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Selection of valve type: (1) Determine the nominal pressure; instead of using Pmax to determine PN, the appropriate PN is found from a table based on temperature, pressure, and material specifications, ensuring that it meets the PN requirement of the selected valve. (2) The selected valve type has a leakage rate that meets the process requirements. (3) For the selected valve type, its operating pressure difference should be less than the allowable pressure difference of the valve; if this is not possible, special considerations must be given or another valve should be chosen. (4) The temperature of the medium is within the operating temperature range of the valve, and the ambient temperature meets the requirements. (5) Consider the issue of valve clogging based on the cleanliness of the medium. (6) Consider the corrosion resistance of the valve based on the chemical properties of the medium. (7) Consider the erosion and wear resistance of the valve based on the pressure difference and the medium containing hard particles. (8) Performance-to-price ratio considering comprehensive economic effects. Three issues need to be considered: a. Simple structure (the simpler, the higher the reliability), easy maintenance, and availability of spare parts ； b. Service life ； c. Price. (9) Preferred order. Butterfly valve – Single-seat valve – Double-seat valve – Sleeve valve – Angle valve – Three-way valve – Ball valve – Eccentric rotary valve – Diaphragm valve. Selection of actuator: (1) The simplest is the pneumatic diaphragm type, followed by the piston type, and finally the electric type. (2) The main advantage of electric actuators is the convenience of the power source, but they are expensive, and their reliability as well as their resistance to water and explosions are inferior to those of pneumatic actuators; therefore, pneumatic actuators should be given priority. (3) Old electric actuators are bulky; we offer electronic, compact, and highly reliable electric actuators (at a correspondingly higher price). (4) The old ZMA and ZMB film actuators can be phased out, to be replaced by multi-spring lightweight actuators (with improved performance and a weight and height reduction of about 30%). (5) There are many varieties and specifications of piston actuators; older, large, and bulky types are not recommended anymore, and lighter, newer designs should be chosen instead. Material selection: (1) The pressure rating, operating temperature, and corrosion resistance of the valve body should meet or exceed the requirements of the process connection pipes, and prefabricated products from manufacturers should be given priority. (2) Cast iron valves are not suitable for water vapor, wet gases with high moisture content, and flammable and explosive media. (3) When the ambient temperature is below –20°C (especially in the north), cast iron valves are not suitable. (4) In the rectangular coordinate system defined by medium temperature and pressure difference, where cavitation and erosion are severe, areas outside the line connecting the points with a temperature of 300°C and a pressure difference of 1.5 MPa require the use of wear-resistant materials for the throttle sealing surfaces, such as cobalt-based alloys or Stellite alloys deposited on the surface. (5) For highly corrosive media, the selection of corrosion-resistant alloys requires choosing appropriate materials based on factors such as the type, concentration, temperature, and pressure of the medium. (6) The valve body and the throttling element are treated separately; the inner wall of the valve body experiences a low throttling velocity and can tolerate a certain degree of corrosion, with an erosion rate of around 1 mm per year ； The throttle element is subject to high-speed erosion and corrosion. (7) When selecting the lining material (rubber, plastic), the temperature, pressure, and concentration of the working medium must all fall within the range suitable for that material; At the same time, consideration must also be given to the physical and mechanical stresses exerted on it during valve operation, such as shear stress. (8) Vacuum valves should not employ a valve body lined with rubber or plastic. (9) Rubber-lined materials should not be used for the two-position cut-off valves in water treatment systems. (10) Selection of typical corrosion-resistant alloy materials for typical media: a. Sulfuric acid: 316L, Hastelloy, Alloy 20. b. Nitric acid: aluminum, C4 steel, C6 steel. c. Hydrochloric acid: Hastelloy B. d. Hydrofluoric acid: Monel. e. Acetic acid, formic acid: 316L, Hastelloy. f. Phosphoric acid: for nickel alloys and Hastelloy. g. Urea: 316L. h. Caustic soda: Monel. i. Chlorine: Hastelloy C. j. Seawater: Inconel, 316L. (11) To date, the most versatile corrosion-resistant material is tetrafluoro, known as the \"king of corrosion resistance\". Therefore, fully fluorinated