The variety of valves available to employ on a project is staggering, and usually the choice of which to use is based on the technical application, the ease of use by operators and maintenance personnel, and the cost.
In application, valves are used for two purposes:
- On/Off service, in which the valve is intended to be full open to permit flow, or full-closed to stop flow.
- Throttling service, in which the valve is required to modulate the flow through i in order to satisfy a process condition such as regulating flow rate or pressure.
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Valve Body is the main shell of the valve and serves as the main element of the valve assembly. It provides the main framework that holds different components together. Valve body also connects the inlet and outlet from the adjoining pipe. It resists the fluid pressure and is usually made of cast or forged steel. Valve body material includes stainless steel, forged carbon steel, brass bronze and nickel alloys. The inlet and out on the valve body could be flanged, bolted, threaded or welded joint.
Valve Bonnet provides the opening int he body of the valve and is the 2nd most important boundary of valve. Bonnets are available in many designs and models. Bonnet act as a cover on the valve body. Bonnet is cast or forged and is usually of the same material as valve body. Bonnets are necessary to assemble the internal components of the valve and to service them. Since they are an opening to the valve body, they are considered a pressure barrier. They are attached to valve body by bolts and sealing mechanism.
Valve Trim is the internal components of the valve that can be removed or replaced. These parts includes valve seats, disk, guids, bushings, glands and internal springs. The most important valve trim component is disc and seat which comes in variety of material and shapes, depending on the type of valve. They provide the basic valve functioning of valve. Valve trim can provide linear or rotational movement to provide flow control. For example in ball valve, the perforated ball rotates to allow or restrict fluid flow. In case of gate valve, the valve trim provides linear up and down motion of the knife shape get to allow or restrict flow of fluid. Since valve trims are directly in the path of fluid, they come in different material and cladding to provide chemical and mechanical strength.
Valve Disc provides on/off and throttling action to the fluid flow. The disc could be a ball, plug, wedge shaped or any shape that matches with the profile on the seat to create a pressure barrier.
Valve Seat provides mating surface to the valve disc to create a seal. A valve may have one or multiple seats.Seats could be made of elastomer or thermoplastic. In case of high temperature or pressure application the seat could be all metallic, lappaed to mate with the disc or cladded with chemical resistant material.
Valve Stem provides the required movement to the sealing element. It is connected to the valve handwheel, level or actuator on one end and to the valve disc on the other end.
Stems are also forged and connected to disc by thread or other means. It also have sealing element to prevent leakage on the valve bonnet/body. Main types of valve stems are;
- Rising stem with outside screw and yoke
- Non Rising Stem with inside screw
- Sliding Stem
- Rotary Stem
Valve Actuators are devices that provides the linear or rotary motion to valve stem to provide the require on/off or throttling operation. The valve actuation could be manual in case of level or hand wheel or it could be electrical or pneumatic in case of bigger and heavier valve stems.
Commonly used valve actuators includes, gear, electric motor, pneumatic, hydraulic or solenoid operated valves.
Classification of Valves
Linear Motion Valves: These valves close with a linear motion of disc relative to seat. This includes gate valve, globe valve and check valve.
Rotary Motion Valves: This is when the disc rotates relative to seat to allow or restrict flow of fluid. Examples are ball valve, butterfly valve and swing check valves.
Pressure and temperature ratings of valves are designated by class numbers. ASME B16.34, Valves-Flanged, Threaded, and Welding End is one of the most widely used valve standards. It defines three types of classes: standard, special, and limited. ASME B16.34 covers Class 150, 300, 400, 600, 900, 1500, 2500, and 4500 valves.