In fluid piping systems, valves are control elements whose main functions are to isolate equipment and piping systems, regulate flow, prevent backflow, and regulate and discharge pressure.

Valves can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, slurries, oils, liquid metals and radioactive media. Since the selection of the most suitable valve for a piping system seems to be very important, it becomes essential to understand the characteristics of valves and the steps and basis for selecting valves.

Classification of valves

First, the valve can be divided into two main categories:

The first category of automatic valves: rely on the medium (liquid, gas) the ability to act on its own valve.

Such as check valves, safety valves, control valves, traps, pressure reducing valves.

The second type of drive valve: with the help of manual, electric, hydraulic, pneumatic to manipulate the action of the valve.

Such as gate valves, globe valves, throttle valves, butterfly valves, ball valves, plug valves and so on.

Second, according to the structural characteristics, according to the direction of the closing member relative to the valve seat can be divided:

1. gate shape: the closing member moves along the center of the valve seat;

2. gate-shaped: the closing member moves along the center of the vertical valve seat;

3. Plunger and ball: the closing member is a plunger or ball, rotating around its own centerline;

4. Spin-open shape: the closing member rotates around the axis outside the valve seat;

5. disc-shaped: the closing member of the disk, around the valve seat within the axis of rotation;

6. Slide valve shape: the closing member slides in the direction perpendicular to the channel.

Third, according to the use, according to the different uses of the valve can be divided:

1. open and close: used to connect or cut off the pipeline medium, such as globe valves, gate valves, ball valves, butterfly valves and so on.

2. Check: used to prevent the medium backflow, such as check valves.

3. Adjustment: used to adjust the pressure and flow of the medium, such as regulating valves, pressure reducing valves.

4. Distribution: used to change the direction of flow of media, distribution of media, such as three-way plug, distribution valves, slide valves.

5. Safety valve: in the medium pressure exceeds the specified value, used to discharge excess media to ensure the safety of piping systems and equipment, such as safety valves, accident valves.

6. Other special purposes: such as traps, vent valves, sewage valves, etc..

Fourth, according to the drive mode, according to the different drive mode can be divided:

1. manual: with the help of handwheels, handles, levers or sprockets, etc., there are manpower drive, transmission of large torque is equipped with worm gears, gears and other deceleration devices.

2. Electric: with the help of motor or other electrical devices to drive.

3. Hydraulic: with the help of (water, oil) to drive.

4. Pneumatic: driven by compressed air.

V. Pressure, according to the nominal pressure of the valve can be divided:

1. Vacuum valves: absolute pressure <0.1Mpa that is, 760mm Hg high valves, usually with mm Hg or mm water column pressure.

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

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

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

5. ultra-high pressure valve: nominal pressure PN ≥ 100.0MPa valve.

Sixth, according to the temperature of the medium, according to the temperature of the medium when the valve works can be divided:

1. Ordinary valves: valves suitable for medium temperature -40 ℃ ~ 425 ℃.

2. High-temperature valves: for media temperature 425 ℃ ~ 600 ℃ valve.

3. Heat-resistant valves: for media temperature 600 ℃ or more valves.

4. Low-temperature valves: for media temperature -150 ℃ ~ -40 ℃ of the valve.

5. Ultra-low-temperature valves: for media temperature -150 ℃ below the valve.

Seven, according to the nominal diameter, according to the nominal diameter of the valve can be divided:

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

2. Medium-caliber valves: nominal diameter DN50 ~ 300mm valve.

3. Large caliber valves: nominal diameter DN350 ~ 1200mm valve.

4. Very large caliber valves: nominal diameter DN ≥ 1400mm valve.

Eight, according to the pipeline connection, according to the valve and pipeline connection can be divided:

1. flange connection valve: valve body with a flange, and the pipeline using a flange connection valve.

2. Threaded connection valve: valve body with internal or external threads, and the pipeline using threaded connections.

3. Welding connection valve: valve body with weld, and the pipeline using welding connection valve.

4. Clamp connection valve: valve body with clamp port, and the pipeline using clamp connection valve.

5. ferrule connection valve: the use of ferrule and pipe connection valve.

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Characteristics of valves

Valve characteristics generally have two kinds, use characteristics and structural characteristics.

Use characteristics: it determines the main use of the valve performance and use of the scope of use, belonging to the use of the valve characteristics: the category of the valve (closed-loop valves, regulating valves, safety valves, etc.); product type (gate valves, globe valves, butterfly valves, ball valves, etc.); the valve's main parts (valve body, bonnet, valve stem, valve valve flap, sealing surface) of the material; valve drive, etc..

