Applications of valves in industrial production and experience in selection

Thread Content

With the continuous development of modern industry, as well as the increasing diversity of process controls and the complexity of operational conditions, there is a growing demand for control valves. In fluid piping systems, valves serve as important control elements that are used to open or close the flow of medium in the pipes, change the direction of its flow, regulate its pressure and flow rate, and ensure the proper operation of pipeline equipment. 　　The 100,000-ton-per-year TDI production plant of Gansu Yinguang Juyin Chemical Co., Ltd. is equipped with tens of thousands of various valves. However, issues such as leaks, spills, and drips often occur due to reasons like improper manufacturing, inappropriate selection during use, or inadequate maintenance. As a result, problems such as low product quality, increased energy consumption, equipment corrosion, and even plant shutdowns have become quite common. This places new demands on those who operate valves, carry out maintenance, and work as engineering technicians. In addition to operating the valves correctly and performing timely maintenance, it has become essential to understand the classification, characteristics, and structure of valves, as well as the procedures and criteria for selecting valves. 　　1 Classification of valves There are various types of valves, including straight-through valves (single-seat, double-seat), ball valves, tee valves, three-way valves, eccentric rotary valves, and more. Overall, they can be divided into two main categories: automatic valves and actuated valves. Automatic valves are valves that operate automatically relying on the inherent properties of the medium (liquid, gas). Actuated valves are valves whose operation is controlled manually, electrically, hydraulically, or pneumatically. 　　2 Characteristics and Structure of Valves 　　(1) Characteristics of valves. 1) Usage characteristics: Types of valves, such as: shut-off valves, control valves, safety valves, etc ； Product type, such as: gate valve, butterfly valve, ball valve, etc ； The main components of a valve include: the valve body, valve cover, valve stem, valve core, sealing surfaces, etc. 2) Structural characteristics: The structural length and overall height of the valve, as well as the connection methods to pipes, such as flange connections, threaded connections, welded end connections, etc ； The forms of the sealing surface include: threaded rings, surfacing welding, spray welding, valve body itself, etc ； Valve stem structural forms, etc. 　　(2) Structure of the valve. There are many types of valves, which are mainly composed of components such as upper and lower valve covers, the valve body, the valve core, the valve seat, and packing. Depending on different application requirements, they come in various structural forms. 1) Upper valve cover design of the valve. The upper valve cover is located between the actuator and the valve body, and there are four common configurations available to meet different operating temperatures and sealing requirements. ①Standard type, suitable for high-temperature environments, with an operating temperature range of -20 to 200°C. ②Heat absorption (dissipation) type; the operating temperature of the medium ranges from -60 to 450°C. The valve cover is equipped with multiple layers of heat absorption (dissipation) fins to ensure that the sealing packing can operate within the allowable temperature range. ③Long-neck type, suitable for deep-freezing applications; the operating temperature range of the medium is -250 to -60°C. ④It features a bellows seal for the movable parts of the valve stem, which prevents leakage of the medium as well as risks of poisoning and explosions. It is suitable for highly toxic, volatile, permeable, or valuable process media. 2) Valve core. ①Straight-stroke spools can be divided into five types: flat type, plunger type, window type, multi-stage type, and sleeve type. ②Angle travel valve cores include eccentric rotating valve cores, disc-type valve cores, and spherical valve cores, among which spherical valve cores are further divided into “O”-shaped spherical valve cores and “V”-shaped spherical valve cores. 　　3 Steps and Principles for Valve Selection 　　(1) Steps for valve selection. 1) First, it is necessary to determine the purpose of the valve in the equipment or apparatus, and identify its operating conditions, including the flowing medium, operating pressure, operating temperature, etc. 2) Methods of connecting the valve to the pipe (flange connection, threaded connection, welding, etc.) and the nominal diameter of the connected pipe. 3) Confirm the control method selected for the valve (manual, electric, pneumatic, hydraulic, etc.) as well as the type of valve (gate valve, globe valve, ball valve, butterfly valve, diaphragm valve, throttle valve, safety valve, pressure reducing valve, etc.). 