In fluid pipeline systems, valves play a crucial role. They are not only control components, but also key equipment for isolating equipment and pipeline systems, regulating flow, preventing backflow, and regulating and discharging pressure. Valves have a wide range of applications and can control the flow of various types of fluids such as air, water, steam, etc. At the same time, their selection is particularly important. Therefore, a deep understanding of the classification, characteristics, and selection criteria of valves is crucial for ensuring the smooth operation of pipeline systems.

Valves can be roughly divided into two categories: automatic valves and actuated valves. Automatic valves rely on the ability of the medium itself to operate, such as check valves, safety valves, etc; The driving valves are operated manually, electrically, hydraulically, or pneumatically, such as gate valves, ball valves, etc. In addition, valves can be classified in more detail based on their structural characteristics, applications, and driving methods.

After understanding these classification information, we can have a clearer understanding of the important role of valves in fluid pipeline systems and how to choose suitable valves based on actual needs. At the same time, this also provides us with a starting point for in-depth learning of valve knowledge, helping us grasp more key information about valves.

Vacuum valves, with an absolute pressure less than 1Mpa, i.e. less than 760mm mercury column, are often expressed as pressure in mm mercury column or mm water column. Low pressure valves, with a nominal pressure PN not exceeding 6Mpa, including steel valves. The medium pressure valve has a nominal pressure PN between 5 and 4 MPa. High pressure valves have a nominal pressure PN range of 0 to 0Mpa. And the nominal pressure PN of the ultra-high pressure valve reaches or exceeds 100Mpa.

In addition, valves can also be classified according to the temperature of the medium. For example, ordinary valves are suitable for fluids with medium temperatures between -40 ℃ and 425 ℃; High temperature valves are suitable for fluids between 425 ℃ and 600 ℃, while heat-resistant valves can cope with high temperatures above 600 ℃. Similarly, low-temperature valves and ultra-low temperature valves are suitable for fluids with medium temperatures ranging from -150 ℃ to -40 ℃ and below -150 ℃, respectively.

In terms of size, valves can be divided into small caliber, medium caliber, large caliber, and extra large caliber, which mainly depends on the size of their nominal diameter DN. For example, small-diameter valves typically have a DN of less than 40mm, while large-diameter valves have a DN of 1400mm or more.

In addition, the connection method between valves and pipelines is also an important basis for classification. For example, a flange connected valve is connected to a pipeline through a flange on the valve body; Threaded valves are connected to pipelines through internal or external threads. At the same time, there are various methods such as welding connection, clamp connection, and sleeve connection.

Finally, the characteristics of valves include usage characteristics and structural characteristics. The usage characteristics mainly involve the type of valve, product type, material of main components, and transmission mode, which determine the main performance and scope of use of the valve. The structural characteristics mainly focus on the internal structure and design features of the valve.

It determines some structural characteristics of the installation, maintenance, and upkeep methods of valves, including the structural length and overall height of the valve, the connection method with the pipeline (such as flange connection, threaded connection, etc.), the form of the sealing surface (such as insert ring, threaded ring, etc.), and the structural form of the valve stem (such as rotating rod, lifting rod, etc.).

When selecting valves, certain steps and criteria need to be followed. Firstly, it is necessary to clarify the purpose of the valve in the equipment or device, and determine its working conditions, including applicable media, working pressure, and working temperature. Secondly, it is necessary to determine the nominal diameter and connection method of the pipeline connected to the valve. In addition, it is necessary to determine the method of operating the valve, such as manual, electric, etc. At the same time, select appropriate shell and interior materials based on the medium transported by the pipeline, working pressure, and working temperature. Next, determine the type and type of valve, as well as the parameters that need to be considered, such as allowable flow resistance, discharge capacity, etc. In addition, it is necessary to determine the geometric parameters of the valve, such as structural length, flange connection form, and dimensions. Finally, existing materials such as valve product catalogs and samples can be utilized to select appropriate valve products.

In terms of selecting valves, it is necessary to comprehensively consider the purpose, operating conditions, and control methods of the valve, as well as the properties of the working medium, fluid characteristics requirements, installation dimensions, and external dimensions requirements. These steps and guidelines will help you make informed valve choices.

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

When determining valve parameters, special attention should be paid to: if the valve is to be used for control purposes, the operating method, maximum and minimum flow requirements, pressure drop during normal flow, pressure drop during closure, and additional parameters such as the maximum and minimum inlet pressure of the valve must be clearly defined.

Following the above criteria and steps for selecting valves, in order to make a wise decision, we must have a detailed understanding of the internal structure of various types of valves. After all, valves are the ultimate control components of pipeline systems, and their opening and closing components determine the flow pattern of the medium inside the pipeline, while the shape of the flow channel endows valves with specific flow characteristics.

This is crucial when selecting the most suitable valve for installation. At the same time, we should also follow some principles to guide our choices:

Valves used for cutting off and opening media

Valves with straight through flow channels are usually the preferred choice for shut-off and open media due to their low flow resistance. In contrast, downward closing valves (such as globe valves and plunger valves) are less commonly used due to their tortuous flow paths and higher flow resistance. But in situations where higher flow resistance is allowed, this type of valve still has some applicability.

