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A valve is a device that regulates the flow of substances (either gases, fluidized solids, slurries, or liquids) by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but usually are discussed separately.
Valves are used in a variety of applications including industrial, military, commercial, residential, and transportation. Plumbing valves are the most obvious in everyday life, but many more are used.
Some valves are driven by pressure only, they are mainly used for safety purposes in steam engines and domestic heating or cooking appliances. Others are used in a controlled way, like in Otto cycle engines driven by a camshaft, where they play a major role in engine cycle control.
[edit] Application
A large variety of valves are available and have many applications with sizes ranging from small to large. The cost of valves ranges from very cheap simple disposable valves, in some items to very expensive valves for specialized applications. Often not realized by some, small valves are even inside some common household items including liquid or gel mini-pump dispenser spigots, spray devices, some rubber bulbs for pumping air, etc., manual air pumps and some other pumps, and laundry washers. Valves are almost as ubiquitous as electrical switches. Often a valve is part of some object, the valve body and the object made in one piece; for example, a separatory funnel. Faucets, taps, and spigots are all variations of valves. Many fluid systems such as water and natural gas lines in houses and other buildings have valves. Fluid systems in chemical and power plants and other facilities have numerous valves to control fluid flow.
[edit] Valve parts
[edit] Body
The majority of the valve consists of the valve body, including most of the exterior. The valve body is the vessel or casing that holds the fluid going through inside the valve. Valve bodies are most commonly made of various metals or plastics, although valve bodies fused with glass laboratory items in one piece are also made of glass.
[edit] Ports
The body consists of two or more openings, called ports from which movement occurs from one opening to the next. These ports are controlled by the valve to control the fluid flow. Valves with two or three ports are the most common, while valves consisting of four or more ports are not as frequently used. Extra ports that are not needed can be closed off by the valve. Manufacturing of valves often occurs with the intent that they will be connected with another specific object. These objects can vary, but generally these include some type of piping, tubing, or pump head. In some cases, a valve port is immediately connected to a spray nozzle or container. To make a connection, valves are commonly measured by the outer diameter the ports they connect to. For example, a 1-inch valve is sized to connect to 1-inch outer diameter tubing.
Combined with a valve, ports have the ability to act as faucets, taps, or spigots, all while one or more of its remaining ports are left unconnected. Most valves are built with some means of connection at the ports. This includes threads, compression fittings, glue or cement application (especially for plastic), flanges, or welding (for metals).
[edit] Discs and rotors
Inside the valve body, flow through the valve may be partly or fully blocked by an object called a disc. 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 can be called a rotor. Ball valves are valves which use spherical rotors, except for the interior fluid passageways. Plug valves use cylindrically-shaped 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.
[edit] 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, particularly when the valve is shut (closed). 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.
[edit] 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.
[edit] Bonnet
A bonnet basically 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 were put into the body and then the bonnet was 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.
[edit] 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,
[edit] 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, and nickel. They can also be made of different types of plastic, such as ABS, PVC, PP or PVDF.
[edit] 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.
[edit] 2-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 like 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 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.
[edit] 3-way valves
3-way valves have three ports. 3-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 ("hot tank").
In valves having more than 3 ports, even more flow-directing schemes are possible. Such valves are often rotor valves or ball valves. Slider valves have been used also.
[edit] 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 commode 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.
[edit] 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.
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