Editing Valve (section)

==Valve operating positions==
[[File:Sea water cock.JPG|thumb|A [[seacock]] for cooling [[seawater]], on a marine [[diesel engine]]. Seacocks are usually ball valves, but may be other types as well.]]
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 valve]]s and [[non-return valve]]s 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 valve]]s''. 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 meter]]s.

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 valve]]s'', which are only opened while a sample is taken. Other examples of normally-closed valves are [[Shutdown valve|emergency shutdown 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 regulator]]s 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===
[[File:3-way.svg|120px|thumb|right|Schematic 3 way ball valve: L-shaped ball right, T-shaped left]]
Valves with three ports serve many different functions. A few of the possibilities are listed here.

[[Ball valve#Multiport|Three-way ball valves]] come with 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 valve]]s'' automatically connect the higher pressure inlet to the outlet while (in some configurations) preventing flow from one inlet to the other.

''[[Tap (valve)#immutable link from Valve|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 valve]]s'' 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===
{{Main|Four-way valve}}
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 (material)|sample]] without affecting the pressure of a hydraulic system and to avoid [[degassing]] (no leak, no gas loss or air entry, no external contamination)....