Valves used to control flow

Usually, we tend to choose valves that are easy to adjust the flow rate to control it. Downward closing valves, especially globe valves, are very suitable for this purpose because there is a proportional relationship between the seat size and the stroke of the closing element. In addition, rotary valves and flex valve body valves can also be used for throttling control, but are usually only applicable within a limited range of valve diameters. It should be noted that gate valves, due to their special opening method, are usually not used for flow control.

Valve for directional diversion

This type of valve requires three or more channels to meet the demand for directional diversion. Plug valves and ball valves are highly suitable for this purpose due to their unique structure. However, in some cases, the function of directional diversion can also be achieved by appropriately connecting two or more other types of valves.

Valves suitable for media with suspended particles

When there are suspended particles in the medium, priority should be given to valves that have a wiping effect when the closure slides along the sealing surface. If the movement of the closing element towards the valve seat is vertical, it may clamp particles, so this type of valve is only suitable for media with basic cleanliness or sealing surface materials that allow particles to be embedded. Ball valves and plug valves will wipe the sealing surface during the opening and closing process, making them very suitable for media with suspended particles.

Next, we will further explore the selection instructions for various valves:

Selection of Gate Valves

Gate valves are a good choice, especially suitable for media such as steam and oil, as well as media containing granular solids and high viscosity. For media with solid particles, gate valve bodies are usually equipped with one or two blow holes. If the medium is in a low-temperature state, a specialized low-temperature gate valve should be selected.

Selection of globe valve

Globe valves are suitable for pipelines with no special requirements for fluid resistance or minimal pressure loss, as well as pipelines or devices for high-temperature and high-pressure media. It is also commonly used on steam and other medium pipelines with DN<200mm. Small valves such as needle valves, instrument valves, etc. can also use globe valves. It should be noted that although globe valves can be used for flow or pressure regulation, if the requirements for regulation accuracy are not high and the pipeline diameter is small, globe valves or throttle valves should be selected. For highly toxic media, it is recommended to use a bellows sealed globe valve. However, globe valves are not suitable for media with high viscosity and media containing particles that are prone to precipitation, nor are they suitable as vent valves or valves for low vacuum systems.

Selection of ball valves

Ball valves perform well in low temperature, high pressure, and high viscosity media. They are widely used in media with suspended solid particles and can be used for powder and granular media according to the requirements of sealing materials. Although full channel ball valves are not suitable for flow regulation, they are suitable for situations that require quick opening and closing, making them easy to cut off in emergency situations. In pipelines with strict sealing performance, severe wear, constricted channels, rapid opening and closing, high pressure cutoff (large pressure difference), low noise, gasification phenomenon, low operating torque, and low fluid resistance, ball valves are the ideal choice. In addition, ball valves are also suitable for lightweight structures, low-pressure shut-off, and corrosive media. For low-temperature and cryogenic media, ball valves are the preferred choice, especially in pipeline systems and devices for low-temperature media, low-temperature ball valves with valve covers should be selected. When selecting a floating ball valve, the valve seat material should be able to withstand the load of the ball and working medium. Large caliber ball valves require greater force during operation, and it is recommended to use worm gear transmission for ball valves with DN ≥ 200mm. Fixed ball valves are suitable for larger diameters and higher pressures. In addition, ball valves on pipelines for highly toxic materials and flammable media in the process should have fire-resistant and anti-static structures.

Throttle valves are suitable for situations where the medium temperature is low and the pressure is high. They are mainly used to regulate flow and pressure, but are not suitable for media with high viscosity and solid particles, and should not be used as block valves.

Plug valves are suitable for situations that require quick opening and closing, and are usually not suitable for steam and high-temperature media. For media with low temperature, high viscosity, or media with suspended particles, a plug valve is a good choice.

Butterfly valves are suitable for situations with large diameters (such as DN>600mm) and short structural length requirements, especially for flow regulation and rapid opening and closing. It is generally used for media such as water, oil, and compressed air with temperatures ≤ 80 ℃ and pressures ≤ 0MPa. Due to the relatively large pressure loss of butterfly valves, they are more suitable for pipeline systems with less stringent pressure loss requirements.

Finally, check valves are mainly suitable for clean media and should not be used for media containing solid particles and high viscosity. When selecting, the specific type of check valve should also be determined based on the size of the pipe diameter.

When DN ≥ 450mm, it is recommended to use a buffer type check valve; If the DN is within the range of 100 to 400mm, the same applies to clamp check valves. The rotary check valve can withstand a working pressure of up to 42MPa, and its shell and seal materials are flexible, suitable for various working media and temperature ranges, from water and steam to corrosive media, oil and drugs, etc. The working temperature range of the medium can reach -196 to 800 ℃.

The following are the key points for selecting diaphragm valves. Diaphragm valves are suitable for oil products, water, acidic media, and media containing suspended solids with working temperatures below 200 ℃ and pressures below 0MPa, but are not suitable for organic solvents and strong oxidizing agents. For grinding granular media, it is recommended to use weir type diaphragm valves and refer to their flow characteristics table. If the medium has viscosity, cement slurry or sedimentation characteristics, a straight through diaphragm valve should be selected. Unless there are specific requirements, diaphragm valves are generally not recommended for use in vacuum pipelines and vacuum equipment.

Given the diversity and complexity of valve applications, choosing the right valve is crucial as it directly affects the safety and performance of the system. Therefore, learning to choose the appropriate valve based on actual needs is an essential skill for every engineer.