The main classification methods for valves

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　　Knowledge 1: Classification of valves 1. Classification by purpose and function Shut-off valves are mainly used to stop or allow the flow of media. These include gate valves, globe valves, diaphragm valves, ball valves, plug valves, disc valves, plunger valves, ball plug valves, and needle-type control valves. 　　Regulating valves are mainly used to regulate the flow rate, pressure, etc. of a medium. Including control valves, throttle valves, pressure reducing valves, etc. 　　Check valves are used to prevent the backflow of fluid. Including check valves of various structures. 　　Diverter valves are used to separate, distribute, or mix media. Including distribution valves and steam traps of various structures, etc. 　　Safety valves are used for providing safety protection when the pressure of the medium exceeds a certain level. Including various types of safety valves. 　　2. Classification by main parameters
(1) Classification by pressure
Vacuum valves: Valves whose operating pressure is lower than standard atmospheric pressure. 　　Low-pressure valves: Valves with a nominal pressure PN of less than 1.6 MPa. 　　Medium-pressure valves are valves with a nominal pressure of PN 2.5~6.4 MPa. 　　High-pressure valves: Valves with a nominal pressure of PN10.0~80.0 MPa. 　　Ultra-high pressure valves: Valves with a nominal pressure PN greater than 100 MPa. 　　(II) Classification by medium temperature: High-temperature valves are those with a temperature t greater than 450°C. 　　Medium-temperature valves: Valves with a temperature range of 120°C < t < 450°C. 　　Normal temperature valves – valves for temperatures ranging from -40 “C to less than 120 “C. 　　Cryogenic valves – valves for temperatures where -100 °C < t < -40 °C. 　　Ultra-low temperature valves: valves for temperatures below -100 “C. 　　(III) Classification by valve body material
Non-metallic material valves: such as ceramic valves, fiberglass-reinforced plastic valves, and plastic valves. 　　Metal material valves: such as copper alloy valves, aluminum alloy valves, lead alloy valves, titanium alloy valves, Monel alloy valves, cast iron valves, carbon steel valves, cast steel valves, low-alloy steel valves, and high-alloy steel valves. 　　Valves with metal valve bodies lined: such as lead-lined valves, plastic-lined valves, and enamel-lined valves. 　　3. General classification method: This classification method divides items based on both principles and functions as well as structure, and it is currently the most widely used classification method internationally and domestically. They generally include gate valves, globe valves, throttle valves, control valves, plug valves, diaphragm valves, cock valves, ball valves, butterfly valves, check valves, pressure relief valves, safety valves, steam traps, control valves, foot valves, filters, and drain valves, among others. 　　II. How to Select Valves 　In my opinion, when selecting valves, it’s better to opt for higher-quality valves even if the pipes are of lower quality; leaks and other issues are often related to both the pipe fittings and the valves themselves. 　　When selecting valves: 1. Bypass circuits must be installed for pressure regulators, balance valves, etc ； 　　2. For fully open or fully closed applications, it’s best to use ball valves or gate valves ； 　　3. Use globe valves as little as possible ； 　　4. Attention should be paid to the calculation of valve resistance ； 　　5. The electric valve must be chosen carefully. 　　A. Selection of valves based on the medium’s on/off characteristics
Butterfly valve
In a butterfly valve, the disc is mounted in the diameter direction of the pipe. Inside the cylindrical passage of the butterfly valve body, the disc-shaped butterfly plate rotates around its axis through an angle ranging from 0° to 90°. When it rotates to 90°, the valve is in the fully open position. 　　Butterfly valves have a simple structure, small size, and light weight; they consist of only a few parts. Moreover, it can be quickly opened and closed by simply rotating 90°, making the operation simple; at the same time, this valve boasts excellent fluid control properties. When the butterfly valve is in its fully open position, the thickness of the butterfly disc is the only resistance to the flow of the medium through the valve body; as a result, the pressure drop across this valve is very small, giving it excellent flow control properties. Butterfly valves come in two types of sealing: elastic sealing and metallic sealing. Elastic-sealing valves, where the sealing ring can be embedded in the valve body or attached around the butterfly disc. 　　Valves with metal seals generally have a longer lifespan than those with elastic seals, but it is difficult to achieve complete sealing. Metal seals can withstand higher operating temperatures, while elastic seals have the drawback of being limited by temperature. If a butterfly valve is to be used for flow control, the key is to correctly select its size and type. The structural principle of butterfly valves makes them particularly suitable for manufacturing large-diameter valves. Butterfly valves are widely used not only in general industries such as petroleum, gas, chemicals, and water treatment, but also in the cooling water systems of thermal power plants. 　　