Valve selection

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I. Steps and criteria for valve selection before procurement: In fluid piping systems, valves serve as control elements; their main functions are to isolate equipment from piping systems, regulate flow rates, prevent backflow, and regulate as well as release pressure. Since it is very important to select the most suitable valves for pipeline systems, it is equally crucial to understand the characteristics of valves as well as the steps and criteria for choosing them. To date, the valve industry has been able to produce a wide range of valve products, including 12 major categories such as gate valves, globe valves, throttle valves, plug valves, ball valves, electric valves, diaphragm valves, check valves, safety valves, pressure relief valves, steam traps, and emergency shut-off valves; there are over 3,000 models and more than 4,000 different specifications available ； The maximum operating pressure is 600 MPa, the maximum nominal diameter reaches 5350 mm, the highest operating temperature is 1200°C, and the lowest operating temperature is -196°C. The applicable media include water, steam, oils, natural gas, highly corrosive media (such as concentrated nitric acid and medium-concentration sulfuric acid), flammable media (such as benzene and ethylene), toxic media (such as hydrogen sulfide), explosive media, and radioactive media (such as metallic sodium and pure water in closed circuits). The materials for valve pressure-bearing components include cast copper, cast iron, ductile iron, high-silicon cast iron, cast steel, forged steel, high and low alloy steels, stainless acid-resistant steel, Hastelloy, Inconel, Monel alloy, duplex stainless steel, titanium alloys, etc. It is also capable of producing various electric, pneumatic, and hydraulic valve actuation devices. Faced with such a large variety of valve types and such complex operating conditions, to select the most suitable valve products for installation in a piping system, I believe it is necessary to first understand the characteristics of valves ； Secondly, one should master the steps and criteria for selecting valves ； Furthermore, the principles for selecting valves should be followed. 1. Valves generally have two types of characteristics: operational characteristics and structural characteristics. Usage characteristics: They determine the main performance and application scope of the valve. The usage characteristics of a valve include its category (valves for closing circuits, control valves, safety valves, etc.) ； Product type (gate valve, globe valve, butterfly valve, ball valve, etc.) ； Materials of the main components of the valve (valve body, valve cover, valve stem, valve disc, sealing surface) ； Valve actuation methods, etc. Structural characteristics: It defines certain structural features related to the installation, maintenance, and servicing of valves. These structural characteristics include the valve’s structural length and overall height, as well as the methods of connection to pipelines (flange connection, threaded connection, clamp connection, external threaded connection, welded end connection, etc.) ； Form of the sealing surface (insert ring, threaded ring, surfacing, spray welding, valve body itself) ； Valve stem structure types (rotating rod, lifting rod), etc. 2. The steps and criteria for selecting a valve are generally as follows: ⑴ Selection steps 1. Determine the purpose of the valve in the equipment or device, and identify its operating conditions: the medium it will handle, operating pressure, operating temperature, etc. 2. Determine the nominal diameter and connection method of the pipeline connected to the valve: flange, thread, welding, etc. 3. Determine the method of operating the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrically actuated or electro-hydraulically actuated, etc. 4. The materials for the housing and internal components of the valve to be selected are determined based on the medium transported through the pipeline, the operating pressure, and the operating temperature: gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloys, etc. 5. Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap, etc. 6. Determine the valve parameters: For automatic valves, it is necessary to first determine the allowable flow resistance, discharge capacity, back pressure, etc., based on various requirements, and then determine the nominal diameter of the pipe and the diameter of the valve seat hole. 7. Determine the geometric parameters of the valve to be selected: structural length, flange connection type and dimensions, dimensions in the vertical direction of the valve when it is open or closed, dimensions and quantity of the bolt holes for connection, and overall external dimensions of the valve. 8. Utilize existing materials: valve product catalogs, valve product samples, etc., to select appropriate valve products. ⑵The basis for selecting valves: While understanding the steps for selecting valves, it is also necessary to gain further insight into the criteria used for making such selections. 1. The purpose of the valve selected, its operating conditions, and the method of control. 