Structural characteristics: it determines the valve installation, repair, maintenance and other methods of some structural characteristics, belonging to the structural characteristics of: the structural length of the valve and the overall height, and the pipeline connection form (flange connection, threaded connection, clamped hoop connection, external threaded connection, welded end connection, etc.); the sealing surface of the form (inlays, threaded circle, surfacing, welding, spray welding, the valve body body); the valve stem structural form (rotating stem, lifting) and so on. (rotating stem, lifting stem) and so on.

Selection of the steps and basis of the valve

Selection steps:

1, clear the use of the valve in the equipment or device, to determine the working conditions of the valve: applicable media, working pressure, working temperature, etc..

2, to determine the nominal diameter of the pipeline connected to the valve and connection: flange, thread, welding, etc..

3, to determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage.

4, according to the pipeline medium, working pressure, working temperature to determine the selected valve shell and inner parts of the material: grey cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloy and so on.

5, choose the type of valve: closed-circuit valves, regulating valves, safety valves and so on.

6, to determine the type of valve: gate valves, globe valves, ball valves, butterfly valves, throttle valves, safety valves, pressure reducing valves, steam traps and so on.

7, to determine the parameters of the valve: for automatic valves, according to the different needs of the first to determine the allowable flow resistance, discharge capacity, back pressure, etc., and then determine the nominal diameter of the pipeline and the diameter of the valve seat hole.

8, to determine the geometric parameters of the selected valve: structural length, flange connection form and size, open and close the valve after the height direction of the size, connecting the size and number of bolt holes, the entire valve dimensions.

9, the use of existing information: valve catalogues, valve product samples and other selection of appropriate valve products.

Select the basis of the valve:

In understanding the steps to master the selection of valves at the same time, should further understand the basis for selecting the valve.

1, the use of the selected valve, the use of working conditions and manipulation control.

2, the nature of the working medium: working pressure, working temperature, corrosive properties, whether it contains solid particles, whether the medium is toxic, whether it is flammable, explosive media, the viscosity of the medium and so on.

3, the requirements of the valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing grade and so on.

4, the installation size and dimensions requirements: nominal diameter, and pipeline connection and connection size, dimensions or weight limitations.

5, the reliability of the valve products, service life and explosion-proof performance of electric devices and other additional requirements.

In the selected parameters should be noted:

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, the pressure drop in normal flow, the pressure drop when closed, the maximum and minimum inlet pressure of the valve.

According to the above selection of valves based on the basis and steps, reasonable and correct selection of valves must also be a detailed understanding of the internal structure of various types of valves, in order to be able to prioritise the selection of valves to make the right choice.

The ultimate control of the pipeline is the valve. Valve opening and closing parts of the control of the medium in the pipeline beam way, the shape of the valve flow path so that the valve has a certain flow characteristics, in the choice of piping system is most suitable for the installation of the valve must take this into account.

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Selection of valves should follow the principle

1, cut-off and open medium with the valve

The flow path is straight through the valve, its flow resistance is small, usually selected as a cut-off and open medium with the valve. Downward closed valve (globe valve, piston valve) due to its winding flow path, flow resistance is higher than other valves, so less selection. Allow a higher flow resistance of the occasion, you can choose to close the valve.

2, control the flow of valves

Usually choose easy to adjust the flow of the valve as a control flow. Downward closing type valve (such as globe valves) is suitable for this purpose, because of its seat size and the closing member of the travel between the proportional relationship. Rotary valves (plug valves, butterfly valves, ball valves) and flexure body type valves (clamp valves, diaphragm valves) can also be used for throttling control, but are usually only applicable in a limited range of valve sizes. Gate valve is a disc-shaped gate to the circular valve seat mouth to do cross-cutting movement, it is only close to the closing position, to better control the flow, so usually not used for flow control.

3、Directional shunt with the valve

According to the need for commutation and diversion, this 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 commutation diversion are selected one of these valves. But in some cases, other types of valves, as long as the two or more valves properly connected to each other, can also be used for reversing the flow.

4, with suspended particles of the medium with the valve

When the medium with suspended particles, the most suitable for the use of its closing member along the sealing surface of the sliding with the wiping effect of the valve. If the closing member of the seat of the back and forth movement is vertical, then it may be clamped particles, so this valve unless the sealing surface material can allow embedded particles, otherwise only suitable for basic clean media. Ball valves and plug valves in the opening and closing process of the sealing surface are wiping effect, so it is suitable for use in the medium with suspended particles.

Valve selection instructions

1, gate valve selection

     In general, the gate valve should be preferred. In addition to the gate valve for steam, oil and other media, but also for the medium containing granular solids and viscosity of the medium, and is suitable for venting and low vacuum system valves. For media with solid particles, the gate valve body should be with one or two blowholes. For low-temperature media, should be used for low-temperature special gate valve.

