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The importance of understanding valve characteristics: 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 and release pressure. Since it is very important to select the most suitable valves for piping 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 sodium metal 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 various 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 the 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 (closed-loop valves, control valves, safety valves, etc.) ； Product type (gate valve, globe valve, butterfly valve, ball valve, etc.) ； Materials of the main valve components (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 upkeep of valves. These structural characteristics include the valve’s structural length and overall height, as well as the methods of connection to pipes (flange connection, threaded connection, clamp connection, external threaded connection, welded end connection, etc.) ； Form of the sealing surface (flange, 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 of the pipeline connected to the valve and the connection method: 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 Select the type of valve: closed-loop valves, control valves, safety valves, 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, 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 pipeline and the diameter of the valve seat hole. ⑧ 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 bolt holes for connection, and overall external dimensions of the valve. ⑨ Use existing materials: valve product catalogs, valve product samples, etc., to select the appropriate valve products. ⑵ 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 valves: flow resistance, discharge capacity, flow characteristics, sealing grade, etc. ④ Requirements for installation dimensions and external dimensions: nominal diameter, connection method and connection dimensions to the pipeline, external dimensions or weight limits, etc. ⑤ Additional 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: operating 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 medium 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. The following are the principles to be followed when selecting valves: (1) Valves for shutting off and opening the medium. Valves with a straight-through flow path have low flow resistance, and they are usually chosen as valves for shutting off and opening the medium. Downward-closing valves (gate valves, plug 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-valve types can be used. ⑵ Valves for controlling flow Valves that are easy to adjust for controlling flow are usually chosen for this purpose. 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 generally only applicable within a limited range of valve port sizes. 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. ⑶ Valves for reversing and diverting flow: Depending on the requirements for reversing and diverting flow, such valves can have three or more channels. Plug valves and ball valves are more suitable for this purpose; therefore, most valves used for reversing flow are chosen from these two types. 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. ⑷ Valves for media containing suspended particles When the medium contains suspended particles, valves in which the closing element slides along the sealing surface while performing a wiping action are the most suitable. If the closing element moves vertically back and forth against the valve seat, it may trap particles; 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 scrub 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.