2. Properties of the working medium: operating pressure, operating temperature, corrosion resistance, presence of solid particles, toxicity of the medium, whether it is flammable or explosive, viscosity of the medium, etc. 3. Requirements for the fluid properties of the valve: flow resistance, discharge capacity, flow characteristics, sealing grade, etc. 4. Requirements for installation dimensions and external dimensions: nominal diameter, method and connection dimensions for connecting to pipes, external dimensions or weight limits, etc. ⑤Additional requirements for the reliability, service life of valve products, and the explosion-proof performance of electric actuators. (When selecting parameters, note that if the valve is to be used for control purposes, the following additional parameters must be determined: operation method, maximum and minimum flow requirements, pressure drop under normal flow conditions, pressure drop when closed, and the maximum and minimum inlet pressures of the valve.) ) Based on the criteria and steps for selecting valves mentioned above, to choose valves in a reasonable and correct manner it is also necessary to have a detailed understanding of the internal structure of various types of valves, so as to make the right decision regarding which valve should be given priority. The final control of a pipeline is the valve. The valve disc controls the manner in which the fluid flows within the pipeline, and the shape of the valve’s flow channel determines its flow characteristics; this factor must be taken into account when selecting the most suitable valve for installation in a piping system. 3. The following are the principles to be followed when selecting valves: (1) Valves used for shutting off and releasing the medium have a straight-through flow path, resulting in low flow resistance; such valves are usually chosen for this purpose. Downward-closing valves (gate valves, plunger valves) are less commonly used because of their tortuous flow paths, which result in higher flow resistance compared to other valves. In applications where a higher flow resistance is acceptable, closed-type valves can be used. ⑵Valves used for controlling flow usually choose those that are easy to adjust the flow rate for such purposes. Downward-closing valves (such as globe valves) are suitable for this purpose, as the size of their seat is proportional to the travel of the closing element. Rotary valves (plug valves, butterfly valves, ball valves) and flexibly-bodied valves (clamping valves, diaphragm valves) can also be used for throttling control, but they are typically only suitable for a limited range of valve diameters. A gate valve uses a disc-shaped gate that moves transversely across a circular valve seat; it can only control flow effectively when it is near the closed position, which is why it is generally not used for flow control. ⑶The valve used for reversing and shunting flow has three or more channels, as required by the needs of such reversal and shunting. Plug valves and ball valves are more suitable for this purpose; therefore, most valves used for direction reversal and flow splitting are chosen from these types of valves. However, in some cases, other types of valves can also be used for reversing and diverting flow, as long as two or more such valves are properly connected to each other. ⑷Valves for media containing suspended particles: When the medium contains suspended particles, it is most suitable to use valves whose closing elements slide along the sealing surface, thereby providing a wiping action. If the closing element moves vertically back and forth against the valve seat, it is possible for particles to get trapped; therefore, such valves are suitable only for basically clean media, unless the material of the sealing surface can tolerate the presence of particles. Ball valves and plug valves clean the sealing surfaces during opening and closing, making them suitable for use with media containing suspended particles. Currently, in pipeline systems across industries such as oil and chemicals, as well as in other sectors, the application of valves, their frequency of operation, and the level of service required vary greatly. To control or eliminate even the slightest leaks, valves remain the most important and crucial equipment. The ultimate control of pipelines is the valve, and its performance in terms of reliability and service across various fields is unparalleled. 4. Common types and uses of valves There are many types of valves used in engineering. Due to differences in the pressure, temperature, and physicochemical properties of fluids, different control requirements are needed for fluid systems. Among them, gate valves, globe valves (throttle valves, needle valves), check valves, plug valves, ball valves, butterfly valves, and diaphragm valves are the most widely used in chemical processing plants. 　　 4.1 Gate Valves Generally used to control the flow of fluids; they exhibit low fluid resistance, good sealing performance, are not restricted in terms of the direction of flow, require little external force to open or close, and have a relatively short structural length. 　　