What is a gate valve used for?
What is a gate valve?
In daily life, gate valves are a very common thing. It is a door with main opening and closing parts. The opening and closing parts of the gate valve are the ram, and the direction of movement of the ram is perpendicular to the direction of the fluid. The main function of a gate valve is to achieve fully open or fully closed control of the fluid, and is not suitable for flow regulation. The ram has two sealing surfaces that squeeze against each other when the valve is closed, creating a seal that cuts off the flow of fluid. The opening and closing of the gate valve is achieved by rotating the valve stem, which is connected to the gate, and when the stem rotates, the gate is driven up or down, thereby controlling the flow of fluid. It is mainly used as a cutting medium in pipelines, i.e., fully open or fully closed. Ordinary gate valves cannot be used as throttle valves. It can be used for high temperature and high pressure, and can also be used for various media. In pipelines that transport slurries and viscous fluids, gate valves are not usually used.
Design & parts of a gate valve
Gate valves are generally composed of a valve body, a valve disc, a valve stem, a sealing ring and a driving device. The valve body is the main body of the valve, which bears the pressure of the fluid; The disc plays the role of opening and closing; The valve stem connects the disc and the control device, and undertakes the force transmission during rotation; Seals are used to ensure the tightness of the valve when it is closed. With these basic structures, gate valves are able to efficiently control the flow of fluids.
The material of the gate valve usually affects its performance, corrosion resistance, pressure resistance and applicable environment.
The following are several commonly used gate valve materials:
Cast iron gate valve: Cast iron is a common valve material, which has good mechanical properties and pressure resistance, and is suitable for non-corrosive fluids such as water and gas. Cast iron valves are inexpensive, but their corrosion resistance is relatively poor, making them unsuitable for use in some highly corrosive environments.
Stainless steel gate valve: Stainless steel gate valve has excellent corrosion resistance and high temperature resistance, and is suitable for use in petroleum, chemical and other industries. It has strong antioxidant properties and long service life, but its relative price is high.
Copper Gate Valve: Due to its good thermal conductivity and antibacterial properties, copper gate valves are widely used in the food and pharmaceutical industries, especially when used to transport drinking water. Copper valves are more resistant to corrosion, but they can still be affected in certain environments.
Plastic gate valve: Plastic gate valve is usually used for chemically corrosive fluids, with good corrosion resistance and lightweight characteristics, and is suitable for some lightweight material piping systems. The disadvantage is that the pressure bearing capacity is relatively weak, and the use environment needs to be carefully selected.
Types of gate valves
In terms of structural form, the main difference is the form of the sealing element used. According to the form of sealing elements, gate valves are often divided into several different types, such as: wedge gate valve, parallel gate valve, parallel double gate gate valve, wedge double gate gate valve, etc. The most commonly used forms are wedge gate valves and parallel gate valves. Wedge gate valves (including solid wedge, flexible wedge, and split wedge designs) use two tilting seats and an inclined gate or barrier that is deliberately slightly misaligned with the seat. In some applications, this tends to achieve a tighter seal than parallel versions. Parallel gate valves, sometimes referred to as parallel gates, double gates, or parallel sliding gates, have a flat gate or barrier for the flow of liquid perpendicular to two parallel seats.
Parallel or wedge-shaped gate valves
The closing member of the wedge gate valve is wedge-shaped. The purpose of using the wedge shape is to increase the auxiliary sealing load so that the metal-sealed wedge gate valve can not only ensure the sealing of high medium pressure, but also seal the low medium pressure. In this way, the potential sealing degree that the metal-sealed wedge gate valve can achieve is higher than that of the ordinary metal-sealed parallel gate valve. However, the inlet sealing load generated by the wedging action of the metal-sealed wedge gate valve is often insufficient to achieve the inlet sealing. The valve body of the wedge gate valve is provided with a guide mechanism to prevent the gate from rotating when opening or closing, thereby ensuring the corresponding alignment of the sealing surface and preventing the gate from rubbing against the valve seat before reaching the closed position, thereby reducing the wear of the sealing surface. The disadvantage is that the wedge gate valve cannot be provided with a diversion hole like the parallel gate valve with a diversion hole, and the thermal expansion of the valve stem will also overload the sealing surface. Moreover, the sealing surface of the wedge gate valve is more likely to be mixed with solid particles carried by the flowing medium than the parallel gate valve. Compared with the parallel gate valve. The electric drive device used in wedge gate valves is more complicated because the electric drive device limits not the stroke but the torque. The wedge gate valve must have a large enough closing torque to make the gate wedge into the valve seat to achieve sealing when the valve is closed. In order to open under full pressure difference and allow for the increased opening and closing torque due to thermal expansion of valve components, the drive device must have sufficient torque margin.
