control valve overview
The regulating valve is used to regulate the flow, pressure and liquid level of the medium. According to the signal of the regulating part, the opening of the valve is automatically controlled, so as to achieve the regulation of medium flow, pressure and liquid level. The control valve is divided into electric control valve, pneumatic control valve and hydraulic control valve.
The regulating valve is composed of electric actuator or pneumatic actuator and regulating valve. Regulation is generally divided into two types: straight through single seat type and straight through double seat type. The latter has the characteristics of large flow capacity, small imbalance and stable operation, so it is usually especially suitable for occasions with large flow, high pressure drop and less leakage.
The flow capacity CV is one of the main parameters for selecting the regulating valve. The definition of the flow capacity of the regulating valve is: when the regulating valve is fully open, the pressure difference between the two ends of the valve is 0.1MPa, and the fluid density is 1g / cm3, the flow rate of the regulating valve per hour is called the flow capacity, also known as the flow coefficient, which is expressed in CV, and the unit is t / h. The CV value of the liquid is calculated according to the following formula.
The nominal diameter DN of the regulating valve can be determined by checking the table according to the circulation capacity CV value.
The flow characteristic of the regulating valve is the relationship between the relative flow of the medium through the regulating valve and its opening under the condition that the pressure difference between the two ends of the valve remains constant. The flow characteristics of the control valve are linear, equal percentage and parabola. The significance of the three injection characteristics is as follows:
(1) Equal percentage characteristic (logarithm) the relative stroke of equal percentage characteristic does not have a linear relationship with the relative flow. The change of flow caused by the change of unit stroke at each point of the stroke is directly proportional to the flow at this point, and the percentage of flow change is equal. So its advantage is that when the flow rate is small, the flow rate changes little and the flow rate is large, the flow rate changes greatly, that is to say, it has the same regulation accuracy in different openings.
(2) Linear characteristic (linear) the relative stroke of the linear characteristic is in line with the relative flow. The change of flow rate caused by the change of unit stroke is constant. When the flow is large, the change of flow relative value is small. When the flow is small, the change of flow relative value is large.
(3) The parabolic characteristic flow rate is proportional to the two sides of the stroke, and has the intermediate characteristics of linear and equal percentage characteristics.
From the analysis of the above three characteristics, it can be seen that in terms of its regulation performance, the equal percentage characteristic is the best, its regulation stability and regulation performance are good. The parabolic characteristic is better than the linear characteristic. Any flow characteristic can be selected according to the different requirements of the application.