Valve Fundamentales

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

They perform any of the following functions.

•  Starting and stopping or isolating fluid flow. In an open valve, fluid flows in a direction from higher pressure to lower pressure. 
• Controlling or varying (throttling) the amount of fluid flow by change of direction or restriction. 
• Checking the flow or controlling the direction of fluid flow and preventing backflow.
• Regulating downstream system or process pressure.
• Relieving component or piping system of a certain pressure.

The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.

Valves may be operated manually, either by a handle, lever, pedal or wheel. Valves may also be automatic, driven by changes in pressure, temperature, or flow. These changes may act upon a diaphragm or a piston which in turn activates the valve, examples of this type of valve found commonly are safety valves fitted to hot water systems or boilers.

Valves are quite diverse and may be classified into a number of basic types. 

Classification based on Valve Types :

Isolation Valves or Block Valves

• Gate Valves
• Ball Valves
• Butterfly Valves
• Plug Valves

Non Returning Valves

• Check Valves

Controlling or Regulating / Throttling Valves

• Globe Valves
• Diaphragm Valves
• Pinch Valves
• Needle Valves

Pressure and Safety Relieve Valves

• Pressure Reducing Valves
• Control Valves

Classification based on how they are actuated :

• Hydraulic
• Pneumatic
• Manual
• Solenoid Valve
• Motor

Classification based on Valve Operating Positions :

• Two Port Valves
• Three Port Valves
• Four Port Valves

Classification based on Mechanical Motions :

• Linear Motion Valves

The valves in which closure member moves in a straight line to allow, stop or throttle the flow.

• Rotary Motion Valves

The valves in which the closure member moves along angular or circular path. 

• Quarter Turn Valves

Some Rotary Motion Valves requires approximately a Quarter Turn, 0 to 90 Degree, motion of the stem to go fully open from a fully closed position or vice versa.   

Valve Types	Linear Motion	Rotary Motion	Quarter Turn
Gate	YES	NO	NO
Globe	YES	NO	NO
Plug	NO	YES	YES
Ball	NO	YES	YES
Butterfly	NO	YES	YES
Swing Check	NO	YES	NO
Diaphragm	YES	NO	NO
Pinch	YES	NO	NO
Safety	YES	NO	NO
Pressure Relief	YES	NO	NO

Regardless of type, all valves have the following basic parts: the body, bonnet, trim (internal elements), actuator, and packing.

Valve Components

Valve Body

The valve body sometimes called the shell, is the primary boundary of a pressure valve.
It serves as the main element of a valve assembly because it is the framework that holds all the parts together. 

The body, the first pressure boundary of a valve, resists fluid pressure loads from connecting piping. It receives inlet and outlet piping through threaded, bolted and welded joints.

Valve Bonnet

The cover or opening in the valve body is the bonnet, and it is the second most important part boundary of a pressure valve.
Bonnet acts as a cover of the valve body, is cast of forged of the same material as of the body. It is commonly connected to body by a threaded, bolted or welded joint.
During the manufacturing of the valve, the internal components, such as stem, disc etc. are put into the body and then bonnet is attached to hold all the parts together inside.

Valve Trim

Valve Trim is the collective name for the replaceable parts, in a valve. A typical trim design includes a disk/wedge, seat, stem and sleeves needed to guide the stem.
A valve's performance is determined by the disc and seat interface and the relation of the disc position to the seat.
Because of the trim, basic motions of the flow control are possible.

Valve Disc

The disc is the part which allows, throttles, or stops flow depending on its position. In case of a Plug or a Ball Valve, the disc is called a plug or a ball.
The disc is the third most important primary pressure boundary.
With the valve closed, full system pressure is applied across the disc, and for this reason the disc is pressure related component.
Discs are usually forged, and in some designs, hard surfaced to provide good wear properties. Most valves are named by the design of their discs.

Valve Seat

The seat or seal rings provide the seating surface for the disc. A valve may have one or more seats.
In the case of a Globe or a Swing Check valve, there is usually one seat, which forms a seal with the disc to stop the flow.
In case of Gate valve there are two seats, one on the upstream side and the other on the downstream side. The Valve disc has two seating surfaces, that come in contact with the valve seats to form a seal for stopping the flow.

Valve Stem

The valve stem provides the necessary movement to the disc, plug or the ball for opening or closing the valve, and is responsible for proper positioning of the disc. It is connected to the valve hand wheel, actuator, or the lever at one end and at the other end to the valve disc.