Editing Valve (section)

==Components==
[[File:Valve cross-section.PNG|frame|right|Cross-sectional diagram of an open '''[[globe valve]]:'''
{{olist
|'''body'''
|'''ports'''
|'''seat'''
|'''stem'''
|'''disc''' when valve is open
|'''handle''' or '''handwheel''' when valve is open
|'''bonnet'''
|'''packing'''
|'''gland nut'''
|'''fluid flow''' when valve is open
|position of disc if valve were shut
|position of handle or handwheel if valve were shut}}]]

The main parts of the most usual type of valve are the body and the bonnet. These two parts form the casing that holds the fluid going through the valve.

===Body===
The valve's body is the outer casing of most or all of the valve that contains the internal parts or ''trim''. The bonnet is the part of the [[casing (borehole)|encasing]] through which the [[stem (bicycle part)|stem]] (see below) passes and that forms a guide and seal for the stem. The bonnet typically screws into or is bolted to the valve body.

Valve bodies are usually [[metal]]lic or [[plastic]]. [[Brass]], [[bronze]], [[gunmetal]], [[cast iron]], [[steel]], [[alloy steel]]s and [[stainless steel]]s are very common.<ref>{{cite web|url=https://www.actuation.co.uk/valve-body-types|title=Types of valve bodies and their specifications - Actuation Valves|website=www.actuation.co.uk}}</ref> Seawater applications, like desalination plants, often use duplex valves, as well as super duplex valves, due to their corrosion resistant properties, particularly against warm seawater. [[Alloy 20]] valves are typically used in sulphuric acid plants, whilst [[monel]] valves are used in [[hydrofluoric acid]] (HF Acid) plants. [[Hastelloy]] valves are often used in high temperature applications, such as nuclear plants, whilst [[Hydrogen valve|inconel]] valves are often used in hydrogen applications. Plastic bodies are used for relatively low pressures and temperatures. [[PVC]], [[Polypropylene|PP]], [[PVDF]] and [[Fiberglass|glass-reinforced nylon]] are common plastics used for valve bodies.{{Citation needed|date=November 2009}}

===Bonnet===
A '''bonnet''' acts as a cover on the valve body. It is commonly semi-permanently screwed into the valve body or bolted onto it. 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 valve]]s usually do not have bonnets. Many ball valves do not have bonnets since the valve body is put together in a different style, such as being screwed together at the middle of the valve body.

===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 its ports to pipes or other components. Connection methods include [[Screw thread|threading]]s, [[compression fitting]]s, [[glue]], [[cement]], [[flange]]s, or [[welding]].

===Handle or actuator===
A handle is used to manually control a valve from outside the valve body. Automatically controlled valves often do not have handles, but some may have a handle (or something similar) anyway to manually override automatic control, such as a [[Check valve#Stop-check valve|stop-check valve]]. An actuator is a mechanism or device to automatically or remotely control a valve from outside the body. Some valves have neither handle nor actuator because they automatically control themselves from inside; for example, check valves and [[relief valve]]s may have neither.

===Disc===
[[File:Nozzle check valve disc-The-Alloy-Valve-Stockist.JPG|150px|right|thumb|Valve disc]]
A disc, also known as a 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. Depending on the type of valve, a disc can move linearly inside a valve, or rotate on the stem (as in a [[butterfly valve]]), or rotate on a [[hinge]] or [[trunnion]] (as in a check valve). 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 valve]]s use spherical rotors with a cylindrical hole drilled as a fluid passage. Plug valves use cylindrical or conically tapered rotors called plugs.{{Ambiguous|date=July 2011}} 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===
[[File:Gordon Power Station Control Valve.jpg|150px|thumb|right|The shut off butterfly valve for a [[Francis turbine]] at [[Gordon Power Station, Tasmania]]]]
[[File:Valve-balls-The-Alloy-Valve-Stockist.jpg|150px|left|thumb|[[Ball valve]]]]

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 or trunnion, 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 [[leak]]age.
*''Soft seats'' are fitted to the valve body and made of softer materials such as [[PTFE]] or various [[elastomer]]s such as [[Nitrile rubber|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, and check valves are usually hard seated while butterfly, ball, plug, and diaphragm valves are usually soft seated.

===Stem===
The '''stem''' transmits motion from the handle or controlling device to the disc. The stem typically passes through the [[Hood (vehicle)|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 (Angle valve using torque reactor pin and Hub Assembly). 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|date=July 2011}} [[O-ring|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.

===Gaskets===
[[Gasket]]s are the mechanical seals, or packings, used to prevent the leakage of a gas or fluids from valves.

===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 [[Acrylonitrile butadiene styrene|ABS]], PVC, PP or PVDF.

===Spring===
[[File:Check-valve-springs-in-inconel-The-Alloy-Valve-Stockist.JPG|150px|thumb|left|[[Inconel]] X750 [[Spring (device)|spring]]]]
Many valves have a [[Spring (device)|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 spring]]s are normally used. Typical spring materials include [[Galvanization|zinc plated steel]], stainless steel, and for high temperature applications [[Inconel]] X750.

===Trim===
The internal elements of a valve are collectively referred to as a valve's '''trim'''. 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).