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Valve
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- For other uses, see Valve (disambiguation). For the electronic component, see Thermionic valve. For the game development company see Valve Corporation.
A valve is a device that regulates the flow of a fluid (gases, fluidized solids, slurries, or liquids) by opening, closing, or partially obstructing various passageways.
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Encyclopedia
- For other uses, see Valve (disambiguation). For the electronic component, see Thermionic valve. For the game development company see Valve Corporation.
A valve is a device that regulates the flow of a fluid (gases, fluidized solids, slurries, or liquids) by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category.
Valves are also found in the human body. For example, there are several which control the flow of blood in the chambers of the heart and maintain the correct pumping action (see heart valve article).
Valves are used in a variety of contexts, including industrial, military, commercial, residential, and transportation.
Oil and gas, power generation, mining, water reticulation, sewerage and chemical manufacturing are the industries in which the majority of valves are used.
Plumbing valves, such as taps for hot and cold water are the most noticeable types of valves. Other valves encountered on a daily basis include gas control valves on cookers and barbecues, 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 diaphram 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 steam 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, lifters and or push rods where they play a major role in engine cycle control.
Applications
A huge variety of valves are available, and valves have infinite applications and sizes ranging from .004" (0.1 mm) to 24" (600 mm). Special valves can be manufactured to have a diameter exceeding 200" (5000 mm).
Valves range from inexpensive, simple, disposable valves to components in exotic items that in some instances cost thousands of dollars (US$) per inch (25 mm) of diameter.
Disposable valves may be found inside common household items including liquid or gel mini-pump dispensers and aerosol spray cans.
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Types and designations
Valves can be categorized into the following design types, and although there are hundreds of variations, they all fit into these basic types:
- Gate
- Poppet
- Plug
- Globe
- Check
- Butterfly
- Diaphragm
- Ball
- Needle
Valves may be classified by how they are operated:
- manual
- Solenoid
- Hydraulic/Pneumatic
Valve parts
Body and Bonnet
The main part of the valve consists of the valve body and bonnet. These two parts form the casing that holds the fluid going through the valve. Valve bodies are usually metallic. Brass, bronze, cast iron, steel, alloy steels and stainless steels are very common. 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 is the part of a valve casing through which the stem passes and that forms a guide and seal for the stem.
Ports
Integral to the valve body are the passages that allow flow into and out of the valve. These are called ports. These ports are obstructed or opened up by the valve member or disc to control the fluid flow. Valves with two or three ports are the most common, while valves with multiple ports (up to 20) are used in special applications.
Nearly all valves are built with some means of connection at the ports.
These include
Threads (male or female); BSP or NPT are most common.
Compression fittings, to suit tube s/s or copper.
Glue or cement application (especially for plastic) almost always a socket type connection (not a butt)
Flanges ANSI, BS, DIN, or JIS. (US, British, European, Japanese standards)
Welding either Socket type or Butt type welds.
Disc / Rotor / Valve Member
Inside the valve body, flow through the valve may be partly or fully blocked by an object called a disc or valve member. Although valve discs of some kinds of valves are traditionally disc-shaped, discs can come in various shapes. Although the valve body remains stationary within the fluid system, the disc in the valve is movable so it can control flow. A round type of disc with fluid pathway(s) inside which can be rotated to direct flow between certain ports is usually called a ball. Ball valves are valves which use spherical rotors, except for the interior fluid passageways. Plug valves use cylindrical or conically tapered rotors called plugs. Other round shapes for rotors are possible too 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 valve seat is the interior surface in the body which contacts or could contact the disc to form a seal which should be leak-tight when the valve is shut. If the disc moves linearly as the valve is controlled, the disc comes into contact with the seat when the valve is shut. When the valve has a rotor, the seat is always in contact with the rotor, but the surface area of contact on the rotor changes as the rotor is turned. If the disc swings on a hinge, as in a swing check valve, it contacts the seat to shut the valve and stop flow. In all the above cases, the seat remains stationary while the disc or rotor moves. The body and the seat could both come in one piece of solid material, or the seat could be a separate piece attached or fixed to the inside of the valve body, depending on the valve design.
Seats can be integral to the valve body, that is "hard" metal or plastic. Nearly all metal seated valves leak, even though some leaks are extremely small.
"Soft" seats can be fitted to the valve body and made of materials such as PTFE or various elastomers such as NBR, EPDM, FKM. Each of these soft materials is limited by temperature (rough maximum temperatures are listed below)
NBR 80 °C
EPDM 120 °C
FKM 170 °C
PTFE 200 °C
The advantage of soft seats is that they are more likely to offer 100% tight shutoff when valve is closed.
There are advantages of Hard seated Valves as well in applications where there is heavy erosion due to the material flowing from the pipes then the metal seated valves are preferred over soft seated valves.
Metal seated Valves have longer life as well.
Gate Valves, Globe Valves, Check Valves are usually hard seated Valves and Butterfly Valves, Ball Valves, Plug Valves, Diapharagm Valves are Usually soft seated Valves.
Though there are some special cases where we do have hard seated Butterfly Valves and Hard seated Ball Valves as well.
Stem
The stem is a rod or similar piece spanning the inside and the outside of the valve, transmitting motion to control the internal disc or rotor from outside the valve. Inside the valve, the rod is joined to or contacts the disc/rotor. Outside the valve the stem is attached to a handle or another controlling device. Between inside and outside, the stem typically goes through a valve bonnet if there is one. 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 can be a linear push or pull motion, a rotating motion, or some combination of these. A valve with a rotor would be controlled by turning the stem. 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 seal fluid inside the valve in spite of turning of the stem. Some valves have no external control and do not need a stem; for example, most check valves. Check valves are valves which allow flow in one direction, but block flow in the opposite direction. Some refer to them as one-way 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 (also called '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 (also called 'back seated'). These terms do not apply to valves with no stem nor to valves using rotors.
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.
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 steel (zink plated), stainless steels 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
Valve positions are operating conditions determined by the position the disc or rotor in the valve. Some valves are made to be operated in a gradual change between two or more positions.
Two-way valves
2-port valves are commonly called 2-way valves. Operating positions for such 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, which maybe qualitative descriptions in between. 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 with fluid systems, 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. Then, isolation valve(s) are shut 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-way valves
Three-way valves have three ports. Three-way valves are commonly made such that flow coming in at one port can be directed to either the second port in one position or the third port in another position or in an intermediate position so all flow is stopped. 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. Other kinds of 3-port valves can be designed for other possible flow-directing schemes and positions; for example, see Ball valve.
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-way valves
The Four-way valve is a fluid control valve whose body has four ports equally spaced round the valve chamber and the plug 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).
It was also used to control the flow of steam to the cylinder of early double-acting steam engines, such as those designed by Trevithick. This use of the valve is possibly attributable to Papin.
Control
Many valves are controlled manually with a handle attached to the valve stem. If the handle is turned a quarter of a full turn (90°) between operating positions, the valve is called a quarter-turn valve. Butterfly valves, ball valves, and plug valves are often quarter-turn valves. Valves can also be controlled by devices called 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 centralized control room, or because manual control is too difficult; for example, the valve is large. Pneumatic actuators and hydraulic actuators need pressurized 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. In an open valve, fluid flows in a direction from higher pressure to lower pressure.
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.
Types of valves
Generic type valves
- 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.
- Expansion valve, used pressure reduction of fluid in general. Type of restrictor valve to reduce refrigerant pressure and temperature in short period of time in adiabatic expansion.
- 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.
Specific valve 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 reducing valve (PRV), or pressure regulator: 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
- 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
- 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
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