The commonly used butterfly valves are wafer-type butterfly valves and flange-type butterfly valves. A wafer-type butterfly valve is connected between two pipe flanges using bolts, while a flanged butterfly valve has flanges on it, with the flanges at both ends of the valve being secured to the pipe flanges using bolts. The strength performance of a valve refers to its ability to withstand the pressure of the medium. Valves are mechanical components that are subjected to internal pressure; therefore, they must possess sufficient strength and stiffness to ensure they do not crack or deform over long periods of use. 　　Ball valve The ball valve evolved from the plug valve. It has the same 90-degree rotation action, but the difference is that the plug body is spherical, with a circular hole or passage running through its axis. The ratio between the sphere and the channel opening should be such that when the sphere rotates 90 degrees, the entire spherical surface becomes visible at both the inlet and outlet, thereby blocking the flow. 　　A ball valve can be closed tightly with just a 90-degree rotation and a very small torque. The completely equal inner chamber of the valve provides a flow path with very low resistance and a direct route for the medium. It is generally believed that ball valves are most suitable for direct on-off operation, but recent developments have designed them to also serve the purpose of throttling and controlling flow. The main advantages of ball valves are their compact structure, ease of operation and maintenance. They are suitable for use with common working media such as water, solvents, acids, and natural gas, as well as for media under harsh operating conditions, such as oxygen, hydrogen peroxide, methane, and ethylene. The body of a ball valve can be integral or assembled. 　　Globe valve The axis of the valve stem in a globe valve is perpendicular to the sealing surface of the valve seat. The opening or closing stroke of the valve stem is relatively short, and it features a very reliable shut-off action; this makes such valves ideal for cutting off, regulating, and throttling the medium. 　　Once the disc of a globe valve is in the open position, there is no longer any contact between its seat and the sealing surface of the disc, resulting in a very reliable shut-off action; this makes such valves highly suitable for shutting off, regulating, or throttling fluids. 　　Once a globe valve is in the open position, there is no longer any contact between its seat and the sealing surface of the valve disc; as a result, mechanical wear on these sealing surfaces is minimal. Moreover, since the seat and valve disc of most globe valves can be easily repaired or replaced without the need to remove the entire valve from the pipeline, this makes them particularly suitable for situations where the valve is welded to the pipeline. The flow direction of the medium changes as it passes through such valves; therefore, the flow resistance of globe valves is higher than that of other valves. 　　The commonly used globe valves include the following: 1) Angle-type globe valve ； In angular stop valves, the fluid only needs to change direction once, resulting in a lower pressure drop across the valve compared to stop valves with conventional designs. 2) DC globe valve ； In straight-through or Y-pattern globe valves, the flow passage within the valve body forms an oblique line with the main flow path. This results in less disruption to the flow pattern compared to conventional globe valves; consequently, the pressure loss across the valve is also reduced accordingly. 3) Plunger-type globe valve: This type of globe valve is a variant of the conventional globe valve. In this valve, the valve disc and seat are usually designed based on the plunger principle. The valve disc is polished to form a plunger that connects with the valve stem, and sealing is achieved through two elastic sealing rings fitted over the plunger. The two elastic sealing rings are separated by a collar, and the sealing rings around the plunger are pressed in place by the load applied to the valve cover by the valve cover nut. The elastic sealing ring can be replaced and is made from a variety of materials. This valve is primarily used for opening or closing, but it also features a specially designed plunger or special rings that allow it to be used for regulating flow. 　　Globe Valve A globe valve is used to shut off the flow of a medium; when it is fully open, the flow path is completely unobstructed, resulting in the lowest pressure loss for the flowing medium. Gate valves are typically suitable for applications where frequent opening and closing are not required, and where the gate plate needs to remain fully open or fully closed. Not suitable for use as a regulator or throttle. For high-speed flowing media, partial opening of the gate plate can cause vibration in the gate, and this vibration may damage the sealing surfaces of the gate plate and the valve seat; moreover, throttling exposes the gate plate to erosion by the medium. In terms of structural form, the main difference lies in the type of sealing element used. Based on the design of the sealing element, gate valves are often classified into several different types, such as wedge gate valves, parallel gate valves, parallel double-plate gate valves, and wedge double-plate gate valves. The most commonly used types are wedge gate valves and parallel gate valves. 　　