26

2021

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03

Speaking of "valve", this article is a textbook

Author:


A valve that operates on its own based on the ability of the medium (liquid, gas) itself. Such as check valves, safety valves, regulating valves, traps, pressure reducing valves, etc.


Classification of valves

 
 

Valves can generally be divided into two categories:

1

automatic valve

     A valve that operates on its own based on the ability of the medium (liquid, gas) itself. Such as check valves, safety valves, regulating valves, traps, pressure reducing valves, etc.

 

2

Drive valve

     Valves that operate manually, electrically, hydraulically, or pneumatically. Such as gate valves, globe valves, throttle valves, butterfly valves, ball valves, plug valves, 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. Portal shape: the closing member moves along the center of the valve seat;

  2. Gate shape: the closing member moves along the center of the vertical valve seat;

  3. Plugs and Balls: The closing member is a plunger or ball that rotates about its own centerline;

  4. Swing type: the closing member rotates around the axis outside the valve seat;

  5. Disc: The disc of the closure that rotates about an axis within the valve seat;

  6. Spool type: The closing member slides in the direction perpendicular to the channel.

 

    According to the purpose, the valve can be divided into:

  1. For breaking: used to connect or cut off pipeline media, such as stop valves, gate valves, ball valves, butterfly valves, etc.

  2. Check use: Used to prevent medium from flowing back, such as check valves.

  3. Adjustment: Used to adjust 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 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.

  6. Other special uses: such as drain valves, vent valves, sewage valves, etc.

 

    According to the driving mode, it can be divided into:

  1. Manual: With the help of handwheels, handles, levers or sprockets, it is driven by manpower, and equipped with reduction devices such as worm gears and gears when transmitting larger torques.

  2. Electric: Driven by a motor or other electrical device.

  3. Hydraulic: driven by (water, oil).

  4. Pneumatic: driven by compressed air.

 

    Press pressure, according to the nominal pressure of the valve, it can be divided into:

  1. Vacuum valve: a valve with a maximum pressure <0.1Mpa, that is, a valve with a height of 760mm mercury. The pressure is usually expressed in mm mercury or mm water.

  2. Low pressure valve: valves with nominal pressure PN ≤ 1.6Mpa (including steel valves with PN ≤ 1.6MPa)

  3. Medium pressure valve: valve with nominal pressure PN2.5-6.4MPa.

  4. High-pressure valve: valve with nominal pressure PN10.0-80.0MPa.

  5. Ultra-high pressure valve: valve with nominal pressure 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 valves: suitable for valves with medium temperature -40℃~425℃.

  2. High temperature valve: suitable for valves with medium temperature of 425℃~600℃.

  3. Heat-resistant valve: suitable for valves with medium temperature above 600°C.

  4. Low temperature valve: suitable for valves with medium temperature -150℃~-40℃.

  5. Ultra-low temperature valve: suitable for valves with medium temperature below -150℃.

 

    According to the nominal diameter, the valve can be divided into:

  1. Small diameter valve: valve with nominal diameter DN<40mm.

  2. Medium diameter valve: valve with nominal diameter DN50~300mm.

  3. Large diameter valve: valve with nominal diameter DN350~1200mm.

  4. Extra large diameter valve: valve with nominal diameter DN≥1400mm.

 

    According to the connection method with the pipeline, the valve can be divided into:

  1. Flange connection valve: The valve body is equipped with a flange and is flanged to the pipeline.

  2. Threaded connection valve: The valve body has internal threads or external threads and is threadedly connected to the pipe.

  3. Welded connection valve: The valve body has a welding port and is welded to the pipeline.

  4. Clamp connection valve: A valve with a clamp on the valve body and connected to the pipeline by a clamp.

  5. Card sleeve connection valve: a valve that uses a card sleeve to connect to the pipeline.

 

 

 

Valve characteristics

 

 

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

 

1

Usage features

    It determines the main performance and scope of use of the valve. The use characteristics of the valve include:

  • Type of valve (closed-circuit valve, regulating valve, safety valve, etc.);

  • Product type (gate valve, globe valve, butterfly valve, ball valve, etc.);

  • Materials of the main parts of the valve (valve body, valve cover, valve stem, valve disc, sealing surface);

  • Valve transmission mode, etc.

