Overview of gate valves：
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Gate valves are one of the most common types of valves in the isolation category. They can be found in every refinery, chemical plant, power station, and industrial production facility, and can be manufactured in sizes ranging from DN6 to . gate valves are chosen for the primary purpose of fluid shutoff. It is for this reason that gate valves are often referred to as "cut-off" or "blocking" valves.
Gate valves are generally used in the fully open or fully closed operation. When fully open, the pressure drop through the gate valve is very small; when fully closed, the pressure sealing performance is very good. Due to the fit of the gate and seat ring, there is almost no leakage when the gate valve is closed. However, in the case of very low back pressure (generally not less than 0.34 bar, 1 bar = 105 Pa), some leakage may occur.
Another advantage of gate valves is that they usually open and close very slowly, a feature that prevents fluid water hammer effects from damaging the piping system.
The main limitation of gate valves is that they cannot be used in throttling conditions. When gate valves are used in dry throttling service, the fluid often has a high flow rate near the seat, which can cause erosion. Also in the partially open state, the gate will swing under the impact of the medium, which will cause damage to the seal. Usually, gate valves are more prone to seat and gate seal surface wear than globe valves, and the repair of such wear is difficult to be accomplished by means of grinding and polishing.
Because the structural characteristics of the gate valve are very suitable for large-scale devices, its application range is expanding, and to high temperature and high-pressure development. The maximum diameter of the gate valve has reached DN, in the ammonia plant has been a large number of high-pressure gate valve PN320, for oil and gas drilling wellhead gate valve working pressure up to 138MPa. manufacturing a high-temperature and high-pressure gate valve is the key to solving the problem of wedging the gate plate due to temperature changes, as well as ensuring its reliable sealing performance. To this end, the gate and seat sealing structure should be studied and improved. Resilient seats and resilient floating seats have begun to be applied to enable the gate valve to achieve inlet sealing, outlet sealing, or double sealing of import and export.
Composition of gate valve：
Gate valve, also called the gate valve, mainly consists of the valve body, bonnet, gate, stem, seat and seal packing, and other parts.
The gate valve is the opening and closing member (gate), driven by the valve stem, along the seat (sealing surface) to do a linear movement of the valve. Generally, the gate valve can not be used to regulate the flow, only a cut-off device can be used, either completely open or completely closed. Among the various types of valves, the gate valve is the most widely used one. It is suitable for both normal temperature and pressure conditions, as well as high and low temperature and high and low-pressure conditions, and can be applied to a variety of different media through the selection of different materials.
The advantages of the gate valve:
①Small fluid resistance. Gate valve body's internal media channel is straight, the media flow through the gate valve does not change its flow direction, so the fluid resistance is small.
② structure length (the distance between the two ends connected to the pipe) is small. As the gate is disc-shaped, is placed vertically in the body of the valve, and the globe valve flap (also disc-shaped) is placed parallel to the body of the valve, so compared with the globe valve, its structure length is smaller. Such as the PN16, and DN150 globe valve, the structure length of 480mm, while the same parameters of the gate valve structure length are only 350mm.
③ opening and closing less effort. When opening and closing the gate movement direction is perpendicular to the direction of media flow, while the shut-off valve flap is usually closed in the direction of movement and seated in the opposite direction of media flow, and therefore must overcome the force of the medium. Therefore, compared with the shut-off valve, the opening, and closing of the exposition valve is more labor-saving.
④ Media flow direction is not restricted. The medium can flow from both sides of the gate valve in any direction through the gate valve and can achieve the purpose of connection or cut off. Easy to install, suitable for the flow direction of the medium may change in the pipeline.
⑤ When fully open, the sealing surface is subject to minimal erosion of the working medium.
⑥The structure of the body is relatively simple, and the manufacturing process is good.
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Disadvantages of the gate valve:
①Sealing surface is easy to produce abrasions. When opening and closing the gate and the valve seat in contact with the relative sliding between the two sealing surfaces, under the action of the medium thrust easy to produce abrasion, thus destroying the sealing performance, and affecting the service life.
② more parts, the structure is more complex, manufacturing and maintenance are more difficult, compared with the cost of the globe valve are higher.
③ shape size and opening height are larger, and the space required for installation is larger.
④Generally gate valves have two sealing pairs, which adds some difficulties to processing, grinding, and maintenance.
⑤ The operating stroke is large, and the opening and closing time is long. Since the gate needs to be completely lifted to the top of the seat channel when opening, and all the gate needs to fall down to block the seat channel when closing, so the opening and closing stroke of the gate is large, and the opening and closing time is long.
Ball valves and gate valves both belong to the same family: shut-off valves. When inside a hydraulic system, these mechanical devices interrupt or regulate the flow, depending on whether the valves are fully open or closed, or if they are used in intermediate positions.
It is important to specify that ball valves, given their fast opening and closing mechanism, are not used to regulate the flow but only to stop or interrupt it.
Gate valves, instead, can be partially open during use, even if it should be remembered that in this case the liquid can become disturbed and friction losses can occur.
Gate valves are used to isolate specific areas of the water supply network: during maintenance, in order to do maintenance, with new installations, or for deviating the flow through the conduit. A completely free passage and minimum pressure drop are the strong points of gate valves. If used in an intermediate position, these valves choke the flow in the conduits.
Gate valves get their name from the shape of their closing disc, which can be straight, like a guillotine, or inclined (wedge). The first solution is preferable above all for high delivery capacities, but in both cases it is always the closing disc that determines valve closing or opening according to whether it is raised or lowered. The valve is opened and closed by the rotary movement of a threaded stem connected to the closing disc. To move from closed to open and vice versa, the handwheel needs to be turned a few times. It is therefore a slow operation, which has the advantage of avoiding dangerous hydraulic shocks.
Commonly used in civil and industrial applications, ball valves get their name from their round closing disc; the disc has a bore that is coaxial to the flow. When the valve is open, the hollow ball becomes a part of the conduit and allows the fluid to flow; when it is closed, the flow is completely blocked. To open or close a ball valve, just give it a quarter turn, making the ball rotate along the same axis as the fluid flow, and as such the piping. Even though this quick movement can be made manually (think about taps in a home), it can just as easily be automated, for example using a pneumatic actuator, in the case of industrial systems.
(read also Motorised valves with pre-assembled pneumatica actuator: the advantages of a perfect combination)
As anticipated, the quick movement can cause hydraulic shock; the wave generated by the movement of the fluid when stopped quickly can damage the piping, a problem that is solvable by installing check valves along the piping.
(read also How to prevent water hammer consequences and avoid damages)
In conclusion, here is an outline of the strong points, and differences between, these two types of valve:
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