Valve selection is the top priority of chemical design, so the chemical design needs to be based on the valve type, metal material, use, characteristics, etc., for the correct selection of valves, the only way to ensure the safety and reliability of chemical production and improve the economic efficiency of chemical enterprises. Based on this, the basic principles of valve selection in chemical design are explained. The selection strategies of several valves, such as gate valves, ball valves, check valves, globe valves, throttle valves, etc., are introduced, and the considerations of valve selection are summarized.
In the fluid piping system, the valve belongs to an indispensable and important control element, accounting for up to 30%-50% of the pipeline cost. A reasonable selection of valves can be completed to open and close, adjust the flow and throttling, as well as to prevent the backflow of media, adjust the pipeline pressure, and perform other operations. A valve has a high degree of complexity of the components through the different parts of the common transfer composition, and the technical content is significantly higher. With the rapid development of the chemical industry, chemical production equipment stored in the transport of media has toxic, corrosive and flammable, and explosive hazards; operating conditions are relatively complex and harsh, and the temperature, pressure, and other relatively high valve failure problems, may cause media leakage, serious can cause major dangerous accidents, the environment, the economy, and another serious impact. Therefore, in the chemical design, it is necessary to make a good selection of commonly used valves, saving costs and, at the same time, fully protecting the chemical safety production.
1. The importance of valve selection in chemical design
Chemical design, commonly used valve selection, is one of the most important to provide a reliable guarantee of chemical safety production, is also conducive to cost control, and fully ensures economic benefits. Chemical pipeline, the valve function is powerful. It can complete the opening and closing, adjust the flow and throttling, prevent the backflow of media, and adjust the pipeline pressure and other operations. At the same time, in the operation of the chemical system, but also through the valve to complete effective control. Therefore, the chemical design period, based on the principles and points of selection, the specific situation to be considered comprehensively to complete a reasonable selection of commonly used valves to ensure the safety of chemical products while effectively saving costs. In the chemical design process, the variety and specifications of commonly used valves are relatively large, such as globe valves, butterfly valves, and gate valves. To fully ensure the quality of chemical design, the need to have a full understanding of common valve types’ uses and characteristics, etc., a reasonable choice of the corresponding valve types to ensure the quality of chemical systems, to promote the design of the service life can be truly improved.
2. The basic principles of valve selection commonly used in chemical design
2.1 Clarify the function of the valve, positioning the valve purpose
Chemical design, commonly used during the selection of valves, the need for its specific function to be fully defined, and accurate positioning of the valve purpose. On this basis, clear the key role played by the valve, fully understand the valve corresponding to the control mode and working pressure and other basic parameters, and master the role of valve function, taking into account the specific circumstances, so that the common valve to complete a reasonable selection.
2.2 Familiar with the characteristics of the valve
Valve varieties are different, valve types also have obvious differences, and their functional role differs. The different types of valves, regardless of the characteristics, production, or application, also show a corresponding difference between the differences. Therefore, during the chemical design, designers need to focus on the characteristics of the valve to consider and fully understand the characteristics and types of valves to ensure that the valves selected meet the site’s requirements to provide safety for chemical production.
2.3 Determine the valve end connection
Valve end connection involves threaded, flange, and welded connection forms, of which threaded connection and flange connection forms, are used more. Through the threaded connection, the nominal valve diameter has certain standard requirements, DN ≤ 50 mm; if the diameter significantly exceeds the standard, the connection parts' installation and sealing are bound to increase the difficulty significantly. In the flange connection, the installation and disassembly operation is relatively easy, and the scope of application is relatively wide, with different diameters and pressures of the pipeline connection can be used. Welded connections have more stringent standard requirements for the conditions; the connection is more reliable. But in the welded connection, the valve disassembly and reinstallation are more difficult, which also has constraints on its scope of application; in reliable long-term operation or under strict conditions, and higher temperatures, the welded connection form can be applied.