Valve stems can be either exposed or concealed. Exposed-stem gate valves are suitable for corrosive media; in chemical engineering, exposed-stem gate valves are predominantly used. Dark stem gate valves are mainly used in water systems, and are often employed in applications with low pressure and non-corrosive media, such as certain cast iron and copper valves. The structural forms of the gate plates include wedge-type gate plates and parallel-type gate plates. 　　Wedge-type gate plates are divided into single-gate and double-gate types. Parallel gate valves are commonly used in oil and gas transmission systems and are not frequently used in chemical plants. 　　 4.2 Stop valves Mainly used for shutting off flow. Gate valves have high fluid resistance, high opening and closing torque, and require a specific flow direction. Compared to gate valves, globe valves have the following main advantages: (1) During the opening and closing process, the friction between the sealing surfaces is less than that of gate valves, making them more wear-resistant. 　　(2) The opening height is smaller than that of gate valves. 　　(3) Globe valves usually have only one sealing surface, feature good manufacturing processes, and are easy to maintain. 　　Like gate valves, stop valves are also divided into those with exposed stems and those with concealed stems, which will not be discussed further here. Depending on the valve body structure, globe valves come in straight-through, angle, and Y types. The straight-through type is the most widely used, while the angled type is used in situations where the fluid flow direction changes by 90°. 　　Additionally, throttle valves and needle valves also belong to the category of stop valves, with stronger control capabilities than ordinary stop valves. 　　 4.3 Check Valves Check valves, also known as one-way valves, are used to prevent fluid from flowing in the reverse direction. Therefore, when installing a check valve, it is important to ensure that the flow direction of the fluid matches the direction indicated by the arrow on the valve. There are many types of check valves, and different manufacturers offer various products, but structurally they are mainly divided into swing-type and lift-type. Swing check valves mainly include single-disc and double-disc types. 　　 4.4 Butterfly Valves Butterfly valves can be used for opening, closing, and throttling liquid media containing suspended solids. They feature low fluid resistance, light weight, compact dimensions, and rapid operation, making them suitable for use in large-diameter pipelines. Butterfly valves have a certain regulating function and can transport slurries. Due to outdated processing techniques in the past, butterfly valves were used primarily in water systems and rarely in process systems. With improvements in materials, design, and manufacturing, butterfly valves are now being used more and more often in process systems. 　　Butterfly valves come in two types: soft-seal and hard-seal. The choice between soft seal and hard seal mainly depends on the temperature of the fluid medium. Relatively speaking, soft seals have better sealing performance than hard seals. 　　Soft seals come in two types: rubber and PTFE (polytetrafluoroethylene) seat. Rubber-seat butterfly valves (with rubber-lined bodies) are commonly used in water systems; they have a medium-sized structure. Such butterfly valves do not require gaskets during pipeline installation, as the flange on the rubber lining can serve as a gasket. PTFE-seated butterfly valves are commonly used in process systems, and generally feature a single-eccentric or double-eccentric design. 　　There are many types of hard-sealed designs, such as rigid fixed seals and laminated seals; due to the differences in design among manufacturers, their leakage rates also vary. The preferred structure for hard-sealed butterfly valves is three-eccentric design, which addresses the issues of thermal expansion compensation and wear compensation. Hard-sealed butterfly valves with double or triple eccentric structures also possess bidirectional sealing capability; the sealing pressure in the reverse direction (from low pressure side to high pressure side) should be no less than 80% of that in the forward direction (from high pressure side to low pressure side). The design selection should be discussed with the manufacturer. 　　 4.5 Plug Valves Plug valves have low fluid resistance, good sealing performance, and a long service life; they can provide sealing in both directions. Therefore, they are often used with materials that are highly hazardous. However, their operating torque is relatively high, and their cost is also high. No liquid accumulates in the body of a plug valve; in particular, it does not contaminate materials in intermittent processing systems. Therefore, plug valves must be used in certain applications. Figure 1 shows the liquid accumulation in the chamber when the plug valve and ball valve are closed. 　 　　The flow channels of plug valves can be straight-through, three-way, or four-way, and are suitable for multi-directional distribution of gaseous and liquid fluids. 　　