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Slide valves with a parallel-faced gate-like closure member are commonly known as parallel gate valves. These valves are equipped with a closure member that resembles a gate and can be either a single disc or twin discs. The main characteristic of these valves is their parallel-faced design, which allows for efficient flow control. The parallel gate valve is widely used in various industries for its reliable performance and durable construction. By rearranging the provided information, we can emphasize the key points about the parallel gate valve and its design features.
The pressure of the fluid is responsible for exerting a force on either the floating disc or the floating seat, which subsequently presses the disc against the seat. In twin disc parallel gate valves, this force can be further enhanced by a spreading mechanism positioned between the discs.
Parallel gate valves offer several advantages, one of which is their ability to allow for a smooth flow as they have a low resistance to flow. In fact, the resistance to flow in parallel gate valves is comparable to that of a short length of straight pipe. This means that fluids can pass through these valves without encountering significant obstructions or turbulence.
Another advantage of parallel gate valves is their capability to handle fluids that contain suspended solids. The design of these valves allows the disc to slide across the seat face, which enables them to effectively handle the flow of fluids carrying solid particles. This makes parallel gate valves suitable for applications where the presence of solids in suspension is a common occurrence.
However, it is important to note that the operation of parallel gate valves also comes with certain limitations. The sliding motion of the disc across the seat face restricts the types of fluids that can be effectively controlled using these valves. Certain fluids with specific characteristics or properties may not be well-suited for use with parallel gate valves. Therefore, it is crucial to consider the specific requirements and limitations of parallel gate valve operation before selecting them for a particular application.
When the pressure of the fluid is too low, it can cause a problem with the metal-to-metal seatings. Specifically, the force that is used to create the seal may not be strong enough to guarantee an effective seal. This can negatively impact the performance of different components and systems that rely on these sealing properties, so it is important to ensure that the fluid pressure is maintained at an appropriate level.
Valve operation that occurs frequently may cause the seating faces to experience excessive wear and tear. The extent of damage largely depends on factors such as fluid pressure, the width of the seating faces, the fluid's lubricity, and the wear resistance of the seating material. Consequently, it is highly recommended to reserve the use of parallel gate valves for situations that require infrequent valve operation.
When subjected to shear forces from high-density, high-velocity flow, loosely guided discs and loose disc components are prone to excessive rattling. The vibrations caused can be vigorous and disruptive.
When it comes to controlling flow, circular disc valves travelling across circular flow passages have their limitations. They only provide satisfactory flow control between the 50% closed and fully closed positions. As a result, parallel gate valves are typically used solely for on-off operations. However, some parallel gate valve designs have adopted V-porting the seat to enable flow control.
Full-bore valves, also known as unicavity valves, offer a distinct advantage compared to other valve types. These valves have a disc which seals the valve body cavity, preventing any kind of contamination or solid ingress from getting in, regardless of whether the valve is open or closed. This feature makes full-bore valves ideal for use in pipelines that require regular cleaning or are prone to clogging. Using full-bore valves can help ensure the smooth and uninterrupted flow of liquid or gas, making them a highly desirable choice for many industrial applications.
Conventional Parallel Gate Valves
This commonly known valve, referred to as a parallel slide gate valve, is quite renowned. Its closure member is comprised of two discs with springs in between. The primary purpose of these springs is to maintain a sliding contact between the upstream and downstream seatings, while also enhancing the seating load when dealing with low fluid pressures. To prevent any undesired spreading during the valve's full open position, the discs are securely held in a belt eye arrangement.
The parallel slide gate valve being discussed here has a flow passage that is venturi shaped, which helps to ensure smooth and efficient flow. When the valve is fully open, an eyelet bridges the gap between the seats to further improve the flow. This design offers a number of benefits including reduced construction costs, lower maintenance requirements and decreased operating effort. The only drawback to this configuration is a slight increase in pressure loss across the valve.
When the valve is nearly closed, the seating stress reaches its peak, resulting in a near maximum pressure drop across the valve. However, it's important to note that only a portion of the total seating area is in mutual contact. As the valve moves from three-quarter closed to nearly closed, the flowing fluid exerts a tilting force on the disc, leading to potential wear in the seat bore and outer edge of the disc. To prevent excessive seating stress and corresponding wear, it is necessary to have appropriately wide seatings.
This requirement presents a paradoxical situation where the seating width needs to be small enough to achieve high seating stress yet wide enough to maintain acceptable seating wear. Despite this paradox, these valves ensure fluid tightness and meet the leakage criteria of the steam class, as long as the fluid pressure remains above a certain threshold.
Parallel slide gate valves have other excellent advantages: the seatings are virtually self-aligning and the seat seal is not impaired by thermal movements of the valve body. Also, when the valve has been closed in the cold condition, thermal extension of the stem cannot overload the seatings. Furthermore, when the valve is being closed, a high accuracy in the positioning of the discs is not necessary, thus an electric drive for the valve can be travel limited. Because an electric drive of this type is both economical and reliable, parallel slide gate valves are often preferred as block valves in larger power stations for this reason alone. Of course, parallel slide gate valves may be used also for many other services such as water, in particular boiler feed water—and oil.
A variation of the parallel slide gate valve used mainly in the U.S. is fitted with a closure member such as the one shown . The closure member consists of two discs with a wedging mechanism in between, which, on contact with the bottom of the valve body, spreads the discs apart. When the valve is being opened again, the wedging mechanism releases the discs. Because the angle of the wedge must be wide enough for the wedge to be self-releasing, the supplementary seating load from the wedging action is limited.
To prevent the discs from spreading prematurely, the valve must be mounted with the stem upright. If the valve must be mounted with the stem vertically down, the wedge must be appropriately supported by a spring.
The performance characteristic attributed to parallel slide gate valves also applies largely to this valve. However, solids carried by the flowing fluid and sticky substances may interfere with the functioning of the wedging mechanism. Also, thermal extension of the stem can overload the seatings. The valve is used mainly in gas, water, and oil services.
Conventional parallel gate valves may also be fitted with soft seatings, as in the valve shown. The closure member consists here of a disc that carries two spring-loaded floating seating rings. These rings are provided with a bonded O-ring on the face and a second O-ring on the periphery. When the disc moves into the closed position, the O-ring on the face of the floating seating ring contacts the body seat and produces the initial fluid seal. The fluid pressure acting on the back of the seating ring then forces the seatings into still closer contact.
Because the unbalanced area on the back of the floating rings is smaller than the area of the seat bore, the seating load for a given fluid pressure and valve size is smaller than in the previously described valves. However, the valve achieves a high degree of fluid tightness by means of the O-ring even at low fluid pressures. This sealing principle also permits double block and bleed.
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