During the valve selection period, in the selection of materials, the working medium needs to be focused on consideration, especially the physical and chemical properties. On this basis, it is also necessary to fully grasp the degree of cleanliness of the medium. And the relevant specifications to be a comprehensive reference. The reasonable choice of valve material can fully guarantee the valve's quality, performance, and service life. The valve body selection, cast iron priority, carbon steel, and stainless steel in order of reasonable choice, the seal material selection, rubber priority, copper, alloy steel, and F4 in order of reasonable choice.
3. Chemical design of common valve type selection strategy
3.1 Gate valve selection
In a chemical production plant, the gate valve belongs to the more common type of valve and will not make the liquid flow through the direction of change. The gate valve is fully open, the resistance coefficient is relatively small, and the caliber, pressure, and temperature range used is relatively wide. The same diameter, compared with the globe valve, can save installation size space, so it is more widely used in the selection. But the gate valve also has some disadvantages, such as a large height and the time required to open and close longer; in the process of opening and closing, the sealing surface will have the possibility of erosion; the media cleanliness has certain standard requirements; in the production of non-metallic corrosion-resistant materials for manufacturing difficulties. In the gate valve open state, located in the back position of the gate, the media can form a vortex phenomenon, resulting in the gate being affected by erosion and vibration; the valve seat sealing surface is more likely to occur corresponding damage. Therefore, the need to regulate the flow is not suitable for selecting gate valves; in the fully open, circle-closed state, the fluid opening and closing for effective control are suitable for selecting gate valves.
According to the thread position of the stem to be divided, the gate valve has two forms, the bright and dark stem. In the corrosive media environment, priority is given to the open stem gate valve. In contrast, the concealed stem gate valve can be used in the face of a low-pressure and non-corrosive media environment. Divided by gate structure, gate valves can be divided into wedge-type and parallel-type gate valves. The wedge-type gate valve can be divided into single and double gates. For the parallel type gate valve, the focus of use in the oil and gas transmission system, chemical installations are used relatively less.
About the gate valve application, the medium for steam, oil and gas or high-temperature oil, or frequent switch position, you can reasonably choose the gate valve. Still, in the case of easy coking, the medium is less applicable. Easy to coke high-temperature media conditions, can reasonably choose wedge type single inter-plate gate valve, steam, oil, and other media can reasonably choose wedge type double gate valve.
3.2 Ball valve selection
Low temperature, high temperature, viscosity, and other media can reasonably choose the ball valve. With the presence of suspended solid particles in the media, most ball valves show good applicability to the seal material as the standard, the same can be used in granular, powdery, and other media, and the ball valve can be reasonably selected. Full-channel ball valves are used less in flow regulation, more in the case of rapid opening and closing, and can quickly complete the accident emergency cut-off operation. Chemical pipelines have strict standards, such as sealing, wear, and tear. High-pressure shut-off, opening, and closing action is a rapid and indented channel that can be reasonably applied to the ball valve. Regarding the media, such as corrosive, light structure and low-pressure shut-off, etc., the same applies to using ball valves. In low-temperature, deep cold media, ball valves can also play an important role. In piping systems and installations for cryogenic media, cryogenic ball valves are required with corresponding valve covers. The use of floating ball valves has corresponding standard requirements for the seat material, which must effectively carry the load generated by the ball and the working medium. When using large-diameter ball valves, the turbine drive form is required in the case of DN ≥ 200mm due to the larger force required to operate. In the caliber, pressure standards are larger, and a fixed ball valve is needed. In addition, for the combustible, toxic media pipeline, the ball valve must set the corresponding fire and anti-static structure to ensure safety fully.