2

Structural properties

    It determines some structural characteristics of 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, threaded connection, clamp connection, external threaded connection, welded end connection, etc.);

  • The form of the sealing surface (inlaid ring, threaded ring, overlay welding, spray welding, valve body);

  • Valve stem structural form (rotating stem, lifting stem), etc.

 

 

 

Steps and basis for selecting valves

 

 

1

Select 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 pipe connected to the valve: flange, thread, welding, etc.

(3) Determine the method of operating the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage, etc.

(4) Determine the materials of the shell and internal parts of the selected valve according to the medium transported by the pipeline, working pressure, and working temperature: gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloy, etc. .

(5) Select 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, height dimension of the valve after opening and closing, size and number of connecting bolt holes, external dimensions of the entire valve, etc.

(9) Use existing information: valve product catalogs, valve product samples, etc. to select appropriate valve products.

 

2

Basis for selecting valves

 

  While understanding the steps for selecting valves, you should also further understand the basis for selecting valves.

(1) The purpose, operating conditions and operation control method 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 grade, etc.

(4) Installation dimensions and overall dimension requirements: nominal diameter, connection method and connection size to the pipeline, overall dimensions or weight limit, etc.

(5) Additional requirements for the reliability, service life of valve products and the explosion-proof performance of electric devices.

 

Pay attention to the following when selecting parameters:

If the valve is to be used for control purposes, the following additional parameters must be determined: method of operation, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop when closed, maximum and minimum inlet pressure of the valve.

According to the above-mentioned basis and steps for selecting valves, when selecting valves reasonably and correctly, you must also have a detailed understanding of the internal structures of various types of valves so that you can make the right choice for the preferred valve.

 

The final control of a pipeline is the valve. The valve opening and closing parts control the flow pattern of the medium in the pipeline. The shape of the valve flow channel gives the valve certain flow characteristics. This must be taken into consideration when selecting the most suitable valve for installation in the pipeline system.

 

Principles to follow when selecting valves:

1. Valves used to cut off and open media

The valve with a straight-through flow path has a small flow resistance and is usually selected as a valve for shutting off and opening the medium. Downward closing valves (stop valves, plunger valves) are rarely used due to their tortuous flow paths and higher flow resistance than other valves. In situations where higher flow resistance is allowed, closed valves can be selected.

 

2. Valves used to control flow

Valves that are easy to adjust the flow are usually selected for flow control. Downward-closing valves (such as globe valves) are suitable for this purpose because their seat size is directly proportional to the stroke of the closing member. Rotary valves (plug valves, butterfly valves, ball valves) and deflection valves (pinch valves, diaphragm valves) can also be used for throttling control, but usually only within a limited valve diameter range. The gate valve uses a disc-shaped gate plate to make a transverse movement to the circular valve seat opening. It can better control the flow only when it is close to the closed position, so it is usually not used for flow control.

 

3. Valve for reversing and diverting flow

This valve can have three or more channels depending on the need for reversing the flow. Plug valves and ball valves are more suitable for this purpose. Therefore, most valves used for reversing and diverting flow choose one of these types of valves. However, in some cases, other types of valves can also be used for reversing and diverting flow as long as two or more valves are properly connected to each other.

 

4. Valves for media with suspended particles

When the medium contains suspended particles, it is most suitable to use a valve whose closing member slides along the sealing surface with a wiping effect. If the back-and-forth movement of the closing member against the valve seat is vertical, particles may be trapped. Therefore, this type of valve is only suitable for basically clean media unless the sealing surface material can allow embedded particles. 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

 

 

1

Gate valve selection

       In general, gate valves should be preferred. In addition to being suitable for steam, oil and other media, gate valves are also suitable for media containing granular solids and high viscosity, and are suitable for valves in venting and low vacuum systems. For media containing solid particles, the gate valve body should be equipped with one or two purge holes. For low-temperature media, low-temperature special gate valves should be selected.

