1. From the use of the function to select the control valve (1) regulation 1) require the regulator valve action smooth. 2) adjust the performance when the opening is relatively small. 3) Select the desired flow characteristics. 4) to meet the adjustable ratio. 5) resistance, flow ratio (valve rated flow parameters and nominal size ratio). 6) fast adjustment. (2) The amount of leakage and cut off the pressure of the two are indivisible, interrelated two factors. Leakage should meet the technical requirements, and the reliability of sealing surface protection measures; cut off the pressure difference (when the valve closing pressure) must be made to allow the selected control valve have enough output force to overcome, otherwise it will lead to the choice of implementation Agency too big or too small. (3) anti-blocking control valve through the medium even if it is clean, there is also the problem of congestion, which is due to the pipeline oxide and other dirt was brought into the control valve medium, resulting in blockage, it should consider the control valve Anti-blocking performance. Usually the angle of the control valve than the straight stroke of the control valve anti-blocking performance is much better, it is easy to block the occasion suggested the use of angular stroke control valve. (4) Corrosion! Including erosion resistance, cavitation, corrosion, mainly related to the choice of valve material life and service life, also involves economic issues. Should choose the valve has good corrosion resistance, and the price is reasonable. (5) pressure and temperature! This involves the nominal pressure regulator, the choice of operating temperature. Mainly appropriate choice of the main valve material and the inner material. Such as the maximum working pressure of cast iron 1.6MPa, the maximum operating temperature of 200 ℃; carbon steel maximum working pressure of 42.0MPa, the maximum operating temperature of 425 ℃; austenitic steel maximum working pressure of 32.0MPa, the operating temperature range -196 ~ 600 ℃; The maximum working pressure of the heat-resisting steel (molybdenum steel and molybdenum-chromium steel with the mass fraction of molybdenum not less than 0.4%) is 42MPa 'and the maximum working temperature is 570 ℃. 2. Comprehensive economic results to determine the type of control valve to meet the requirements of the use of the above functions, there are several types of control valve. At this point should be a comprehensive economic results to determine the type of a control valve. To this end, at least the following four issues should be considered: 1) high reliability. 2) long service life. 3) easy maintenance, adequate spare parts. 4) product performance and reasonable price. 3. Control valve type selection order According to the advantages and disadvantages of the control valve function and the above principles, the control valve selection sequence is as follows: Full-function ultra-light control valve-V-shaped opening ball valve → butterfly valve → sleeve control valve → single seat control valve → Two-seat control valve → Eccentric rotary valve → O-shaped ball valve → angle valve → three-way valve → diaphragm valve.