Plug valves can be divided into two types: unlubricated and lubricated. The oil-sealed plug valve with forced lubrication creates an oil film between the plug and the sealing surfaces of the valve due to the forced lubrication. This provides better sealing performance, makes it easier to open and close the device, and prevents damage to the sealing surfaces. However, it is necessary to consider whether lubrication could contaminate the material, and regular maintenance is required; therefore, lubrication-free types should be preferred. 　　The sealing of the plug valve’s sleeve is continuous, surrounding the entire plug; as a result, the fluid does not come into contact with the shaft. Additionally, the plug valve has a diaphragm made of metal composite material as a secondary seal; therefore, it can effectively prevent external leakage. Generally, plug valves have no packing. However, when there are special requirements (such as prohibiting any external leakage), packing is needed to serve as a third layer of sealing. 　　The design of the plug valve allows for on-line adjustment of the sealing seat. Due to prolonged operation, the sealing surfaces will wear out. Since the plug is conical, it can be pressed down using the bolts on the valve cover, thereby ensuring a tight fit with the valve seat and achieving a sealed connection. 　　 4.6 Ball valves The function of ball valves is similar to that of plug valves (ball valves are a derivative product of plug valves). Ball valves have good sealing performance, so they are widely used. Ball valves open and close quickly, have a lower torque required for operation than plug valves, experience very low resistance, and are easy to maintain; they are suitable for use in pipelines carrying slurries, viscous fluids, or media that require high-level sealing performance. And due to their low cost, ball valves are more widely used than plug valves. Ball valves can generally be classified by the structure of the ball, the structure of the valve body, the flow channel design, and the material of the valve seat. 　　Based on the ball structure, they are divided into floating ball valves and fixed ball valves. The former is mostly used for small diameters, while the latter is used for large diameters; generally, the boundary is at DN200 (CLASS 150) and DN150 (CLASS 300 and CLASS 600). 　　Based on the valve body structure, there are three types: single-piece, two-piece, and three-piece. The one-piece type comes in two variants: top-mounted and side-mounted. 　　Based on the flow channel shape, there are full-bore and reduced-bore types. Reduced-bore ball valves require less material than full-bore ball valves and are cheaper; if the process conditions permit, they can be considered as a preferred choice. Ball valve flow channels can be divided into straight-through, three-way, and four-way types, and are suitable for multi-directional distribution of gaseous and liquid fluids. Based on the valve seat material, they are divided into soft-seal and hard-seal types. When used with flammable media or in environments where combustion is possible, soft-sealed ball valves should have anti-static and fire-resistant designs, and the manufacturers’ products must pass anti-static and fire resistance tests, such as those specified in API607. This also applies to soft-sealed butterfly valves and plug valves (plug valves can only meet the external fire protection requirements in fire tests). 　　 4.7 Diaphragm Valve The diaphragm valve can provide bidirectional sealing and is suitable for low-pressure, corrosive slurries or viscous suspended fluid media. Moreover, since the operating mechanism is separated from the medium channel and the fluid is cut off by an elastic diaphragm, it is particularly suitable for media used in the food and pharmaceutical industries. The operating temperature of a diaphragm valve depends on the heat resistance of the diaphragm material. Structurally, they can be divided into straight-through type and weir type. 5. Materials The valve materials include the valve body, internal components, gaskets, packing, and fastening materials, etc. Since there are many types of valve materials, and due to space constraints, this article only provides a brief introduction to typical valve housing materials. Black metal shell materials include cast iron, carbon steel, stainless steel, and alloy steel. 5.1 Cast Iron Gray cast iron (A1262B) is generally used in low-pressure valves and is not recommended for use in process pipelines. The properties (strength and toughness) of ductile iron (A395) are better than those of gray cast iron. 5.2 Carbon Steel The most common carbon steel materials used in valve manufacturing are A216 WCB (cast) and A105 (forged). Special attention should be paid to the fact that prolonged operation of carbon steel at temperatures above 400°C can affect the lifespan of the valve. For valves used in low-temperature conditions, A3522LCB (cast) and A3502LF2 (forged) are commonly used. 5.3 Austenitic stainless steels Austenitic stainless steel materials are typically used in environments with corrosive conditions or ultra-low temperature conditions. Commonly used castings include A351-CF8, A351-CF8M, A351-CF3, and A351-CF3M; commonly used forgings include A182-F304, A182-F316, A182-F304L, and A182-F316L. 5.4 Alloy steel materials For valves used in low-temperature applications, A352-LC3 (cast) and A350-LF3 (forged) are commonly used. 　　