Basic knowledge and application of valves

In the fluid piping system, the valve is the control element, its main role is to isolate the equipment and piping system, regulate the flow rate, prevent backflow, 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, mud, oil, liquid metal and radioactive media. Because it is very important to choose the most suitable valve for the pipeline system, it is also very important to understand the characteristics of the valve and the steps and basis for selecting the valve.

First, the classification of valves

Valves can be divided into two categories:
1, automatic valve
A valve that operates by itself depending on the ability of the medium (liquid, gas) itself.
Such as check valves, safety valves, regulating valves, steam traps, pressure reducing valves, etc.
2. Drive the valve
Valves that operate manually, electrically, hydraulically, and pneumatically.
Such as gate valve, globe valve, throttle valve, butterfly valve, ball valve, plug valve and so on.

According to the structural characteristics, according to the direction of movement of the closure relative to the seat can be divided into:
(1) Closure type: the closure moves along the center of the valve seat;
(2) Gate type: the closing part moves along the center of the vertical seat;
(3) Cock and ball: the closing part is a plunger or ball, rotating around its own center line;
(4) Swing shape: the closing part rotates around the axis outside the seat;
(5) Disc: the disc of the closing part rotates around the shaft in the seat;
(6) Slide valve type: the closing part slides in the direction perpendicular to the channel.
According to the use, according to the different uses of the valve can be divided into:
(1) Breaking: used to connect or cut off pipeline media, such as globe valves, gate valves, ball valves, butterfly valves, etc.
(2) Check: used to prevent the backflow of the medium, 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, distribution 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 media to ensure the safety of pipeline systems and equipment, such as safety valves and accident valves.
(6) Other special uses: such as steam trap, blowdown valve, blowdown valve, etc.
According to the driving mode, according to different driving modes can be divided into:
(1) Manual: With the help of hand wheels, handles, levers or sprockets, etc., there are human drivers, and the transmission of large torque fashion has worm wheels, gears and other decelerating devices.
(2) Electric: driven by a motor or other electrical device.
(3) Hydraulic: driven by (water, oil).
(4) Pneumatic: driven by compressed air.
According to pressure, according to the nominal pressure of the valve can be divided into:
(1) Vacuum valve: absolute pressure <0.1Mpa, that is, 760mm Hg high valve, usually with mm Hg or mm water column to indicate the pressure.
(2) Low pressure valve: nominal pressure PN≤1.6Mpa valve (including PN≤1.6MPa steel valve)
(3) Medium pressure valve: nominal pressure PN2.5-6.4mpa valve.
(4) High pressure valve: nominal pressure PN10.0-80.0mpa valve.
(5) Ultra-high pressure valve: nominal pressure PN≥100.0MPa valve.
According to the temperature of the medium, according to the medium temperature when the valve is working, it can be divided into:
(1) Ordinary valve: suitable for medium temperature -40℃ ~ 425℃ valve.
(2) High temperature valve: suitable for medium temperature 425℃ ~ 600℃ valve.
(3) Heat-resistant valve: suitable for medium temperature above 600℃ valve.
(4) Low temperature valve: suitable for medium temperature -150℃ ~ -40℃ valve.
(5) ultra-low temperature valve: suitable for medium temperature below -150℃ valve.
According to the nominal diameter, according to the nominal diameter of the valve can be divided into:
(1) Small diameter valve: nominal diameter DN<40mm valve.
(2) Medium diameter valve: nominal diameter DN50 ~ 300mm valve.
(3) Large diameter valve: nominal diameter DN350 ~ 1200mm valve.
(4) Large diameter valve: nominal diameter DN≥1400mm valve.
According to the connection with the pipeline, according to the connection between the valve and the pipeline can be divided into:
(1) Flange connection valve: The valve body has a flange, and the pipe is connected with a flange valve.
(2) Threaded valve: valve body with internal thread or external thread, threaded connection with the pipe valve.
(3) Welded connection valve: the valve body is equipped with a welding port and the valve is welded with the pipeline.
(4) Clamp connection valve: the valve body is equipped with a clamp, and the pipe is connected with a clamp valve.
(5) Sleeve connected valve: the valve connected with the pipe by the sleeve.