 

2

Globe valve selection

     The stop valve is suitable for pipelines with lax requirements on fluid resistance, that is, pressure loss is not considered much, and pipelines or devices with high temperature and high pressure media. It is suitable for steam and other medium pipelines with DN <200mm; small valves can use cut-off valves. Valves, such as needle valves, instrument valves, sampling valves, pressure gauge valves, etc.; stop valves have flow adjustment or pressure adjustment, but the adjustment accuracy is not required, and the pipeline diameter is relatively small, so a stop valve or throttling valve should be used Valve; for highly toxic media, a bellows-sealed stop valve should be used; however, the stop valve should not be used for media with high viscosity and media containing particles that are prone to sedimentation, nor should it be used as a vent valve and a valve in a low vacuum system.

 

3

Ball valve selection instructions

      Ball valves are suitable for low temperature, high pressure and high viscosity media. Most ball valves can be used in media with suspended solid particles, and can also be used in powdery and granular media according to the sealing material requirements; full-channel ball valves are not suitable for flow regulation, but are suitable for occasions requiring rapid opening and closing, which is easy to implement. Emergency cut-off in accidents; usually recommended in pipelines with strict sealing performance, wear, shrinkage channels, rapid opening and closing movements, high-pressure cutoff (large pressure difference), low noise, gasification phenomenon, small operating torque, and small fluid resistance. Use ball valves; ball valves are suitable for light structures, low-pressure cut-offs, and corrosive media; ball valves are also the most ideal valves for low-temperature and cryogenic media. For piping systems and devices with low-temperature media, low-temperature ball valves with valve covers should be used; choose When using a floating ball valve, its seat material should bear the load of the ball and the working medium. Large-diameter ball valves require greater force during operation. Ball valves with DN ≥ 200mm should use worm gear transmission; fixed ball valves are suitable for larger diameters and In situations with high pressure; in addition, ball valves used in process pipelines for highly toxic materials and flammable media should have fire-proof and anti-static structures.

 

 

4

Throttle valve selection instructions

      The throttle valve is suitable for occasions where the medium temperature is low and the pressure is high. It is suitable for parts that need to adjust the flow rate and pressure. It is not suitable for media with high viscosity and solid particles, and is not suitable for use as a isolation valve.

 

 

5

Plug valve selection instructions

     Plug valves are suitable for occasions requiring rapid opening and closing. They are generally not suitable for steam and medium with higher temperature. They are used for medium with lower temperature and high viscosity, and are also suitable for medium with suspended particles.

 

6

Butterfly valve selection instructions

     Butterfly valves are suitable for applications with large diameters (such as DN>600mm) and short structural length requirements, as well as situations where flow adjustment and fast opening and closing are required. They are generally used for water, oil and compression products with temperatures ≤80°C and pressures ≤1.0MPa. Air and other media; because the pressure loss of butterfly valves is relatively large compared to gate valves and ball valves, butterfly valves are suitable for pipeline systems with loose pressure loss requirements.

 

 

7

Check valve selection instructions

    Check valves are generally suitable for clean media and are not suitable for media containing solid particles and high viscosity.

  • When DN ≤ 40mm, a lift check valve should be used (only allowed to be installed on horizontal pipelines);

  • When DN=50~400mm, a swing-type lift check valve should be used (it can be installed on both horizontal and vertical pipelines. If installed on a vertical pipeline, the medium flow direction 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;

 

     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 depending on the materials of the shell and seals. 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

     The diaphragm valve is suitable for oil, water, acidic media and media containing suspended solids with an operating temperature of less than 200°C and a pressure of less than 1.0MPa. It is not suitable for organic solvents and strong oxidant media. Weir type diaphragm valves should be selected for abrasive granular media. When selecting a weir type diaphragm valve, refer to its flow characteristics table; viscous fluids, cement slurries and precipitating media should use straight-through diaphragm valves; except for specific requirements, diaphragm valves should not be used in vacuum pipelines and vacuum equipment.

 

Valve applications, operating frequencies and services are ever-changing. To control or eliminate even small leaks, the most important and critical equipment is the valve. Learning to select the correct valve is essential.