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

Valves can be used to control air, water, steam, all kinds of corrosive media, mud, oil, liquid metal and radioactive media and other types of fluid flow. As it is very important to select the most suitable valve for the piping system, it is also very important to understand the characteristics of the valve and the steps and basis for selecting the valve.

Classification of valves

1 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.

2 Drive valve

With manual, electric, hydraulic, pneumatic operation 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 of movement of the closing parts relative to the seat, it can be divided into:

Interceptor type: the closing piece moves along the center of the seat;

Gate shape: the closing parts move along the center of the vertical seat;

Cock and ball: The closing piece is a plunger or ball that rotates around its own center line;

Swing: the closing piece rotates around the axis outside the seat;

Disc: the disc of the closing member, revolving around the shaft in the seat;

Slide valve: The closing member slides in a direction perpendicular to the channel.

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

On and off: used to put through or cut off pipeline medium, such as globe valve, gate valve, ball valve, butterfly valve, etc.

Check: Used to prevent dielectric backflow, such as a check valve.

Regulating with: used to adjust the pressure and flow of the medium, such as regulating valve, reducing valve.

Distribution: used to change the flow direction of the medium, distribution 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 the excess medium to ensure the safety of pipeline system and equipment, such as safety valve and accident valve.

Other special uses: such as trap, blowoff valve, blowdown valve, etc.

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

Manual: with the help of hand wheel, handle, lever or sprocket, etc., manpower drive, drive larger torque fashion worm gear, gear and other reduction devices.

Electric: Driven by a motor or other electrical device.

Hydraulic: To drive with the aid of (water, oil).

Pneumatic: Driven by compressed air.

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

Vacuum valve: Valve with nominal pressure PN<0.1Mpa or 760mm Hg, usually expressed as mm Hg or mm water.

Low pressure valve: nominal pressure PN≤1.6Mpa valve (including PN≤1.6MPa steel valve)

Medium pressure valve: nominal pressure PN2.5-6.4mpa valve.

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

Super high pressure valve: nominal pressure PN≥100.0MPa valve.

According to the temperature of the medium, it can be divided into:

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

High temperature valve: suitable for medium temperature 425℃ ~ 600℃ valve.

Heat resistant valve: Suitable for medium temperature of 600℃ above the valve.

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

Ultra-low temperature valve: suitable for medium temperature -150℃ below the valve.

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

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

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

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

Large diameter valve: nominal diameter DN≥1400mm valve.

According to the connection with the pipe, according to the connection between the valve and the pipe can be divided into:

Flanged valve: Valve body with flange, and pipe using flange connection valve.

Threaded valve: Valve with internal or external threads in the body and threaded to the pipe.

Welded valve: Valve body with welding, welded with the pipe of the valve.

Clamp connection valve: Valve body with clamp mouth, and pipe with clamp connection valve.

Bushing valve: Valve connected with pipe by bushing.

Valve characteristics

1 Usage characteristics

It determines the main performance and scope of use of the valve, which belongs to the use characteristics of the valve are:

 Types of valves (closed-circuit valves, regulating valves, safety valves, etc.);

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

The main parts of the valve (valve body, valve cover, stem, disc, sealing surface) materials;

Valve transmission mode, etc.

2 Structural characteristic

It determines some structural characteristics of valve installation, repair, maintenance and other methods, which belong to the structural characteristics:

The structural length and overall height of the valve, and the connection form of the pipeline (flange connection, thread connection, clamp connection, external thread connection, welding end connection, etc.);

Sealing surface form (insert ring, thread ring, surfacing welding, spray welding, valve body);

Valve stem structure form (rotating rod, lifting rod), etc.

Procedure and basis for valve selection

1 Selection step

（1）Clear valve in equipment or device use, determine the valve working conditions: applicable media, working pressure, working temperature, etc.

（2）Determine the nominal diameter and connection mode of the pipe connected to the valve: flange, thread, welding, etc.

（3）Determine how to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical or electro-hydraulic linkage, etc.

（4）According to the pipeline transmission medium, working pressure, working temperature to determine the selected valve shell and internal materials: gray cast iron, malleable iron, ductile cast 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 valve parameters: for automatic valves, according to different needs 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 seat hole.

（8）Determine the geometric parameters of the selected valve: structural length, flange connection form and size, open and closed valve height direction size, connecting bolt hole size and number, the overall valve appearance size.

（9）Select the appropriate valve product using the available information: valve product catalog, valve product sample, etc.

2 Selection of valve basis

In understanding the selection of valve steps, but also should further understand the basis of valve selection.

（1）The use of selected valves, operating conditions and control mode.

（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 flammable, explosive medium, viscosity of the medium and so on.

（3）Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, seal grade, etc.

（4）Mounting and overall dimensions requirements: nominal diameter, connection to pipe and connection size, overall dimensions or weight limits, etc.

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

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

According to the above selection of valve 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 choose the preferred valve to make the right choice.

The ultimate control of the pipe is the valve. The valve opening and closing parts control the flow stream mode of the medium in the pipe. The shape of the valve channel enables the valve to have certain flow characteristics, which must be taken into account when selecting the most suitable valve for installation in the pipe system.

Selection of valves should follow the principle:

1、Valves for closing and opening media

The flow channel is a straight through valve, its flow resistance is small, usually selected as a cut-off and open medium valve. Downward closed valve (globe valve, plunger valve) because of its tortuous flow path, flow resistance is higher than other valves, so less choice. Closed valves can be used where high flow resistance is allowed.