corrosion-resistant valves (patented products from Hualin Institute) should be chosen first; alloys should only be used as a last resort (e.g., when the temperature is >180°C and PN > 1.6). Selection of flow characteristic: The options provided below are preliminary; for detailed choices, refer to the specialized documentation: (1) Choose logarithmic characteristic when S > 0.6. (2) Choose logarithmic characteristic for operation at low opening angles and when the unbalanced force varies greatly. (3) Choose a linear function when the required response speed of the adjusted parameter is fast, and a logarithmic function when it is slow. (4) The pressure regulation system can have a linear characteristic. (5) The liquid level control system can have a linear characteristic. Mode of operation selection: (1) Abroad, it is common to use \"open under fault\" or \"closed under fault\" to indicate this, that is, fault open and fault closed, which is exactly the opposite of the terms \"air open\" and \"air closed\" used in China; fault open corresponds to an air-closed valve, while fault closed corresponds to an air-open valve. (2) New lightweight valves and miniaturized valves no longer emphasize the positive or negative action of the actuator, hence it must be indicated in the footnote. 1) First, select the spring range, and also determine the operating spring range. (2) Determining the working spring range involves calculating the output force required to overcome the unbalanced force. If there are difficulties, the conditions (mainly the pressure difference when the valve is closed) should be provided to the manufacturer, who will assist in performing the calculations and adjusting the spring and operating range before the product leaves the factory (currently, many manufacturers do not carry out such calculations at all). Flow direction selection: (1) At the throttle port, when the medium flows in the direction toward the valve core, it is considered an open flow; when it flows in the direction away from the valve core, it is considered a closed flow. (2) The selection of flow direction mainly pertains to single-seal control valves, which are divided into three main categories: single-seat valves, angle valves, and single-seal sleeve valves. The others are for specified flow direction (such as two-seat valves, V-ball valves) and arbitrary flow (such as O-ball valves). (3) When dg＞15, flow-through is usually chosen; for small-diameter valves with dg≤15, especially high-pressure valves, flow-sealed mode can be selected to improve lifespan. (4) Flow closure is optional for the two on-off valves. (5) If oscillation occurs in a flow-blocking valve, switching to a flow-allowing type can eliminate it. Selection of packing: (1) For control valves, tetrafluoroethylene “V”-type packing and graphite “O”-type packing are commonly used. (2) Tetrafluorine fillers have low friction, but are not resistant to temperature changes and have a short lifespan ； Graphite fillers have high friction, but good heat resistance and a long service life ； Graphite packing is recommended for valves used at high temperatures and those equipped with positioners. (3) If the tetrafluoride filler is replaced frequently, graphite filler can be considered. Selection of accessories: (1) The main accessories for control valves include: positioners, converters, actuators, pressure boost valves, hold-down valves, pressure relief valves, filters, oil misters, limit switches, position transmitters, solenoid valves, and handwheel mechanisms. (2) The attachments serve a supplementary function and ensure the proper operation of the valve. Add only what is necessary; do not add anything unnecessary. Adding attachments when not necessary increases the price and reduces reliability. (3) The main function of a positioner is to increase output force and speed of movement; it is not necessary to have one when these functions are not required, and having a positioner does not necessarily mean it is beneficial. (4) For fast-response systems, there is no need for a valve with rapid operation; a converter can be used instead. (5) For strict explosion-proof environments, optional: electrical converter + pneumatic positioner. (6) Reliable solenoid valves should be selected to prevent them from failing to operate when needed. (7) It is recommended not to use a handwheel mechanism in important applications to prevent accidental operation. (8) It is best for the manufacturer to provide them and assemble them on the valve for delivery, in order to ensure the reliability of the connection between the system and the assembly. (9) When placing an order, the name, model, specifications, input signals, output signals, etc. of the attachments should be provided. (10) Once again, please note the importance of these \"small components\", especially their reliability; if necessary, Hualin can provide Japanese pneumatic components such as solenoid valves.