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Professional manufacturing Professional valve

Views: 8     Author: Site Editor     Publish Time: 2021-08-16      Origin: Site

Learn about valves, better choose valves

In the fluid piping system, the valve is the control element, and its main function is to isolate the equipment and the piping system, adjust the flow, prevent backflow, adjust and drain the pressure. Valves can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media. Since it is very important to choose the most suitable valve for the piping system, it is also important to understand the characteristics of the valve and the steps and basis for selecting the valve. 

Classification of valves Valves can be divided into two categories:

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.

Drive the valve With the help of manual, electric, hydraulic, pneumatic to control the action of the valve. Such as gate valve, globe valve, throttle valve, butterfly valve, ball valve, plug valve, etc. According to the structural characteristics, according to the direction in which the closing part moves relative to the valve seat, it can be divided into: Sectional door shape: the closing piece moves along the center of the valve seat; Gate shape: The closing piece moves along the center of the vertical valve seat; Cock and ball: The closing part is a plunger or ball, which rotates around its centerline; Swing shape: the closing piece rotates around the axis outside the valve seat; Dish shape: the disc of the closing part, which rotates around the axis in the valve seat; Sliding valve shape: The closing piece slides in a direction perpendicular to the channel. According to the purpose, according to the different purposes of the valve can be divided into: For breaking: used to connect or cut off pipeline media, such as globe valves, gate valves, ball valves, butterfly valves, etc. Non-return use: used to prevent the medium from flowing back, such as a check valve. Adjustment: used to adjust the pressure and flow of the medium, such as regulating valves and pressure reducing valves. Distribution: used to change the flow direction of the medium and distribute the medium, such as three-way cock, distribution valve, slide valve, etc. Safety valve: When the medium pressure exceeds the specified value, it is used to discharge excess medium to ensure the safety of the pipeline system and equipment, such as safety valves and emergency valves. Other special purposes: such as traps, vent valves, drain valves, etc. According to the driving mode, according to different driving modes, it can be divided into: Manual: With the help of handwheels, handles, levers or sprocket wheels, etc., it is driven by human power, and the transmission of large torque is equipped with worm gears, gears and other deceleration devices. Electric: Driven by motors or other electrical devices. Hydraulic: driven by (water, oil). Pneumatic: Driven by compressed air. According to the pressure, according to the nominal pressure of the valve can be divided into: Vacuum valve: a valve whose absolute pressure is less than 0.1Mpa, that is, 760mm mercury column, usually expressed in mm mercury column or mm water column. Low pressure valve: valves with nominal pressure PN≤1.6Mpa (including steel valves with PN≤1.6MPa) Medium pressure valve: a valve with a nominal pressure of PN2.5-6.4MPa. High-pressure valve: a valve with a nominal pressure of PN10.0-80.0MPa. Ultra-high pressure valve: a valve with a nominal pressure of PN≥100.0MPa.

&middot;According to the temperature of the medium, according to the temperature of the medium when the valve is working, it can be divided into: Ordinary valve: suitable for valves with a medium temperature of -40&deg;C to 425&deg;C. High temperature valve: suitable for valves with a medium temperature of 425&deg;C to 600&deg;C. Heat-resistant valve: suitable for valves with medium temperature above 600. Low temperature valve: suitable for valves with medium temperature -150-40. Ultra-low temperature valve: suitable for valves whose medium temperature is below -150. &middot;According to the nominal diameter, according to the nominal diameter of the valve can be divided into: Small diameter valves: valves with nominal diameter DN<40mm. Medium-caliber valves: valves with a nominal diameter of DN50300mm. Large-caliber valves: valves with a nominal diameter of DN3501200mm. Extra large diameter valve: valve with nominal diameter DN ≥ 1400mm. &middot;According to the connection with the pipeline, according to the connection between the valve and the pipeline, it can be divided into: Flange connection valve: The valve body has a flange and is connected to the pipeline by a flange. Threaded connection valve: The valve body has internal thread or external thread, and the valve is connected with the pipeline by thread. Welded connection valve: The valve body has a welding port and is connected to the pipeline by welding. Clamping clamp connection valve: a valve with clamp opening on the valve body and clamp connection with the pipeline. Ferrule connection valve: a valve that uses ferrule to connect to the pipeline.

Valve characteristics There are generally two types of valve characteristics, use characteristics and structural characteristics.

Usage characteristics It determines the main performance and scope of use of the valve. The characteristics of the valve are as follows: Type of valve (closed circuit valve, regulating valve, safety valve, etc.); Product type (gate valve, globe valve, butterfly valve, ball valve, etc.); The material of the main parts of the valve (valve body, bonnet, valve stem, valve clack, sealing surface); Valve drive 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, and the connection form with the pipeline (flange connection, thread connection, clamp connection, external thread connection, welding end connection, etc.); The form of the sealing surface (inlaid ring, threaded ring, surfacing welding, spray welding, valve body); Stem structure (rotating rod, lifting rod), etc.

&middot;Steps and basis for selecting valves

Selection steps 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. Determine the nominal diameter and connection method of the pipeline connected to the valve: flange, thread, welding, etc. Determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage, etc. Determine the material of the selected valve housing 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. Choose the type of valve: closed circuit valve, regulating valve, safety valve, etc. Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap, etc. 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. 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. Use existing information: valve product catalog, valve product samples, etc. to select appropriate valve products. Basis for choosing valve While understanding and mastering the steps of selecting a valve, it is also necessary to further understand the basis for selecting a valve. The purpose, operating conditions and operating control methods of the selected valve. 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. Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing level, etc. Installation dimensions and external dimensions requirements: nominal diameter, connection to the pipeline and connection dimensions, external dimensions or weight restrictions, etc. Additional requirements for valve product reliability, service life, and explosion-proof performance of electric devices. Attention should be paid to the selection of 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 closing, maximum and minimum inlet pressure of the valve . According to the above-mentioned basis and steps for selecting the valve, the internal structure of various types of valves must be understood in detail when selecting the valve reasonably and correctly, so that the correct choice can be made on the preferred valve. The ultimate control of the pipeline is the valve. The valve opening and closing parts control the flow mode 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.

Principles to be followed when selecting valves:

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.

Valves for flow control 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 cross-cut 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.

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.

Valve for medium 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.

Valve selection instructions

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 in venting 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, special low-temperature gate valves should be used.

Instructions 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.

Ball valve selection instructions 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 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, ball valves used for process highly toxic materials and flammable medium pipelines should have a fire-proof and anti-static structure.

Instructions for 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.

Cock valve selection instructions 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. Butterfly valve selection instructions Butterfly valve is suitable for large diameter (such as DN>600mm) and short structure length, as well as occasions where flow adjustment and fast opening and closing are required. Generally used for water, oil and compression with temperature ≤80 and pressure 1.0MPa Air and other media; because the butterfly valve has a relatively large pressure loss compared to the gate valve and ball valve, the butterfly valve is suitable for piping systems with less stringent pressure loss requirements.

Check valve selection instructions Check valves are generally suitable for clean media, not for media containing solid particles and high viscosity. When DN≤40mm, lift check valve should be adopted (only allowed to be installed on horizontal pipeline); When DN=50400mm, swing-type lifting check valve should be adopted (it can be installed on both horizontal and vertical pipelines, if installed on vertical pipelines, the flow direction of the medium should be from bottom to top); When DN≥450mm, buffer check valve should be adopted; When DN=100400mm, wafer check valve can also be used; The swing check 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 -196800.

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.

Learning to choose the valve correctly is the key to production and life.

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