2、A valve for controlling flow

Usually choose easy to adjust the flow of the valve as the flow control. Downward closing valves, such as globe valves, are suitable for this purpose because their seat size is proportional to the stroke of the closing element. Rotary valves (plug, butterfly, ball) and flex-body valves (pinch, diaphragm) can also be used for throttling control, but usually only in a limited range of valve calibers. Gate valve is a disc-shaped gate to the circular valve seat mouth to do crosscutting motion, it only close to the closed position, can better control the flow, so it is usually not used for flow control.

3、Valve for reversing and diverting

The valve may have three or more channels, depending on the need for reversing shunt. Plug and ball valves are more suitable for this purpose, so most valves used for reversing shunt use one of these valves. In some cases, however, other types of valves may be used for reversing diverting, provided that two or more valves are properly connected to each other.

4、Valves for medium with suspended particles

When the medium with suspended particles, it is best to use the closing parts along the sealing surface of the sliding valve with wiping action. If the back and forth movement of the closing element against the seat is vertical, then it may clamp particles, so this valve unless the sealing material can allow embedding particles, otherwise only suitable for basic clean media. Ball valve and plug valve in the process of opening and closing have a wiping effect on the sealing surface, so it is suitable for use in the medium with suspended particles.

Valve selection instructions

1 Gate valve selection

In general, gate valves should be preferred. In addition to steam, oil and other media, the gate valve is also suitable for containing granular solids and high viscosity media, and suitable for vent and low vacuum system valves. For media with solid particles, the gate valve body shall be provided with one or two purge holes. For low temperature medium, low temperature special gate valve should be selected.

2 Cut-off valve selection instructions

The globe valve is suitable for the fluid resistance requirements are not strict on the pipeline, that is, the pressure loss is not considered, and high temperature, high pressure medium pipeline or device, suitable for DN < 200mm steam and other medium pipeline; Small valve can choose globe valve, such as needle valve, instrument valve, sampling valve, pressure gauge valve, etc.; The globe valve has flow regulation or pressure regulation, but the adjustment accuracy is not high, and the diameter of the pipeline is relatively small, should choose the globe valve or throttle valve; For highly toxic media, the bellows sealed stop valve should be selected; But the globe valve should not be used for medium with large viscosity and medium containing particles easy to precipitate, and should not be used as a vent valve and low vacuum system valve.

3 Ball valve selection instructions

Ball valve is suitable for low temperature, high pressure, viscosity medium. Most ball valves can be used with suspended solid particles in the medium, according to the material requirements of the seal can also be used for powder and granular media; All-channel ball valve is not suitable for flow regulation, but it is suitable for quick opening and closing occasions, which is convenient to realize emergency cutting; Usually in strict sealing performance, wear, shrinking channel, fast opening and closing action, high pressure cut-off (large pressure difference), low noise, gasification phenomenon, small operating torque, small fluid resistance pipeline, recommended to use ball valves; Ball valve suitable for light structure, low pressure cut-off, corrosive media; Ball valve or low temperature, cryogenic medium of the most ideal valve, low temperature medium piping system and device, should be selected with the low temperature ball valve cover; When choosing floating ball valve, the valve seat material should undertake the load of the ball and the working medium. Large diameter ball valve needs greater force in operation. Ball valve DN≥200mm should choose worm gear transmission form; Fixed ball valve is suitable for large diameter and high pressure occasions; In addition, for the process of highly toxic materials, flammable medium pipeline ball valve, should have 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 adjust the flow and pressure parts, not suitable for viscosity and containing solid particles of the medium, not as a partition valve.

5 Specification for type selection of plug valve

The cock valve is suitable for quick opening and closing occasions, generally not suitable for steam and high temperature medium, for low temperature, high viscosity medium, also suitable for medium with suspended particles.

6 Butterfly valve selection instructions

Butterfly valve is suitable for large diameter (such as DN>600mm) and short structural length requirements, as well as the need for flow regulation and rapid opening and closing requirements of the occasion, generally used for temperature ≤80℃, pressure ≤1.0MPa water, oil and compressed air and other media; Because the butterfly valve relative to the gate valve, ball valve pressure loss is larger, so the butterfly valve is suitable for the pressure loss requirements of the pipeline system.

7 Check valve selection instructions

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

When DN≤40mm, it is advisable to use the lifting check valve (only allowed to be installed on the horizontal pipe);

When DN = 50 ~ 400mm, it is appropriate to use swing type lifting check valve (can be installed on the horizontal and vertical pipelines, such as installed on the vertical pipeline, medium flow from the bottom to the top);

When DN≥450mm, the buffer type check valve should be used;

When DN = 100 ~ 400mm can also choose sandwich check valve;

Swing check valve can be made of very high working pressure, PN can reach 42MPa, according to the material of the shell and seal can be applied to any working medium and any working temperature range. Medium for water, steam, gas, corrosive media, oil, medicine, etc. Medium operating 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 matter, not suitable for organic solvents and strong oxidant medium; Weir diaphragm valve should be selected for abrasive granular medium. The weir diaphragm valve should refer to its flow characteristics table; Viscous fluid, cement slurry and precipitating medium should choose straight-through diaphragm valve; Diaphragm valves should not be used on vacuum lines and vacuum equipment unless specified.

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 choose the right valve is crucial.