A valve is a mechanical device that turns on and off, regulates, modulates or isolates the rate, volume, pressure or direction of liquids, gases, slurries or dry materials through a pipeline, chute or similar passageway, according to the Valve Manufacturers Association of America. The globe valve and gate valve are two of the most common valve types in use today.
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Though the history of the valve probably goes as far back as the Greeks and the Egyptians who used primitive valves to assist in providing water for public consumption and crop irrigation, it was the Romans who first used valves in their sophisticated plumbing systems. They were able to provide water to individual buildings and used plug stops and check valves to prevent backflow. During the Renaissance and especially the Industrial Revolution, the use of valves became more prominent and necessary.
The two main types of valves are stop valves or isolation valves, which are used to shut off or regulate the flow of fluid, and check valves, which prevent the backflow of fluid. Globe valves and gate valves, along with butterfly and ball valves, are considered stop valves since they are used to both stop and regulate the flow of fluid and are controlled by the movement of the valve stem.
Valves can range in size from a fraction of an inch to 30 feet in diameter and vary anywhere from simple brass valves to precision-designed, highly sophisticated coolant system control valves constructed from exotic metal alloys such as those used in nuclear reactors. Valves are used to control the thinnest gases, highly corrosive chemicals, superheated steam, abrasive slurries, toxic gases and radioactive materials. While a globe valve can be used for either on-off or throttling, however, a gate valve is designed to be either on or off and is not designed for flow regulation.
A globe valve gets its name from the globular shape of the valve body and is the most commonly used valve. The inlet and outlet openings of the valve body can be arranged in several different ways, depending on its use. The most common configurations are straight flow, angle flow and cross flow. The globe valve utilises a conical plug which moves in and out of the fluid, causing a restriction in flow and a drop in pressure and can be used with a high degree of accuracy.
A gate valve derives its name from the fact that the part of the valve that controls the flow of liquid is similar to a gate opening and closing. A gate valve should be used either fully open or fully closed and is not a good valve to use for controlling pressure because of the design. A gate valve can be damaged due to the liquid flowing against the partially closed gate. Gate valves employ either a rising stem, with the gate travelling up and down the stem of the valve, or a non-rising stem, with the stem and gate both rising and lowering together.
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