The valve can be divided into two categories
The first type of automatic valve: a valve that relies on the ability of the medium (liquid, gas) to act on its own. Such as check valves, safety valves, regulating valves, traps, pressure reducing valves, etc.
The second type of drive valve: a valve that is operated manually, electrically, hydraulically, and pneumatically.Such as gate valve, globe valve, throttle valve, butterfly valve, ball valve, plug valve, etc.
According to structural characteristics, according to the direction of movement of the closing member relative to the valve seat, it can be divided into
1. Sectional door shape: the closing piece moves along the center of the valve seat;
2. Gate shape: The closing piece moves along the center of the vertical valve seat;
3. Cock and ball: The closing part is a plunger or a ball, which rotates around its centerline;
4. Swing shape: the closing piece rotates around the axis outside the valve seat;
5. Dish shape: the disc of the closing part, which rotates around the axis in the valve seat;
6. Slide valve shape: The closing piece slides in the direction perpendicular to the channel.
According to the purpose, the different purposes of the valve can be divided into
1. For breaking: used to connect or cut off pipeline media, such as globe valves, gate valves, ball valves, butterfly valves, etc.
2. Non-return use: used to prevent the medium from flowing back, such as a check valve.
3. For regulation: used to regulate the pressure and flow of the medium, such as regulating valves and pressure reducing valves.
4. Distribution: used to change the flow direction of the medium and distribute the medium, such as three-way cock, distribution valve, slide valve, etc.
5. Safety valve: When the pressure of the medium exceeds the specified value, it is used to discharge excess medium to ensure the safety of the piping system and equipment, such as safety valves and emergency valves.
6. Other special purposes: such as traps, vent valves, drain valves, etc.
According to the driving mode, it can be divided into different driving modes
1. Manual: With the help of handwheel, handle, lever or sprocket, etc., it is driven by manpower, and the transmission torque is equipped with worm gear, gear, and other speed reduction devices.
2. Electric: driven by a motor or other electrical devices.
3. Hydraulic: driven by (water, oil).
4. Pneumatic: driven by compressed air.
According to the pressure, the nominal pressure of the valve can be divided into
1. Vacuum valve: The absolute pressure is less than 0.1Mpa, that is, the valve with a height of 760mm mercury column. Usually, the pressure is expressed by mm mercury column or mm water column.
2. Low-pressure valve: valves with nominal pressure PN≤1.6Mpa (including steel valves with PN≤1.6MPa)
3. Medium pressure valve: a valve with a nominal pressure of PN2.5—6.4MPa.
4. High-pressure valve: a valve with a nominal pressure of PN10.0-80.0MPa.
5. Ultra-high pressure valve: a valve with a nominal pressure of PN≥100.0MPa.
According to the temperature of the medium, according to the temperature of the medium when the valve is working, it can be divided into
1. Ordinary valve: suitable for valves with a medium temperature of -40°C to 425°C.
2. High-temperature valve: suitable for valves with a medium temperature of 425°C to 600°C.
3. Heat-resistant valve: suitable for valves with medium temperature above 600℃.
4. Low-temperature valve: suitable for valves with medium temperature -150℃～-40℃.
5. Ultra-low temperature valve: suitable for valves whose medium temperature is below -150℃.
According to the nominal diameter, the nominal diameter of the valve can be divided into
1. Small-diameter valves: valves with a nominal diameter of DN<40mm.
2. Medium-caliber valves: valves with a nominal diameter of DN50～300mm.
3. Large-diameter valves: valves with a nominal diameter of DN350～1200mm.
4. Extra large diameter valves: valves with nominal diameter DN ≥ 1400mm.
According to the connection with the pipeline, it can be divided according to the connection between the valve and the pipeline
1. Flange connection valve: The valve body has a flange and is connected to the pipeline by flange.
2. Threaded connection valve: The valve body has internal thread or external thread, and the valve is connected with the pipeline by thread.
3. Welding connection valve: The valve body has a welding port and is connected to the pipeline by welding.
4. Clamping clamp connection valve: a valve with clamp opening on the valve body and clamp connection with the pipeline.
5. Ferrule connection valve: a valve that uses ferrule to connect to the pipeline.
There are generally two types of valve characteristics, use characteristics and structural characteristics
Use characteristics: It determines the main performance and scope of use of the valve. The characteristics of the valve are valve category (closed-circuit valve, regulating valve, safety valve, etc.); product type (gate valve, globe valve, butterfly valve, ball valve, etc.); Material of the main parts of the valve (valve body, bonnet, valve stem, valve disc, sealing surface); valve transmission mode, etc.