For valves used in high-temperature conditions, the commonly used materials are A217-WC6 (cast), A182-F11 (forged), A217-WC9 (cast), and A182-F22 (forged). Since WC9 and F22 belong to the 2-1/4Cr-1Mo series, they contain higher levels of Cr and Mo compared to WC6 and F11, which are part of the 1-1/4Cr-1/2Mo series; as a result, they exhibit better resistance to high-temperature creep. 　　 5. Driving method Usually, valves are operated manually. When the nominal pressure of the valve is high or its nominal size is large, making it difficult to operate the valve manually, methods such as gear drive can be used. The selection of the valve actuation method should be determined based on the type of valve, nominal pressure, and nominal size. Table 1 shows the conditions for gear drive that should be considered for different valves. For different manufacturers, these conditions may vary slightly and can be negotiated. 　　 6. Principles for selecting valves 6.1 Main parameters to consider when selecting valves (1) The properties of the fluid to be transported, as these affect the choice of valve type and the material used for its construction. 　　(2) Functional requirement (regulation or shut-off), which mainly affects the valve type selection. 　　(3) Operating conditions (frequency of use), which will affect the valve type and the selection of valve materials. 　　(4) Flow characteristics and friction losses. 　　(5) The nominal size of the valve (valves with very large nominal sizes can only be found in a limited range of valve types). 　　(6) Other special requirements, such as automatic shutdown, balanced pressure, etc. 　　 6.2 Material Selection (1) For small diameters (DN≤40), forgings are generally used, while for large diameters (DN>40), castings are generally used. For the end flanges of forged valve bodies, it is preferable to use fully forged valve bodies; if the flanges are welded to the valve body, then 100% radiographic inspection of the welds should be carried out. 　　(2) The carbon content of carbon steel valve bodies for butt welding and socket welding shall not exceed 0.25%, and the carbon equivalent shall not exceed 0.45%. 　　(3) The recommended operating temperatures for common materials are shown in Table 2. 　 　　Note: When the operating temperature of austenitic stainless steel exceeds 425°C, the carbon content shall not be less than 0.04%. The heat treatment conditions are rapid cooling from above 1040°C (CF8) and from 1100°C (CF8M). 　　(4) When the fluid is highly corrosive and ordinary austenitic stainless steels cannot be used, special materials such as 904L, duplex steels (such as S31803, etc.), Monel, and Hastelloy should be considered. 6.3 Selection of gate valves (1) Rigid single-plate gates are generally used when DN≤50; elastic single-plate gates are generally used when DN>50. 　　(2) For elastic single-disc gate valves in low-temperature systems, an exhaust hole should be provided on the disc on the high-pressure side. 　　(3) In applications requiring low leakage, gate valves with low leakage should be used. There are various structures for low-leakage gate valves; in chemical plants, bellows-type gate valves are generally used, and the minimum cycle life is shown in Table 3. 　　 　　(4) Although gate valves are the most commonly used type in petrochemical production facilities. However, gate valves are not suitable for the following situations: ① Due to their high opening height and the large amount of space required for operation, they are not appropriate for use in areas with limited operating space. 　　② The opening and closing time is long, so it is not suitable for applications requiring rapid opening and closing. 　　③ It is not suitable for fluids with solid sedimentation. Because the sealing surface wears out, the gate cannot be closed properly. 　　④ It is not suitable for flow regulation. When the gate valve is partially open, eddies are generated behind the gate, which can lead to erosion and vibration of the gate, as well as damage to the sealing surface of the valve seat. 　　⑤ Frequent operation of valves can lead to excessive wear on the valve seat surface; therefore, they are generally suitable only for situations where operation is not frequent. 6.4 Selection of Globe Valves (1) Compared with gate valves of the same specification, globe valves have a greater structural length. They are typically used in pipes with a diameter of DN≤250, as globe valves of larger diameters are more difficult to manufacture, and their sealing performance is inferior to that of those with smaller diameters. 　　(2) Due to the high fluid resistance of globe valves, they are not suitable for use with fluids containing suspended solids or those with high viscosity. 　　(3) The needle valve is a globe valve equipped with a fine conical plug, which can be used for fine adjustment of small flow rates or as a sampling valve; it is typically used in small-diameter applications. If the diameter is large, a regulating function is also required; a throttle valve can be used, in which case the valve disc has a parabolic or similar shape. 　　