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There are two criteria for determining the diameter of a control valve: first, when the valve is fully open, it should be able to allow at least 1.25 times the normal flow rate to pass through; this represents a safety factor to ensure that the stop valve can operate in either the fully open or fully closed position； Secondly, considering the characteristics of the valve and economic factors, it is desirable that at normal flow rates, the opening range of the valve be controlled between 30–70% for linear valves, or 30–80% for equal percentage valves. I. Determination of operating conditions 1. Name of the medium, its properties, and key physicochemical parameters 2. Process parameters (flow rate, pressures before and after valves, temperature, etc.) 3. Piping details (type, diameters before and after valves, calculation of system resistance, estimated pressure drop ratio S value, etc.) 4. Type and characteristics of the process control object, such as key control parameters and major disturbance factors 5. Requirements regarding control performance, such as requirements for leakage levels and stability. II. Initial selection of valve type: 1. Select the valve type based on the operating conditions, and determine the flow direction and flow characteristics. 2. Identify the parameters of that product series corresponding to the initially selected valve type, such as DN, PN, Kv, etc. III. Formula for calculating Kv: Kv is the flow coefficient in international units. It is defined as: cubic meters per hour of water at a temperature of 5°C to 40°C, flowing through a control valve under a pressure drop of 105 Pa. Among the several symbols commonly used today, Cv=1.167C and Kv≈C; in addition, Cg is used to represent the flow coefficient for gases, while Cs is used for steam. There are many formulas for calculating the Kv value; the one presented below is a simple formula that requires few physical and chemical parameters. Kv – Rated flow coefficient; △t – Superheat of steam (°C); K – Steam correction coefficient; Q1 – Liquid flow rate (m3/h); P1 – Absolute pressure before the valve (Pa). For steam, K = 19.3. Qg – Gas flow rate (Nm3/h); P2 – Absolute pressure after the valve (Pa). For ammonia steam, K = 25. Gs – Steam flow rate (kg/h); △P – Pressure difference before and after the valve (Pa). For Freon, K = 68.5. r – Relative density (water = 1, at 5–40°C). For methane and ethylene steam, K = 37. G – Specific gravity of gas (air = 1). Pm = (P1 + P2)/2 (Pa). For propane and propylene steam, K = 41.5. t = Temperature of the medium (°C). For butane and isobutylene steam, K = 43.5. IV. Calculation of Kv value: Using the formulas in the table of formulas for calculating Kv values, determine the maximum Kv value as well as the minimum Kv value under normal flow conditions. V. Selection of the diameter 1. Determine the amplification factor m by referring to the “m calculation table” based on the selected valve type and flow characteristics. Starting from the two criteria mentioned earlier, the maximum value of Kv is rounded off; this maximum Kv value is calculated based on the maximum normal flow rate Q and the pressure drop across the valve at that flow rate. The amplification factor m = Kv_selected/Kv_max. It is recommended that for linear valves, m = 1.63, and for equal percentage valves, m = 1.97. 2. Select the larger value for Kv; refer to the DN-Kv relationship in the product specifications, round off the value to the nearest appropriate figure, and determine the corresponding DN and Kv values. 3. Based on the rounded values of Kv/Kv high and Kv/Kv low, refer to the \"m calculation table\" to determine the corresponding maximum and minimum opening degrees of the valve. 4. Verify that the actual adjustable ratio R = 5. 5. If the verification result is satisfactory, the task is complete; otherwise, recalculate and verify again.