Structural characteristics: It determines some structural characteristics of the valve installation, repair, maintenance, and other methods. The structural characteristics include the structural length and overall height of the valve, the connection form with the pipeline (flange connection, threaded connection, clamp connection, External thread connection, welding end connection, etc.); the form of the sealing surface (inlaid ring, threaded ring, surfacing, spray welding, valve body); valve stem structure (rotating rod, lifting rod), etc.
Valve selection steps
1. Clarify the purpose of the valve in the equipment or device, and determine the working conditions of the valve: applicable medium, working pressure, working temperature, etc.
2. Determine the nominal diameter and connection method of the pipeline connecting with the valve: flange, thread, welding, etc.
3. Determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electric linkage or electro-hydraulic linkage, etc.
4. Determine the material of the selected valve shell and internal parts according to the medium, working pressure, and working temperature of the pipeline: gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloy, etc.
5. Choose the type of valve: closed-circuit valve, regulating valve, safety valve, etc.
6. Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap, etc.
7. Determine the parameters of the valve: For automatic valves, first determine the allowable flow resistance, discharge capacity, back pressure, etc., according to different needs, and then determine the nominal diameter of the pipeline and the diameter of the valve seat hole.
8. Determine the geometric parameters of the selected valve: structural length, flange connection form, and size, valve height dimension after opening and closing, connecting bolt hole size and number, overall valve outline size, etc.
9. Use existing information: valve product catalogs, valve product samples, etc. to select appropriate valve products.
The selection basis of the valve
While understanding and mastering the steps of selecting a valve, you should also further understand the basis for selecting a valve.
1. The purpose, operating conditions, and operation control mode of the selected valve.
2. The nature of the working medium: working pressure, working temperature, corrosion performance, whether it contains solid particles, whether the medium is toxic, whether it is a flammable or explosive medium, the viscosity of the medium, etc.
3. Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing level, etc.
4. Requirements for installation dimensions and external dimensions: nominal diameter, connection to the pipeline and connection dimensions, external dimensions or weight restrictions, etc.
5. Additional requirements for the reliability, service life, and explosion-proof performance of electric devices or valve products.
Attention should be paid when selecting parameters
If the valve is to be used for control purposes, the following additional parameters must be determined: operating method, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop when closed, and maximum and minimum inlet pressure of the valve.
According to the above-mentioned basis and steps for selecting valves, it is necessary to have a detailed understanding of the internal structure of various types of valves when choosing a valve reasonably and correctly, so as to make a correct decision on the preferred valve.
The ultimate control of the pipeline is the valve. The valve opening and closing parts control the flow model of the medium in the pipeline. The shape of the valve flow channel makes the valve have a certain flow characteristic. This must be considered when selecting the most suitable valve for the pipeline system.
How to choose the right valve according to function
1. Valves for shutting off and opening the medium
The flow channel is a straight-through valve, and its flow resistance is small, and it is usually selected as a valve for shut-off and open medium. The downward closing valve (stop valve, plunger valve) is less used because of its tortuous flow path and higher flow resistance than other valves. Where a higher flow resistance is allowed, a closed valve can be used.
2. Valves used to control flow
Usually, choose the valve that is easy to adjust the flow as the control flow. Downward closing valves (such as globe valves) are suitable for this purpose because the size of its seat is proportional to the stroke of the closing member. Rotary valves (plug valves, butterfly valves, ball valves) and flex-body valves (pinch valves, diaphragm valves) can also be used for throttling control, but they are usually only applicable within a limited range of valve diameters. The gate valve uses a disc-shaped gate to make a cross-cutting movement to the circular valve seat opening. It can only control the flow better when it is close to the closed position, so it is usually not used for flow control.
3. Valves for reversing and shunting
According to the needs of reversing and shunting, this kind of valve can have three or more channels. Plug valves and ball valves are more suitable for this purpose. Therefore, most of the valves used for reversing and splitting select one of these valves. However, in some cases, other types of valves can also be used for reversing and shunting as long as two or more valves are properly connected to each other.
4. Valves for media with suspended particles
When there are suspended particles in the medium, it is most suitable to use a valve with a wiping effect on the sliding of the closing member along the sealing surface. If the back and forth movement of the closing member to the valve seat is vertical, it may hold particles. Therefore, this valve is only suitable for basic clean media unless the sealing surface material allows particles to be embedded. Ball valves and plug valves have a wiping effect on the sealing surface during the opening and closing process, so they are suitable for use in media with suspended particles.