2、 Globe valve selection instructions

     Globe valve is suitable for the fluid resistance requirements are not strict pipeline, that is, the pressure loss is not a big consideration, as well as high temperature, high pressure media pipeline or device, suitable for DN <200mm steam and other media pipeline; small valves can be used to select the globe valve, such as needle valves, instrumentation valves, sampling valves, manometer valves, etc.; Globe valve flow control or pressure control, but the regulation of the accuracy of the requirements of the high, and the diameter of the pipe and Comparison of hours, it is appropriate to use the globe valve or throttle valve; for highly toxic media, it is appropriate to use the bellows sealed globe valve; but the globe valve should not be used for viscosity of the medium and contain particles easy to precipitate the medium, but also should not be used as a bleeder valve and the valve of the low-vacuum system.

3, ball valve selection instructions

     Ball valves are suitable for low temperature, high pressure, viscosity of the medium. Most ball valves can be used in the medium with suspended solid particles, according to the material requirements of the seal can also be used for powdery and granular media; full-channel ball valves are not suitable for flow regulation, but for the requirements of rapid opening and closing occasions, to facilitate the realization of the accident emergency cut off; usually in the sealing performance is strict, wear and tear, indentation of the channel, open and close quickly, high-pressure cut-off (differential pressure is large), low-noise, aerosolised phenomenon, operating Torque is small, small fluid resistance in the pipeline, the recommended use of ball valves; ball valves are suitable for lightweight structure, low-pressure cut-off, corrosive media; ball valves or cryogenic, deep-cold media, the most ideal valve, cryogenic media piping systems and installations, it is desirable to choose to add the bonnet of the cryogenic ball valves; selection of floating ball ball valves when the seat material should take the ball and the working medium of the load, the large diameter ball valves require a larger operating Force, DN ≥ 200mm ball valve should be selected worm gear drive form; fixed ball valve for larger calibre and higher pressure occasions; in addition, for the process of highly toxic materials, combustible media pipeline ball valve, should have a fire prevention, anti-static structure.

4, throttle valve selection instructions

     Throttle valve is suitable for medium temperature is low, high pressure occasions, suitable for the need to regulate the flow and pressure of the parts, not for viscosity and contain solid particles of the media, should not be used as a disconnecting valve.

5, plug valve selection instructions

     Plug valves are suitable for the requirements of rapid opening and closing occasions, generally not for steam and higher temperature media, for lower temperature, viscosity of the media, but also for the media with suspended particles.

6, butterfly valve selection instructions

     Butterfly valve is suitable for larger calibre (such as DN>600mm) and short structure length requirements, as well as the need for flow regulation and opening and closing requirements for rapid occasions, generally used for temperature ≤ 80 ℃, pressure ≤ 1.0MPa water, oil and compressed air and other media; due to the butterfly valve relative to the gate valve, the ball valve pressure loss is relatively large, the butterfly valve is suitable for pressure loss requirements are not stringent piping system.

7、Check valve selection instructions

     Check valve is generally applicable to clean media, should not be used for solid particles and viscosity of the medium. When DN ≤ 40mm, it is appropriate to use lift check valve (only allowed to be installed in the horizontal pipeline); when DN = 50 ~ 400mm, it is appropriate to use the rotary lift check valve (in the horizontal and vertical pipeline can be installed, such as installation of the vertical pipeline, the media flow direction should be from the bottom to the top); when DN ≥ 450mm, it is appropriate to use the buffer-type check valves; when DN = 100 ~ 400mm may also be Clamped check valve; Swing check valve can be made into a very high working pressure, PN can reach 42MPa, according to the shell and seals of different materials can be applied to any working medium and any working temperature range. The medium is water, steam, gas, corrosive media, oil, pharmaceuticals and so on. Medium working temperature range between -196 ~ 800 ℃.

8, diaphragm valve selection instructions

     Diaphragm valve is suitable for working temperature less than 200 ℃, pressure less than 1.0MPa oil, water, acidic media and media containing suspended solids, not for organic solvents and strong oxidants media; abrasive particulate media should be selected weir diaphragm valve, weir diaphragm valve to refer to the flow characteristics of the table; viscous fluids, cement slurry, and precipitation media should be selected through the type of diaphragm valve; in addition to the specific requirements, diaphragm valve Should not be used in vacuum piping and vacuum equipment.

     Valve applications, frequency of operation and service varies greatly, to control or eliminate even the slightest leakage, the most important, the most critical equipment is still the valve. Learning to select the correct valve is essential