(4) For applications requiring low leakage, low-leakage globe valves should be used. There are various structures for low-leakage stop valves; in chemical plants, bellows-type stop valves are generally used, and the minimum cycle life is shown in Table 4. 　　 　　Bellows-type globe valves are more widely used than bellows-type gate valves, as the bellows in globe valves are shorter and their service life is longer. However, bellows valves are expensive, and the quality of the bellows (such as material and requirements regarding the number of cycles) as well as welding have a direct impact on the valve’s service life and performance; special attention should be paid to these factors when selecting such valves. 6.5 Selection of Check Valves (1) Horizontal lift-type check valves are generally used in applications where DN≤50, and can only be installed in horizontal pipes. Vertical lift check valves are typically used in applications with DN≤100 and are installed on vertical pipes. 　　(2) The lift-type check valve can be designed with a spring, in which case its sealing performance is better than that of one without a spring. 　　(3) The minimum diameter of swing check valves is generally DN>50; they can be used in horizontal pipes as well as in vertical pipes (the fluid must flow from bottom to top), but they tend to cause water hammer effects. Double disc check valves are usually wafer-type; they are the most space-efficient among check valves, making them convenient for pipeline layout, and are particularly widely used in large-diameter applications. Since the valve disc of a standard swing check valve (single-disc type) cannot be fully opened to 90°, resulting in some flow resistance, when process requirements dictate it, special requests can be made (to have the valve disc open completely) or a Y-type lift check valve can be used. 　　(4) In situations where water hammer may occur, check valves equipped with slow-closing devices and damping mechanisms can be considered. This type of valve utilizes the medium in the pipeline for buffering; at the moment the check valve closes, it helps to eliminate or reduce water hammer effects, thereby protecting the pipeline and preventing backflow from the pump. 6.6 Selection of Plug Valves (1) Due to manufacturing issues, unlubricated plug valves with a DN > 250 should not be chosen. 　　(2) When it is required that no fluid accumulates in the valve chamber, a plug valve should be selected. 　　(3) When the sealing performance of the soft-sealed ball valve does not meet the requirements, such as in the case of internal leakage, a plug valve can be used as a substitute. 　　(4) For some operating conditions with frequent temperature changes, ordinary plug valves cannot be used. Due to the different expansion and contraction of valve components and sealing elements caused by temperature changes, prolonged contraction of the packing during thermal cycling can lead to leakage along the valve stem. At this point, special plug valves such as XOMOX’s Severe service series need to be considered; these types of plug valves cannot be manufactured in China yet. 6.7 Selection of Ball Valves (1) Top-mounted ball valves can be repaired online. Three-piece types are generally used for ball valves with threaded and socket weld connections. 　　(2) When the pipeline is equipped with a ball passing system, only full-bore ball valves can be used. 　　(3) Soft seals provide better sealing performance than hard seals, but they cannot be used in high-temperature environments (the heat resistance of various non-metallic sealing materials varies). 　　(4) It shall not be used in applications where accumulation of liquid in the valve chamber is not permitted. 6.8 Selection of Butterfly Valves (1) When it is necessary to disassemble the two ends of the butterfly valve, a threaded lug-type or flanged butterfly valve should be chosen. 　　(2) The minimum diameter of centerline butterfly valves is generally DN50; the minimum diameter of eccentric butterfly valves is generally DN80. 　　(3) When a triple-eccentric PTFE seat butterfly valve is selected, a U-shaped seat is recommended. 6.9 Selection of Diaphragm Valves (1) Straight-through types have low fluid resistance; the diaphragm requires a long stroke to open and close, and their diaphragm lifespan is inferior to that of weir-type valves. 　　(2) The weir type has high fluid resistance, a short opening and closing stroke for the diaphragm, and a longer diaphragm lifespan compared to the straight-through type. 6.10 Influence of other factors on valve selection (1) When the allowable pressure drop in the system is low, valve types with lower fluid resistance should be chosen, such as gate valves and straight-through ball valves. 　　(2) When rapid shut-off is required, globe valves, ball valves, and butterfly valves are preferred. For small diameters, ball valves should be preferred. 　　(3) The vast majority of locally operated valves use hand wheels; for those located at a certain distance from the operation point, sprockets or extended rods can be used. 　　(4) For viscous fluids, slurries, and media containing solid particles, plug valves, ball valves, or butterfly valves are suitable choices. 　　