How to select several common valves
1. Selection of gate valve
In general, gate valves should be the first choice. The gate valve is not only suitable for steam, oil, and other media, but also suitable for media containing granular solids and high viscosity, and is suitable for valves inventing and low vacuum systems. For media with solid particles, the valve body of the gate valve should have one or two purge holes. For low-temperature media, low-temperature dedicated gate valves should be used.
2. Instruction for selection of globe valve
The stop valve is suitable for pipelines that do not require strict fluid resistance, that is, the pressure loss is not considered, and the pipelines or devices for high temperature and high-pressure media are suitable for medium pipelines such as steam with DN<200mm; small valves can be used to stop Valves, such as needle valves, instrument valves, sampling valves, pressure gauge valves, etc.; stop valves have flow regulation or pressure regulation, but do not require high adjustment accuracy, and the diameter of the pipeline is relatively small, so stop valves or throttling should be used Valve: For highly toxic media, a bellows-sealed globe valve should be used; however, globe valves should not be used for media with high viscosity and media that contain particles that are easy to precipitate, nor should they be used as vent valves and valves for low vacuum systems.
3. Description of ball valve selection
The ball valve is suitable for medium with low temperature, high pressure, and high viscosity. Most ball valves can be used in media with suspended solid particles, and can also be used in powder and granular media according to the sealing material requirements; full-channel ball valves are not suitable for flow adjustment, but are suitable for occasions that require fast opening and closing, and are easy to implement Emergency cut-off in case of accident; usually, it is recommended in pipelines with strict sealing performance, abrasion, necking passage, rapid opening and closing action, high-pressure cut-off (large pressure difference), low noise, vaporization phenomenon, small operating torque, and small fluid resistance. Use ball valve; ball valve is suitable for light structure, low-pressure cut-off, corrosive medium; ball valve is also the most ideal valve for low temperature and cryogenic medium, low-temperature ball valve with bonnet should be selected for low-temperature medium piping system and equipment; The seat material of the floating ball valve should bear the load of the ball and the working medium. The large-diameter ball valve requires a greater force during operation. The DN≥200mm ball valve should use the worm gear transmission form; the fixed ball valve is suitable for larger diameters and High-pressure occasions; In addition, the ball valve used in the process of highly toxic materials and combustible medium pipelines should have a fireproof and anti-static structure.
4. Description of throttle valve selection
The throttle valve is suitable for occasions where the medium temperature is low and the pressure is high, and it is suitable for the parts that need to adjust the flow and pressure. It is not suitable for the medium with high viscosity and containing solid particles, and it is not suitable for the isolation valve.
5. Description of plug valve selection
The plug valve is suitable for occasions that require fast opening and closing. Generally, it is not suitable for steam and higher temperature media, for lower temperature and high viscosity media, and also for media with suspended particles.
6. Description of butterfly valve selection
Butterfly valves are suitable for large diameter (such as DN﹥600mm) and short structure length requirements, as well as occasions where flow adjustment and fast opening and closing requirements are required. They are generally used for water, oil, and compression with temperature ≤80℃ and pressure ≤1.0MPa Air and other media; because the pressure loss of butterfly valves is relatively large compared with gate valves and ball valves, butterfly valves are suitable for piping systems with less stringent pressure loss requirements.
7. Check valve selection instructions
Check valves are generally suitable for clean media, but not for media containing solid particles and high viscosity. When DN≤40mm, lift check valve should be used (only allowed to be installed on horizontal pipeline); when DN=50～400mm, swing check valve should be used (installable on both horizontal and vertical pipelines, If installed on a vertical pipeline, the flow direction of the medium should be from bottom to top); when DN≥450mm, buffer check valve should be used; when DN=100～400mm, wafer check valve can also be used; swing check valve The return valve can be made into a very high working pressure, PN can reach 42MPa, and can be applied to any working medium and any working temperature range according to the material of the shell and the seal. The medium is water, steam, gas, corrosive medium, oil, medicine, etc. The working temperature range of the medium is between -196～800℃.
8. Diaphragm valve selection instructions
Diaphragm valve is suitable for oil, water, acid medium and medium containing suspended solids whose working temperature is less than 200℃ and pressure is less than 1.0MPa. It is not suitable for organic solvent and strong oxidant medium; weir-type diaphragm valve should be selected for grinding granular medium. The selection of weir-type diaphragm valve should refer to its flow characteristics table; viscous fluid, cement slurry, and sedimentary medium should use straight-through diaphragm valve; except for specific requirements, diaphragm valve is not suitable for vacuum pipelines and vacuum equipment.
Valve applications, operating frequencies, and services are ever-changing. To control or eliminate even low-level leakage, the most important and critical equipment is the valve. It is very important to learn to choose the valve correctly.