Control valves are used to regulate the flow, pressure and level of the medium. According to adjust the position of the signal, the valve automatically control the opening, so as to achieve the media flow, pressure and liquid level adjustment. Regulating valve sub-electric control valve, pneumatic control valve and hydraulic control valve. Regulating valve by the electric actuator or pneumatic actuator and control valve of two parts. Regulated and usually divided into two types of direct-through single-seat and direct two-seat, the latter has the ability to circulate large, uneven and small operation stability characteristics, it is generally particularly suitable for large flow, high pressure drop and leakage less occasions. Flow capacity Cv is one of the main parameters to select the control valve, the control valve flow capacity is defined as: When the control valve is fully open, the pressure drop across the valve is 0.1MPa, the fluid density of 1g / cm3 hourly flow path The number of flow control valve, known as the flow capacity, also known as flow coefficient, in Cv, said t / h, liquid Cv value calculated according to the following formula. According to the size of flow capacity Cv check the table, you can determine the nominal valve DN. Flow characteristics of the control valve, the pressure drop at both ends of the valve to maintain a constant condition, the medium flow through the control valve and the relative flow of its relationship between the opening. Regulating valve flow characteristics of linear characteristics, such as percentage characteristics and parabolic characteristics of three. The significance of the three kinds of fluency features are as follows: (1) Equal Percentage Characteristics (Logarithmic) The relative and relative flow rates of equal percentage characteristics do not have a linear relationship, and the change of the flow rate caused by the unit stroke change at each point of the stroke corresponds to this point Of the traffic is proportional to the percentage of traffic changes are equal. So it has the advantage of small flow, small changes in traffic, traffic flow, the flow changes, that is, in different degrees, with the same regulation accuracy. (2) Linearity (Linearity) Relativeness and linearity of linearity are linear. Changes in unit stroke caused by changes in the flow is unchanged. When the flow rate is large, the relative value of the flow rate changes little, and when the flow rate is small, the relative value of the flow rate changes greatly. (3) Parabolic characteristics The flow rate varies in proportion to the two sides of the travel, and generally has the intermediate characteristics of the linear and equal percentage characteristics. From the analysis of the above three characteristics can be seen, in terms of its regulatory performance, with equal percentage characteristics for the best, stable regulation, good regulation performance. The parabolic characteristics than the linear properties of the regulatory performance is good, according to the different requirements of the occasion, select any one of the flow characteristics.