B. Valves selected to prevent backflow of the medium This type of valve functions by allowing the medium to flow in only one direction and preventing it from flowing in the opposite direction. Normally, this type of valve operates automatically; under the pressure of fluid flowing in one direction, the valve flap opens ； When the fluid flows in the opposite direction, the fluid pressure together with the weight of the valve disc causes the valve disc to act on the valve seat, thereby stopping the flow. Among them, check valves fall into this category of valves; they include swing-check valves and lift-check valves. A swing check valve has a hinge mechanism, as well as a valve disc that resembles a door and rests freely on an inclined valve seat surface. To ensure that the valve disc always reaches the appropriate position on the valve seat surface, it is designed with a hinge mechanism, allowing sufficient rotation space for the valve disc and enabling it to make true, full contact with the valve seat. The valve disc can be made entirely of metal, or leather, rubber can be inlaid on the metal, or a synthetic covering can be used, depending on the required performance characteristics. When a swing check valve is fully open, the fluid pressure encounters little resistance; therefore, the pressure drop across the valve is relatively small. The disc of the lift-type check valve is seated on the seat sealing surface of the valve body. Apart from the valve disc, which can move up and down freely, this valve operates in a similar manner to a globe valve: fluid pressure lifts the valve disc off the sealing surface of the valve seat, while backflow causes the valve disc to return to the valve seat, thereby stopping the flow. Depending on the operating conditions, the valve disc can be of a fully metal structure, or it can consist of a frame with rubber gaskets or rings embedded in it. Similar to globe valves, the passage through which fluid flows in lift check valves is also narrow. Therefore, the pressure drop across lift check valves is greater than that across swing check valves; moreover, the flow rate through swing check valves is hardly restricted at all. 　　C. Select valves based on the parameters of the medium to be regulated
During the production process, in order to ensure that parameters such as the pressure and flow rate of the medium meet the requirements of the process, regulating mechanisms must be installed to adjust these parameters. The main operating principle of the regulating mechanism is to *change the flow area between the valve disc and the valve seat, thereby achieving the purpose of regulating the aforementioned parameters. Valves of this type are collectively referred to as control valves. Those driven by the power of the medium itself are known as self-acting control valves, such as pressure reducing valves and pressure stabilizing valves. Those driven by external power sources (such as electricity, compressed air, or hydraulic power) are called externally acting control valves, such as electric control valves, pneumatic control valves, and hydraulic control valves. 　　D. Selecting valves based on their driving mechanism Electrically driven valves Electrically driven valves are a common type of valve that uses electric power for operation; such driving systems are often referred to as valve actuators. The advantages of valve actuators include: 1) Fast opening and closing, which allows for **reduction in the time required to open or close the valve ； 2) It can **reduce the workload on operators, and is particularly suitable for high-pressure, large-diameter valves** ； 3) Suitable for installation in locations where manual operation is not possible or where access is difficult; it enables easy remote control, and there are no restrictions on the installation height ； 4) Conducive to the automation of the entire system ； 5) Power sources are easier to obtain than gas and liquid sources, and the installation and maintenance of their wires are also much simpler than those of compressed air and hydraulic pipelines. 　　The disadvantages of electric valve actuators are their complex structure; they are even more difficult to use in humid environments. When used with explosive media, explosion-proof measures must be taken. 　　Depending on the type of valve they drive, valve actuators can be classified into two main categories: Type Z and Type Q. The output shaft of the Z-type valve actuator can rotate many times, making it suitable for driving gate valves, globe valves, diaphragm valves, etc ； The output shaft of the electric actuator for Q-type valves can rotate only 90°, and is suitable for driving plug valves, ball valves, butterfly valves, etc. According to their protection types, there are ordinary type, flame-retardant type (denoted by B), heat-resistant type (denoted by R), and three-in-one type (i.e., for outdoor use, corrosion resistance, and flame retention, denoted by S). 　　