The parallel double-disc gate valve is a valve widely used in water conservancy, chemical industry, urban water supply and drainage and other fields. Due to its special design, it has excellent sealing performance and switching characteristics. In the continuation of this article, we will explore the characteristics and application scenarios of the parallel double-disc gate valve.
The valve body structure of the parallel double-disc gate valve is unique, so that the fluid will not generate eddy currents and resistance when passing through the valve, thereby reducing energy consumption. In addition, the operation mode of the valve is flexible, and it can be switched manually, electrically or pneumatically. During the switching process, the parallel double-disc gate valve can realize the function of quickly cutting off and adjusting the flow, ensuring the stability and reliability of the system.
Metal seated or resilient seated gate valves
Soft seal is the seal between metal and non-metal, hard seal is the seal between metal and metal. Both soft and hard seal gate valves are sealing materials.
1. The sealing materials of soft-sealed gate valves and hard-sealed gate valves are different
1. Soft gate valves are generally made of rubber or polytetrafluoroethylene. Hard gate valves use metal materials such as stainless steel.
2. Soft seal: One side of the sealing pair is made of metal material, and the other side is elastic non-metallic material, which is called “soft seal”. This gate valve has a good sealing effect, but it is not resistant to high temperature, easy to wear, and has poor mechanical properties. For example: steel + rubber, steel + polytetrafluoroethylene, etc.
3. Hard seal: The hard seal is made of metal or other hard materials on both sides. This type of gate valve has poor sealing, but it is resistant to high temperature, wear-resistant, and has good mechanical properties. For example, steel + steel; steel + copper; steel + graphite; steel + alloy steel; and cast iron. Alloy steel spray alloy, etc.
2. The construction technology of soft-sealed gate valves and hard-sealed gate valves is different
The working environment of the machinery factory is complex, mostly ultra-low temperature and low pressure, with large medium resistance and strong corrosiveness. With the advancement of science and technology, hard-sealed gate valves have become popular.
In fact, due to the hardness relationship between metals, hard-sealed gate valves are like soft seals. The valve body needs to be hardened, and the valve plate and valve seat should be continuously ground to achieve sealing. The production cycle of hard-sealed gate valves is long.
3. The use conditions of soft-sealed gate valves and hard-sealed gate valves are different
1. Soft-sealed gate valves can achieve zero leakage, and hard seals can be adjusted as needed;
2. Soft seals may leak at high temperatures, and fire prevention should be paid attention to; hard seals will not leak at high temperatures. The hard seal of the emergency shut-off valve can be used for high pressure, and soft seals cannot be used.
3. For some corrosive media, when soft seals cannot be used, hard seals can be used;
4. At ultra-low temperatures, soft sealing materials will leak, and hard seals will have such problems;
4. The equipment selection of soft-sealed gate valves and hard-sealed gate valves is different
Both sealing levels can reach the level, and the appropriate gate valve is usually selected according to the process medium, temperature, and pressure. Usually, when the medium has solid particles or wear, or the temperature exceeds 200 degrees, it is better to use hard seals. If the shut-off valve torque is large, a fixed hard-sealed gate valve should be selected.
5. The service life of soft seal gate valve and hard seal gate valve is different.
The soft seal has good sealing performance, but the disadvantage is that it is easy to age and wear, and has a short service life. The hard seal has a long service life and poor sealing performance, which can be complementary to the soft seal.
Rising stem or non-rising stem gate valves
According to the valve stem structure, it can be divided into:
- Rising stem gate valve. The valve stem nut is on the valve cover or bracket. When opening and closing the valve, the valve stem is lifted and lowered by rotating the valve stem nut. This structure is beneficial to the lubrication of the valve stem thread, and the degree of opening and closing is obvious, so it is widely used. 2. Concealing stem gate valve. The valve stem nut is in direct contact with the medium in the valve body. When opening and closing, the valve stem is lifted and lowered by rotating the valve stem. The only advantage is that the height of the closed valve always remains unchanged, which is suitable for places with large diameters or limited installation space. The disadvantage is that the valve stem thread cannot be lubricated, and is directly corroded by the medium, which is easy to be damaged, and the valve opening is also difficult to judge.
