Manual Valves Types

Posted: 05/13/2019 05:05:37   Hits: 3
Ball Valves
Ball valves offer very good shut-off capability. A ball valve is a form of quarter-turn (90°) valve to control the fluid flow. This characteristic minimizes valve operation time and decreases the likelihood of leakage due to wear from the gland seal. Ball valves can be divided into two categories: reduced bore type and full bore type. In reduced bore valves, the valve opening is smaller than the diameter of the piping. In full bore valves, the diameter of the valve opening is the same as that of the piping. Balls valves are only recommended for use in the fully open or fully closed position. They are not suited to regulate flow by being kept partially open because ball valves use ring-shaped soft valve seats. When used in the partially open position, pressure is applied to only a portion of the valve seat, which can cause it to deform. If the valve seat deforms, its sealing properties are impaired and it will leak.

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Butterfly valves
In butterfly valves, the flow is regulated through a disc-type element held in place in the center of the valve by a rod. Similar to ball valves, valve operation time is short because the valving element is simply rotated a quarter turn (90°) to open or close the passageway. Butterfly valves are characterized by their simple construction, light weight, and compact design. Their face-to-face dimension is often extremely small, making the pressure drop across a butterfly valve much smaller than globe valves. Materials used for the valving element and sealing can limit their applications at higher temperatures or with certain types of fluids. Butterfly valves are often used in applications for water and air, and in applications with large pipe diameters.

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Globe valves
The globe valve is suitable for use in a wide variety of applications, from flow rate control to open/close operation. In this type of valve, flow rate control is determined not by the size of the opening in the valve seat, but rather by the lift of the valve plug (the distance of the valve plug from the valve seat). One feature of globe valves is that even if used in the partially open position, there is less risk of damage to the valve seat or valve plug by the fluid compared with other types of manual valves. Among the various configurations available, needle type globe valves are particularly well suited for flow rate control. Another point to consider about globe valves is that the pressure drop across the valve is greater than that of many other types of valves because the passageway is S-shaped. Valve operation time is also longer because the valve stem must be turned several times in order to open and close the valve, and this may eventually cause leakage of the gland seal (packing). Furthermore, care must be taken not to turn the valve shaft too far because there is a possibility it could damage the seating surface.

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Gate valves
The construction of a gate valve is similar to that of a floodgate. Flow is controlled by raising or lowering the valving element, which is generally available in three different types: solid (plain), flexible, and split. The latter two types prevent the valving element and body from being deformed due to various operating conditions. Like ball valves, gate valves are not usually used to regulate flow. One of the reasons for this is because the valving element can be damaged when in the partially open position. Similarly, they also limit the pressure drop across the valve when fully open. However, setting the valve to the fully open or closed position requires the handle to be turned many times, which generally makes these valves have the longest operating times among those valve types mentioned here.

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Diaphragm valves
Diaphragm valves use a “pinching” method to stop the valve flow. They are available in two types: weir and straight-way. The most commonly seen of the two is the weir-type. This is because the straight-way type requires additional stretching of the diaphragm, which can shorten the diaphragm's life-span. One of the major advantages of using diaphragm valves is that the valve components can be isolated from the process fluid. Similarly, this construction helps prevent leakage of the fluid without the use of a gland seal (packing) as seen in other types of valves. On the other hand, the diaphragm becomes worn more easily and regular maintenance is necessary if the valve is used on a regular basis. These types of valves are generally not suited for very high temperature fluids and are mainly used in liquid systems.
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2019.05.13
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