(5) For cleanroom systems, plug valves, ball valves, diaphragm valves, and butterfly valves are generally used (special requirements such as polishing specifications and seal requirements must be specified separately). 　　(6) Under normal circumstances, valves with a pressure rating of 900 Class or higher and a DN of 50 or more use Pressure Seal Bonnets; valves with a pressure rating of 600 Class or lower use Bolted Bonnets. For applications where strict leakage prevention is required, Welded Bonnets can be considered. In some low-pressure and normal-temperature utility applications, union bonnet valves can be used, but this design is generally not common. 　　(7) If insulation or heat preservation is required for the valve, the handles of ball valves and plug valves need to be extended at the connection point with the valve stem to avoid the valve’s insulation layer, generally by no more than 150 mm. 　　(8) When the diameter is small, if the valve seat deforms during welding and heat treatment, a valve with a long body or one equipped with short tubes at the ends should be selected. 　　(9) Valves (other than check valves) in low-temperature systems (below -46°C) should adopt a design with an extended valve cover neck. The valve stem should undergo appropriate surface treatment to increase its surface hardness, thereby preventing scratches between the valve stem and the packing as well as the packing gland, which could affect the sealing performance. 　　 When selecting a valve, in addition to the factors mentioned above, considerations such as process requirements, safety, and cost should also be taken into account to make the final decision regarding the type of valve to use. Furthermore, it is necessary to prepare a valve data sheet, which generally should include the following information: (1) the name of the valve, its nominal pressure, and its nominal size. 　　(2) Design and inspection standards. 　　(3) Valve code. 　　(4) Valve structure type, valve cover structure type, and end connection type of the valve. 　　(5) Materials for the valve body, the sealing surfaces of the valve seat and valve disc, materials for internal components such as the valve stem, packing, gasket materials for the valve cover, and fastening materials. 　　(6) Driving method. 　　(7) Packaging and shipping requirements. 　　(8) Requirements for internal and external corrosion protection. 　　(9) Quality requirements and spare parts requirements. 　　(10) Owner’s requirements and other special requests (such as markings, etc.). Valves play an important role in chemical processing systems. The selection of valves for pipelines should take into account various factors such as the phase state of the fluid being transported in the pipeline (liquid, vapor), the solid content, pressure levels, temperature, and corrosion characteristics. Furthermore, operational reliability and faultlessness, cost-effectiveness, and the manufacturing cycle are also important considerations. 　　In previous engineering designs, when selecting valve materials, only the material of the housing was considered, while the selection of materials for internal components was ignored. An inappropriate choice of internal components often leads to seal failures at the valve’s internal sealing areas, the stem packing area, and the valve cover gaskets, which reduces the valve’s lifespan and prevents it from achieving the intended performance levels; this can also increase the risk of accidents. 　　Currently, API valves do not have a unified identification code, while although national standard valves have an identification system, it is not possible to clearly indicate the internal components and other materials, as well as other specific requirements. Therefore, in engineering projects, the required valves should be described in detail by preparing valve data sheets. This facilitates the selection, procurement, installation, commissioning of valves, as well as the management of spare parts, improves work efficiency, and reduces the likelihood of errors. II. Inspection, maintenance, and repair of valves after purchase: Delayed maintenance can lead to valves becoming faulty and leaking, or failing to operate properly. If valves are not regularly inspected and pressure-tested, or if they are not cleaned, pressure-tested, or assessed technically even after years of use, debris can accumulate inside the valves, resulting in poor sealing and severe oil leakage. Valves may not be closed properly after maintenance, or the pipe ends may not be sealed after the valves are removed. Additionally, the gaskets used on valves might be made of materials that are not resistant to oil or pressure. Therefore, it is necessary to strengthen the inspection of valves in order to prevent problems before they occur. III. Main aspects of valve inspection: 1. Check for leaks at the dynamic seal of the valve stem and the static seal of the flange gaskets. 2. Verify that the valve operates properly in its open or closed state. 3. Inspect the valve body for any damage or leakage-like abnormalities. 4. Turn valves that are normally open or closed 1–2 times or perform one opening and closing test. 5. Lubricate the valve stem parts of valves that are normally open or closed. 6. Inspect and adjust the actuator and electrical system of pneumatic valves