A valve actuator generally consists of a transmission mechanism (reducer), a motor, a travel control mechanism, a torque limitation mechanism, a manual-electric switching mechanism, an opening indicator, and so on. 　　Pneumatic and hydraulic valves Pneumatic valves and hydraulic valves use air, water, or oil under certain pressure as a power source; they rely on the movement of cylinders (or hydraulic cylinders) and pistons to operate the valve. Generally, the air pressure used in pneumatic systems is less than 0.8 MPa, while the water or oil pressure in hydraulic systems ranges from 2.5 MPa to 25 MPa. or diaphragm valve ； Rotary gas and liquid actuation devices are used to drive ball valves, butterfly valves, or plug valves. Hydraulic actuators have high driving force and are suitable for driving large-diameter valves. When used to drive plug valves, ball valves, and butterfly valves, the reciprocating motion of the piston must be converted into rotational motion. In addition to being driven by pistons in cylinders or hydraulic cylinders, there are also those driven by pneumatic diaphragms; due to their limited stroke and driving force, they are mainly used in control valves. 　　Manual valves Manual valves are the most basic type of valves in terms of driving mechanism. It includes two types: direct drive using a handwheel, handle, or wrench, and drive through a transmission mechanism. When the starting torque of the valve is high, it can be driven via gear or worm gear transmission in order to eliminate this requirement. Gear drives are divided into straight cylindrical gear drives and bevel gear drives. Gear drives offer a low reduction ratio and are suitable for gate valves and globe valves, while worm gear drives provide a higher reduction ratio and are appropriate for plug valves, ball valves, and butterfly valves. 　　III. Main technical properties of valves Strength properties The strength properties of a valve refer to its ability to withstand the pressure of the medium. Valves are mechanical components that are subjected to internal pressure; therefore, they must possess sufficient strength and stiffness to ensure they do not crack or deform over long periods of use. 　　Sealing performance The sealing performance of a valve refers to the ability of its various sealing elements to prevent the leakage of the medium, and it is the most important technical parameter for valves. There are three sealing areas in a valve: the contact point between the moving part and the two sealing surfaces of the valve seat ； The fit between the packing and the valve stem as well as the packing box ； The connection between the valve body and the valve cover. The leakage in the former case is called internal leakage, which is what is commonly referred to as poor sealing; it affects the valve’s ability to block the flow of the medium. For cut-off valves, internal leakage is not allowed. The leaks in the latter two locations are called external leaks, that is, the medium leaks from inside the valve to outside it. Leaks can result in material loss, environmental pollution, and in severe cases, accidents. For flammable, explosive, toxic, or radioactive media, leaks are absolutely unacceptable; therefore, valves must have reliable sealing properties. 　　Flow performance: When the fluid passes through the valve, a pressure loss occurs (that is, a pressure difference before and after the valve). In other words, the valve presents a certain resistance to the flow of the fluid, and the fluid must expend energy to overcome this resistance. From the perspective of energy conservation, when designing and manufacturing valves, it is necessary to minimize the resistance exerted by the valves on the flowing medium as much as possible. 　　Operational performance Sensitivity and reliability of operation This refers to the degree of sensitivity with which the valve responds to changes in the parameters of the medium. For valves such as throttle valves, pressure reducing valves, and control valves that are used to adjust the parameters of a medium, as well as valves with specific functions like safety valves and steam traps, their functional sensitivity and reliability are very important technical performance indicators. 　　Opening force and opening torque The opening force and opening torque refer to the force or torque that must be applied to open or close a valve. When closing the valve, it is necessary to create a certain sealing pressure between the sealing surfaces of the operating element and the seat. At the same time, it is also required to overcome the friction forces between the valve stem and the packing, between the threads of the valve stem and the nut, at the support points at the end of the valve stem, and in other friction-prone areas. Therefore, a certain closing force and closing torque must be applied. During the process of opening and closing the valve, the required opening and closing forces and torques change, with their maximum values occurring at the final moment of closure or the initial moment of opening. When designing and manufacturing valves, efforts should be made to reduce their closing force and closing torque. 　　Opening and closing speed The opening and closing speed is expressed as the time required for a valve to complete one cycle of opening or closing.