure drop.
• Globe valve, good for regulating flow.
• Knife valve, for slurries or powders on/off control.
• Needle valve for accurate flow control.
• Piston valve, for regulating fluids that carry solids in suspension.
• Pinch valve, for slurry flow regulation.
• Plug valve, slim valve for on/off control but with some pressure drop.
• Spool valve, for hydraulic control
• Thermal expansion valve
• Poppet valve
• Reductive "self" valve

Specific types

• 4-stroke cycle engine valves: an application of piston valve
• Aspin valve: a cone-shaped metal part fitted to the cylinder head of an engine
• Ball cock: often used as a water level controller (cistern)
• Bibcock: provides a connection to a flexible hosepipe
• Blast valve: prevents rapid overpressures in a fallout shelter or a bunker
• Cock: colloquial term for a small valve or a stopcock
• Demand valve: on a diving regulator
• Double beat valve
• Double check valve
• Duckbill valve
• Flipper valve
• Flow control valve: an application which maintains a constant flow rate through the valve
• Heimlich valve: a specific one-way valve used on the end of chest drain tubes to treat a pneumothorax
• Foot valve: a check valve on the foot of a suction line to prevent backflow
• Four-way valve: was used to control the flow of steam to the cylinder of early double-acting steam engines
• Freeze valve: in which freezing and melting the fluid creates and removes a plug of frozen material acting as the valve
• Gas pressure regulator regulates the flow and pressure of a gas
• Heart valve: regulates blood flow through the heart in many organisms
• Leaf valve: one-way valve consisting of a diagonal obstruction with an opening covered by a hinged flap
• Pilot valve: regulate flow or pressure to other valves
• Poppet valve and sleeve valve: commonly used in piston engines to regulate the fuel mixture intake and exhaust
• Pressure regulator or pressure reducing valve (PRV): reduces pressure to a preset level downstream of the valve
• Pressure sustaining valve, or back-pressure regulator: maintains pressure at a preset level upstream of the valve
• Presta and Schrader valves are used to hold the air in bicycle tires
• Reed valve: consists of two or more flexible materials pressed together along much of their length, but with the influx area open to allow one-way flow, much like a heart valve
• Regulator: used in SCUBA diving equipment and in gas cooking equipment to reduce the high pressure gas supply to a lower working pressure
• Rocker valve
• Rotary valves and piston valves: parts of brass instruments used to change their pitch
• Rupture disc: a one time use replaceable valve for rapid pressure relief, used to protect piping systems from excessive pressure or vacuum; more reliable than a safety valve
• Saddle valve: where allowed, is used to tap a pipe for a low-flow need
• Safety valve or relief valve: operates automatically at a set differential pressure to correct a potentially dangerous situation, typically over-pressure
• Schrader valve: used to hold the air inside automobile tires
• Solenoid valve: an electrically controlled hydraulic or pneumatic valve
• Stopcock: restricts or isolates flow through a pipe
• Swirl valve: A specially designed Joule-Thompson pressure reduction/expansion valve imparting a centrifugal force upon the discharge stream for improving gas-liquid phase separation
• Tap (British English), faucet (American English): the common name for a valve used in homes to regulate water flow
• Thermal expansion valve
• Thermostatic Mixing Valve
• Thermostatic Radiator Valve
• Trap primer: sometimes include other types of valves, or are valves themselves
• Vacuum breaker valve: prevents the back-siphonage of contaminated water into pressurized drinkable water supplies

Components

Cutaway of a simple manual ball valve. 1) Body 2) Seat 3) Disc 4) Lever 5) Stem

Body and bonnet

The main parts of a valve are the body and the bonnet. These two parts form the casing that holds the fluid going through the valve. The bonnet is the casing through which the stem (see below) passes and that forms a guide and seal for the stem.

Valve bodies are usually metallic^{[citation needed]}. Brass, bronze, gunmetal, cast iron, steel, alloy steels and stainless steels are very common.^{[citation needed]} Plastic bodies are used for relatively low pressures and temperatures. PVC, PP, PVDF and glass-reinforced nylon are common plastics used for valve bodies.^{[citation needed]}

Bonnet

A bonnet acts as a cover on the valve body. It is commonly semi-permanently screwed into the valve body. 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.

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 it's ports to pipes or other components. Connection methods include threadings, compression fittings, glue, cement, flanges, or welding.

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. 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.^[ambiguous] 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 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, an check valves are usually hard seated while butterfly, ball, plug, and diaphragm valves are usually soft seated.

Maximum temperatures

• NBR 80 °C
• EPDM 120 °C
• FKM 170 °C
• PTFE 200 °C

Stem

The shut off butterfly valve for a Francis turbine at Gordon Power Station, Tasmania

The stem transmits motion from 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. 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.^[ambiguous] 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.

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.

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.

Valve operating positions

A seacock for cooling seawater, on a marine diesel engine.

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.

One-port valves

1-port valves do not allow to direct a fluid or gas in a specific pipe. Rather they can only allow the fluid or gas to move along the pipe or they can stop it from moving. Depending on the valve chosen, the speed on which this is done may be changed.^[ambiguous]

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 some plants^[ambiguous] with fluid systems^[ambiguous], some 2-way valves can be designated as normally shut or normally open during regular operation. Examples of normally shut valves are sampling valves, which are only opened while a sample is taken. Examples of normally open valves are isolation valves, which are usually only shut 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.

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

Three-way valves have three ports. They are commonly made such that flow coming in at one port can be directed by the disc position to one of the two other ports, shut off entirely in an intermediate position. Often such 3-way valves are ball or rotor valves. Many faucets are made so that incoming cold and hot water can be regulated in varying degrees to give outcoming water at a desired temperature.

The "motor valve" on a domestic heating system is an example of a 3-way valve. Depending on demand the motor head rotates the spindle to control the proportion of the flow that goes to the two outlet pipes: One to radiators, one to hot water system. In a conventional system the valve usually sits just after the pump and by the cylinder.

In valves having more than 3 ports, even more flow-directing schemes are possible. Such valves are often rotor valves or ball valves.

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

A sailor aboard a ship operates the wheel controlling a fuel valve.

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. 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.

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

Globe valve

A valve controlled by a wheel

Large butterfly Valve

See also

• Piping
• Plumbing
• Valve stem
• Variable Valve Timing
• Zone valve
• Plastic Pressure Pipe Systems
• Medical Endobronchial valve

External links

Wikimedia Commons has media related to: Valves

• Animations showing Internal Function of Various Types of Valve
• Flow in known Design Types of Shut-off Valves