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VALVE PARTS :
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.
Ports
The body has two or more openings, often called the ports, through which the fluid can enter or leave the valve. Valves with two ports are the most common. Valves with three ports are the next most common. Less often used are valves with four or more ports. Rheodyne makes rather small valves with six ports and Valco makes similar small valves with six or ten ports. Extra ports which are not needed can be closed off. In some cases, a valve port is immediately connected to a spray nozzle or container.
Connections to ports
Although valves are sometimes made connected or integral with some other object like a vessel, many valves are manufactured initially as a separate item to be connected afterwards to some other object such as a pipe,tube, or pump head on at least one of the valve ports either by a manufacturer or the user. In valves which are essentially faucets, taps, spigots, or sampling valves, one (or more) of the ports is often left unconnected or just has a short length of piping. Commonly valves are made with some means of connection at the ports. Means of connections at valve ports can include treads, compression fittings, gluing or cementing (especially for plastic), or welding(for metals). In fluid systems, valves are often connected to piping or tubing. A certain type of valve may come in various sizes. Valve sizes are commonly given by the outer diameter of tubing or pipng the ports connect to; for example, a 1-inch valve is sized to connect to 1-inch outer diameter tubing.
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 as long as the rotor can be turned inside the valve body. However not all round or spherical dics 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 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, 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.
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 the these. A valve with a rotor would be controlled by turning the stem. The valve and stem can be treaded such that the stem can 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 even though they have two ports.
Valves in which the 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. Valves in which the 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. These terms do not apply to valves with no stem nor to valves using rotors.
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.
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.
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®, Hastalloy®, brass, and nickel.
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