3.3 Check valve selection
Clean media conditions, choose to use more check valves. DN ≤ 40mm, suitable for lifting the check valve, must be located in the horizontal pipeline to complete the practical design and installation. DN between 50-400mm, suitable for the use of swing lift check valve, horizontal, vertical pipeline can be reasonably designed and installed, if located in the vertical pipeline, need to ensure that the media flow direction from bottom to top. DN ≥ 450mm case in the case of DN≥450mm, it is suitable to choose a buffer check valve, and in the case of DN between 100-400mm, it is also possible to choose a clip check valve. The use of a swing check valve can withstand the larger working pressure, PN standard up to 42MPa, based on the difference between the material of each part, and can be in a variety of working media and temperature ranges, such as steam, corrosive media and oil and other working media, the temperature between -196-800 ℃, can be considered the use of check valve.
3.4 Stop valve selection
During the chemical design, there are no higher standard requirements for fluid resistance; you can consider the use of shut-off valves. In the face of high-temperature and high-pressure media, the shut-off valve also shows a relatively wide range of use. DN ≤ 200mm, and in the case of a vaporized pipeline, you can choose the shut-off valve. You can also choose the globe valve for specific practice links, small valves such as sampling valves and needle valves, instrumentation valves, or valves for low vacuum systems and exhaust valves. Regarding the globe valve, its functional role is to regulate the pressure and flow control, so the adjustment accuracy is not strictly standard requirements; the pipeline diameter is not large, so you can prioritize the globe valve. In the presence of toxic media, to fully ensure the sealing performance, the bellows globe valve must be used. Media cleanliness is poor, with large particles of precipitation or medium viscosity situation, and is unsuitable for the globe valve. During the chemical design, the sealing performance standard requirements are more stringent to give priority to the use of shut-off valve design; due to the existence of seals, it can fully ensure good sealing performance, gasket change operation is relatively simple, only the installation of the shut-off valve to focus on the valve direction with the media flow direction to maintain the same.
3.5 Throttle valve selection
Based on the structural form level analysis, the throttle valve and the shut-off valve have obvious similarities, while the difference is that the throttle valve does not set the corresponding throttling element. Combined with the overall situation of the valve market, the throttle valve is relatively small but shows obvious advantages, such as safety regulation and energy saving. However, the throttle valve also has certain shortcomings, such as regulation control accuracy needs to be improved. Therefore, throttle valves can be designed for low-temperature and high-pressure media situations during chemical design. In regulating flow, pressure, and other needs, the throttle valve can be used. In the case of poor media cleanliness or high viscosity, it is not suitable to choose a throttle valve.
3.6 Butterfly valve selection
Butterfly valve fluid resistance is relatively small, rapid opening and closing, and weight, structure, and size also show obvious advantages and can effectively save material costs. In the cut-off throttling design, the choice of butterfly valve is more suitable for the demand. Still, in the large flow rate regulation, due to the impact of sealing materials, its use unit range also has certain limitations. In liquid media containing suspended solids and large-diameter pipeline design, butterfly valves can also be selected. Due to the continuous optimization and improvement of materials, design, and process, its performance changes accordingly, and the butterfly valve becomes more widely applicable. With its economy and flow regulation performance, the butterfly valve can be given priority in the case of a suitable butterfly valve. It is also very beneficial to design butterfly valves for cases where the design pressure is not high, but the pipeline diameter is large and fast opening and closing. Butterfly valves can be divided into soft and hard seal types, and the specific application type needs to be selected based on the medium fluid temperature. It should be reminded that its soft seal performance is significantly better than the hard seal. Among the commonly used butterfly valves, the most common is the large orifice valve, mainly used in crude oil and oil, water, and other media, the temperature is within 300 ℃, and the pressure is within 1.0 MPa.