Second, the characteristics of the valve

The characteristics of the valve are generally two kinds, the use characteristics and the structural characteristics.
1. Use characteristics
It determines the main performance and scope of use of the valve, which belongs to the characteristics of valve use:
•Category 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, valve stem, valve disc, sealing surface) materials;
•Valve transmission mode, etc.

2. Structural characteristics
It determines some structural characteristics of the 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 with the pipeline (flange connection, thread connection, clamp connection, external thread connection, welded end connection, etc.);
•The form of sealing surface (insert ring, thread ring, surfacing welding, spray welding, valve body);
•Valve stem structure form (rotating rod, lifting rod), etc.

Third, the selection of valve steps and basis

1. Select the step
(1) Clearly define the use of the valve in the equipment or device, and determine the working conditions of the valve: applicable media, working pressure, working temperature, and so on.
(2) Determine the nominal diameter and connection mode of the pipe connected with the valve: flange, thread, welding, etc.
(3) Determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage.
(4) According to the pipeline transmission medium, working pressure, working temperature to determine the selected valve shell and internal materials: gray cast iron, malleable cast 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 parameters of the valve: For automatic valves, 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 seat hole.
(8) Determine the geometric parameters of the selected valve: structural length, flange connection form and size, the size of the valve height direction after opening and closing, the size and number of the connected bolt holes, and the overall valve appearance size.
(9) Use the existing information: valve product catalog, valve product samples, etc., to select the appropriate valve products.

2, the basis for choosing the valve
While understanding and mastering the steps of selecting valves, we should also further understand the basis for selecting valves.
(1) The use, working conditions and 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 flammable, explosive medium, the viscosity of the medium, and so on.
(3) Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing grade and so on.
(4) Installation size and overall size requirements: nominal diameter, connection with the pipe and connection size, overall size or weight limits.
(5) Additional requirements for the reliability and service life of valve products and the 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: the method of operation, the maximum and minimum flow requirements, the pressure drop for normal flow, the pressure drop at closing, and the 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 make the right choice of the preferred valve.
The ultimate control of the pipe is the valve. The valve opening and closing parts control the flow pattern of the medium in the pipeline, and the shape of the valve channel enables the valve to have a certain flow characteristics, which must be taken into account when selecting the most suitable valve for installation in the pipeline system.

The selection of valves should follow the principles:
(1) Valves for closing and opening media
The flow channel is a straight-through valve, whose flow resistance is small, and the valve is usually selected as a cut-off and opening medium. Downward closed valves (globe valves, plunger valves) are less used because of their tortuous flow channels and higher flow resistance than other valves. In cases where high flow resistance is allowed, closed valves can be selected.
(2) Valves for controlling flow
Usually choose a valve that is easy to adjust the flow rate as a control flow. Downward closing valves, such as globe valves, are suitable for this purpose because their seat size is proportional to the stroke of the closing member. Rotary valves (plug valves, butterfly valves, ball valves) and flex-body valves (clamp valves, diaphragm valves) 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 seat mouth to do cross-cutting movement, it can only be close to the closed position, in order to better control the flow, so it is usually not used for flow control.
(3) Valves for reversing shunt
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. However, in some cases, other types of valves, as long as two or more valves are properly connected to each other, can also be used for reversing shunt.
(4) Valves for medium with suspended particles
When there are suspended particles in the medium, it is best to use the sliding valve with wiping action along the sealing surface. If the closure is vertical in its back and forth motion against the seat, then the particles may be caught, so the valve is only suitable for a basically clean medium unless the sealing surface material allows the embedding of particles. Ball valves and plug valves have a wiping effect on the sealing surface during opening and closing, so they are suitable for use in media with suspended particles.