Gate valve working principle
The working principle of a gate valve is to insert a physical barrier (gate or valve disc) into the pipe, seal it when closed, and raise or lower it when opened. The barrier is raised and lowered by a handwheel on the valve cover, which acts as a valve actuator. It turns clockwise and counterclockwise to start or stop the flow of liquid as needed.
As an important part of controlling the flow of fluid, the choice of its actuation method is crucial.
Manual actuation: simple but laborious
The biggest advantage of manually actuated valves is simplicity and low cost. Imagine that you can easily control the flow of fluid by just turning a valve handle by hand. For some small or infrequently operated equipment, this method is indeed convenient. However, here comes the problem! If the pressure of the fluid is relatively high or the size of the valve is large, manual operation becomes laborious and unsafe.
Have you ever thought about whether it is worth using manual methods in high-pressure environments? Moreover, in some dangerous working environments, manual operation may pose a safety hazard. Considering the application of stainless steel stop valves for oxygen, manual actuation may not be the best choice.
Electric actuation: efficient but requires power
Next, let’s take a look at electric actuation. This method uses an electric motor to control the opening and closing of the valve, which is very convenient to use. You just need to press a button and the valve will respond quickly. On the production line, electric valves can be automated, which greatly improves production efficiency.
But electric drive also has disadvantages, such as dependence on power supply. Imagine if there is a sudden power outage, how will the valve operate? In addition, electric valves may not perform as stably as manual valves in extreme environments, especially in high or low temperature areas.
For stainless steel stop valves for oxygen, electric drive can provide more precise control, but the premise is that you need to ensure the stability of power supply and take into account possible environmental factors.
Hydraulic drive: powerful but costly
Hydraulic drive valves use liquid pressure to open and close the valve. Its advantage is that it can handle high pressure and large flow of fluids, suitable for large equipment and complex systems. You can imagine that the hydraulic system is like a powerful booster that can easily cope with various challenges.
However, the disadvantages of hydraulic drive are also obvious. It is usually more expensive and relatively troublesome to maintain. Moreover, the complexity of the hydraulic system means more failure points, which is not an ideal choice for some small or budget-limited projects.
How to tell if a gate valve is open or closed?
Unlike some other flow valve types, it is not always easy to tell whether a gate valve is open or not. For example, ball valves and butterfly valves are usually closed when the handle is perpendicular to the pipe or flow direction.
Because the handwheel of a gate valve is located above or to the side of the pipe and rotates 360 degrees to open and close the valve, you cannot determine which position they are in based on a glance. Typically, turning a gate valve counterclockwise will open it, while turning it clockwise will close it. These directions are usually marked on the handwheel or other location on the valve housing.
Unless you know for sure that the gate valve is stuck or restricted, you should stop turning the gate valve when you start to feel obvious resistance. Continuing to turn the gate valve against resistance may damage it, or the pipe or air duct on which it is installed. Therefore, it is sometimes necessary to try to turn the gate valve in both directions to determine its current position.
The signal gate valve improves the electrical equipment signal device under the premise of a dedicated manual gate valve.
Working principle of the signal gate valve: The electric signal device is a fully enclosed precision electromechanical box structure, with stainless parts and imported electrical components in the box, and the device is located on the valve body gland that is not easily collided under the operating handwheel. The micro switch and its driving mechanism are installed inside, and the gears are meshed with the gears fixed on the valve stem. When the hand wheel is turned to open and close the gate valve, the signal device outputs the “on” and “off” electrical signals corresponding to the gate valve opening and closing positions. When the 24V voltage is input and the valve is closed to 1/4 of the full opening, the valve closed signal can be transmitted to the control center. The signal gate valve has a signal output within the valve stroke from full opening to full closing. The signal device inputs a 24V AC or DC power supply.
Can you throttle a gate valve?
Gate valves are usually suitable for working conditions where frequent opening and closing are not required and the gate is kept fully open or fully closed. They are not suitable for use as regulation or throttling. For high-speed flowing media, the gate can cause vibration of the gate when the gate is partially open, and the vibration may damage the sealing surface of the gate and the valve seat, while throttling will cause the gate to be eroded by the medium. Ball valves and butterfly valves are suitable for flow regulation. The ball valve evolved from the plug valve. Its opening and closing part is a sphere, which is used to rotate 90° around the axis of the valve stem to achieve the purpose of opening and closing. It is mainly used to cut off, distribute and change the flow direction of the medium on the pipeline. The ball valve designed with a V-shaped opening also has a good flow regulation function. The butterfly valve is a valve that uses a disc-type opening and closing part to reciprocate about 90° to open, close and regulate the fluid channel.