3.7 Plug valve selection
The plug valve is one of the first designed to use the valve; with good sealing performance, it can achieve two-way sealing, opening and closing more labor-saving, and long service life. The more hazardous material system can focus on the use, but the opening and closing torque is relatively large, resulting in a relatively high price and cost. The plug valve cavity will not produce liquid accumulation and will not pollute the intermittent device material. In the chemical pipeline design, plug valves can complete the cut-off and distribution operations and change the direction of media flow. In the multi-channel structure design, a plug valve is also more common; the valve can be connected to more than one flow channel, so the pipeline system design is simplified to avoid the number of valves and connecting accessories and other accessories excessive use. Plug valves are divided into two types, namely, non-lubricated and lubricated. Oil-sealed plug valves can form an oil film under forced lubrication, which has a higher sealing performance, requires relatively less effort when opening and closing, and reduces the damage to the sealing surface. But to focus on the problem of material contamination, the need for good regular maintenance. In general, non-lubricated use is relatively more. The plug valve is more suitable for the media temperature that is not high and high viscosity situation, and switches fast and another operating environment, while steam, higher temperature media usually do not choose plug valve.
3.8 Diaphragm valve selection
Diaphragm valve in the form of structure has obvious special characteristics, through the rubber membrane, plastic membrane, etc., to effectively control fluid movement. The fluid resistance is relatively small for the diaphragm valve and can achieve a two-way seal in low pressure, suspended viscous fluid, and corrosive slurry. Other media can focus on the use. The operating mechanism with the media channel to maintain a safe separation through the resilient diaphragm to complete the effective cut-off operation of the fluid, diaphragm valves in pharmaceutical and health and food and other industrial production system designs are used more. The diaphragm material’s temperature resistance performance determines the appropriate temperature environment of the diaphragm valve. The structure as a standard can be divided into straight-through, weir types. Diaphragm valves are suitable for suspension, oil and acidic media, etc., the temperature does not exceed 200 ℃, the pressure does not exceed 1.0MPa operating environment, and in strong oxidants, organic solvents, and other media environments, usually do not choose diaphragm valves.
4. Valve selection considerations
In chemical design, the reasonable selection of commonly used valves is one of the most important, directly related to chemical safety production and overall efficiency. Therefore, the valve selection needs to focus on the following factors:
- ① The nature of the transported fluid. The valve’s role is to control the fluid effectively; fluid types vary, such as steam, slurry, gas, liquid, etc., and part of the fluid cleanliness is poor, may contain solid suspended particles and dust, etc. Therefore, during the valve selection period, the need to fully analyze the nature of the fluid to ensure that the valve selection is in line with the specific standard requirements.
- ② Valve function. The valve selection period considers the valve’s function, considering the specific conditions of use, such as adjusting the pressure, flow or cut-off, the need for rapid opening and closing, one-way flow or two-way flow, etc. In short, the need for a full understanding of the valve characteristics uses, and functions to provide the basis for the reasonable selection of valves.
- ③ Valve size. Chemical design, the need to combine the processing pipeline's type and size to select the valve's standard size.
- ④ valve operating pressure, temperature. Different types of valves and conditions of use are significantly different; in the face of different pressure and temperature conditions, it is necessary to choose the correct valve type. Need to be based on the process conditions, the valve material, pressure level is clear to provide a reference for valve selection.
- ⑤ Valve pressure loss. Local resistance loss with the valve is directly linked to the existence of various types of valves due to differences in the structure; the size of resistance varies, so the valve selection process must focus on the standard range of resistance allowed.
- ⑥ Valve material. The design combines temperature, pressure level, and fluid characteristics to focus on the selection of valve material. The valve body, valve seat, and valve disc parts involve different materials, especially when conveying corrosive materials, which must be based on the nature of the material to make a reasonable choice to ensure the valve’s performance.
In chemical design, the reasonable selection of commonly used valves on chemical products' safety, quality, and economic efficiency has an important impact and constraints. Therefore, sufficient attention must be given to the chemical design, depending on the specific analysis, the type of commonly used valves, materials, uses and characteristics, as well as advantages and disadvantages to achieve a full grasp of the scientific and reasonable selection method, combined with comparative analysis, etc., to achieve a good selection of commonly used valves to ensure that the valve quality and use of working conditions can fully meet the strict standards of chemical production requirements, to provide chemical production Safety guarantee.
Author: Lin Meng