Valve

A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways.

Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems.

In HVAC ductwork and other near-atmospheric air flows, valves are instead called dampers.

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

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.

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

A common use of the term valve refers to the poppet valves found in the vast majority of modern internal combustion engines such as those in most fossil fuel powered vehicles which are used to control the intake of the fuel-air mixture and allow exhaust gas venting.

PVC, PP, PVDF and glass-reinforced nylon are common plastics used for valve bodies.

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.

Ports are obstructed by the valve member or disc to control flow.

Connection methods include threadings, compression fittings, glue, cement, flanges, or welding.

An actuator is a mechanism or device to automatically or remotely control a valve from outside the body.

Ball valves use spherical rotors with a cylindrical hole drilled as a fluid passage.

The "seat" is the interior surface of the body which contacts the disc to form a leak-tight seal.

Gaskets are the mechanical seals, or packings, used to prevent the leakage of a gas or fluids from valves.

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.

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 (this pin shall be made of an austenitic stainless steel material).

For example, needle valves have elongated conically tapered discs and matching seats for fine flow control.

When there is a problem these valves open (by switching them 'off') causing the unit to be flushed and emptied.

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 ball valves come with T- or L-shaped fluid passageways inside the rotor.

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)....

When a high water level is reached, a mechanism shuts the valve which fills the tank.

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.

[2] Some fluid system designs, especially in chemical or power plants, are schematically represented in piping and instrumentation diagrams.