4) When selecting the material for the valve body assembly, factors such as the corrosivity of the process medium, temperature, pressure, cavitation, and erosion are primarily taken into consideration. When factors such as cavitation and erosion are taken into account, valves made of special alloys or with special structural designs can be selected ； For general corrosion resistance requirements, ordinary stainless steel (1Cr18Ni9Ti) is commonly used for the valve body and valve core. When higher corrosion resistance is needed, duplex stainless steel (Cr18Ni12Mo2Ti) can be employed, or all-titanium material suitable for most corrosive media can also be used. 5) Confirm the valve parameters as well as the geometric parameters of the valve. Valve parameters include: allowable flow resistance, discharge capacity, back pressure, etc., as well as the nominal diameter of the pipeline. The geometric parameters of a valve include: flange spacing, flange connection type and dimensions, the number of bolts used for connection, and the overall external dimensions of the valve. 　　(2) Principles for selecting valves. When the medium contains suspended particles, if the back-and-forth movement of the closing element against the valve seat is vertical, the particles may get trapped; therefore, such valves are suitable only for essentially clean media, unless the material of the sealing surfaces 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. 　　4 Selection guidelines for various types of valves 　　(1) Ball valves. Ball valves can be divided into two types based on the shape of their ball element: “O”-type ball valves and “V”-type ball valves. “A V-type ball valve features a “V”-shaped opening in the spherical valve core. During operation, the actuator rod rotates the valve core through the crank of the ball valve, thereby changing the opening area as the ball rotates. “The V-type ball valve is particularly suitable for media such as fibers, pulp, those at low temperatures, and those with high viscosity, thanks to its simple flow path, high flow capacity, and reliable sealing achieved through the use of soft materials for the valve seat. Based on the material requirements for sealing, it can also be used for controlling powdered and granular media. 　　(2) Butterfly valve. A butterfly valve, also known as a flap valve, is a type of valve with angular motion. It is mainly composed of components such as the valve body, baffle, baffle shaft, and crank. It is suitable for applications with large diameters and short structural lengths, as well as those that require flow regulation and rapid opening/closing. Since butterfly valves have higher pressure losses compared to gate valves and ball valves, they are appropriate for piping systems where strict requirements regarding pressure loss are not necessary. 　　(3) Diaphragm valve. A diaphragm valve consists of components such as a valve body, diaphragm, valve core, valve cover, and valve stem. The diaphragm drives the valve core up and down via the valve stem, thereby changing the flow area and achieving flow regulation. Diaphragm valves have a simple structure, low flow resistance, and high flow capacity; they can be used for regulating fluids with high viscosity or suspended particles. In environments with strong acids, strong bases, or other highly corrosive substances, valve bodies lined with rubber or polytetrafluoroethylene materials can be used. However, the diaphragm and lining materials are not resistant to high temperatures, so the operating temperature should not exceed 150°C. 　　(4) Stop valve. Stop valves are suitable for pipelines where fluid resistance is not a major concern, as well as for pipelines or devices handling high-temperature and high-pressure media ； When high adjustment accuracy is not required and the pipe diameter is relatively small, globe valves are highly recommended. 　　(5) Plug valve. Globe valves are suitable for applications that require quick on/off operation, as well as for media with low temperatures, high viscosity, or suspended particles. It is generally not suitable for use in pipelines carrying steam or media at high temperatures. 　　(6) Selection of gate valves. Gate valves have a wide range of applications; they are suitable not only for media such as steam and oils, but also for media containing particulate solids and those with high viscosity, as well as for use in evacuation and low-vacuum systems. 　　In short, valve applications, operating frequencies, and service requirements vary greatly. To control or eliminate even the slightest leaks, valves remain the most important and critical equipment. Learning how to select valves properly is essential for ensuring the safe and stable operation of production lines. Keywords: control valve, ball valve, solenoid valve, butterfly valve, angle seat valve, electric ball valve, pneumatic ball valve, pneumatic butterfly valve, electric butterfly valve. Shandong Yuhong Valve Co., Ltd. is a provincial high-tech enterprise specializing in the research, design, manufacturing, sales, service, technical exchange, as well as import and export of pneumatic (electric) control valves, ball valves, butterfly valves, and solenoid valves; it operates in the field of high technology on an international scale.