Four, valve selection instructions

1. Selection of gate valve
In general, gate valves should be preferred. In addition to steam, oil and other media, the gate valve is also suitable for media containing granular solids and large viscosity, and is suitable for valves in cavitation and low vacuum systems. For media with solid particles, the gate valve body should be provided with one or two purge holes. For low temperature medium, special low temperature gate valve should be selected.

2, globe valve selection instructions
The stop valve is suitable for pipelines with low fluid resistance requirements, that is, little consideration of pressure loss, and pipelines or devices with high temperature and high pressure media, suitable for steam and other medium pipelines with DN < 200mm; Small valves can be selected globe valves, such as needle valves, instrument valves, sampling valves, pressure gauge valves, etc. The globe valve has flow regulation or pressure regulation, but the adjustment accuracy is not high, and the pipe diameter is relatively small, should choose the globe valve or throttle valve; For highly toxic media, it is appropriate to choose a bellows sealed stop valve; However, the globe valve should not be used for media with large viscosity and media containing particles easy to precipitate, and it should not be used as a valve for blowing off valves and low vacuum systems.

3, ball valve selection instructions
Ball valve is suitable for low temperature, high pressure, 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 material requirements of sealing; The full-channel ball valve is not suitable for flow regulation, but it is suitable for occasions requiring quick opening and closing, which is easy to achieve emergency cutting; Ball valves are usually recommended in pipelines with strict sealing performance, wear, shrinking channels, rapid opening and closing action, high pressure cutoff (large pressure difference), low noise, gasification, small operating torque, and small fluid resistance. Ball valve is 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 a low temperature ball valve cover; When the floating ball valve is selected, the valve seat material should undertake the load of the ball and the working medium. The large-diameter ball valve needs a larger force during operation. The ball valve with DN≥200mm should use worm gear transmission form. Fixed ball valve is suitable for large diameter and high pressure occasions; In addition, the ball valve used for the process of highly toxic materials and combustible media pipelines should have a fireproof and anti-static structure.

4, throttle valve selection instructions
The throttle valve is suitable for occasions where the medium temperature is low and the pressure is high, suitable for parts where the flow and pressure need to be adjusted, not suitable for media with large viscosity and containing solid particles, and should not be used as a block valve.

5, plug valve selection instructions
The plug valve is suitable for occasions requiring rapid opening and closing, generally not suitable for steam and high temperature media, for low temperature, high viscosity media, and also suitable for media 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 fast opening and closing requirements, generally used for temperature ≤80℃, pressure ≤1.0MPa water, oil and compressed air and other media; Because the pressure loss of butterfly valve is relatively large compared with gate valve and ball valve, butterfly valve is suitable for pipeline systems with low pressure loss requirements.

7, check valve selection instructions
Check valves are generally suitable for clean media, and should not be used for media containing solid particles and large viscosity.
•When DN≤40mm, it is recommended to use the lift check valve (only allowed to be installed on the horizontal pipe).
•When DN = 50 ~ 400mm, it is appropriate to use a swing type lift check valve (can be installed on both horizontal and vertical pipelines, such as installed on vertical pipelines, the medium flow direction should be from bottom to top);
•When DN≥450mm, the buffer check valve should be used.
•When DN = 100 ~ 400mm, you can also choose a sandwich check valve;
Swing check valve can be made of 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. The medium is water, steam, gas, corrosive media, oil, medicine, etc. The operating temperature of the medium is between -196 and 800℃.

8, diaphragm valve selection instructions
The diaphragm valve is suitable for oil, water, acidic media and media containing suspended matter with operating temperature less than 200℃ and pressure less than 1.0MPa, and is not suitable for organic solvents and strong oxidant media; Weir diaphragm valve should be selected for abrasive granular medium, and the weir diaphragm valve should refer to its flow characteristic table; Viscous fluid, cement slurry and precipitating medium should choose straight-through diaphragm valve; Except for specific requirements, diaphragm valves should not be used on vacuum tubes and vacuum equipment.
Valve applications, operating frequencies and services are ever-changing, and the most important and critical equipment to control or eliminate even small leaks is the valve. Learning to choose the right valve is crucial.