Functions, characteristics and flow characteristics of control valves
The regulating valve is used to regulate the flow, pressure and liquid level of the medium. According to the adjustment position signal, the valve opening is automatically controlled, and the medium flow, pressure and liquid level are adjusted. The control valve is divided into electric control valve, pneumatic control valve and hydraulic control valve.
The regulating valve consists of an electric actuator or a pneumatic actuator and a regulating valve. The regulator is usually divided into a straight-through single-seat type and a straight-through two-seat type. The latter has the characteristics of large flow, small unbalance, and stable operation, and is usually especially suitable for occasions with large flow, high pressure drop and less leakage.
Flow is one of the main parameters for selecting a control valve. The flow rate of the control valve is defined as: when the control valve is fully opened, 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 control valve per hour is called the flow rate, also known as the flow coefficient, expressed in CV, The unit is T/h calculated according to the following formula. The nominal diameter DN of the regulating valve can be determined by looking up the table according to the flow CV value. The flow characteristic of the regulating valve is the relationship between the relative flow of the medium flowing through the regulating valve and its opening degree under the condition that the pressure difference between the two ends of the valve remains unchanged. The flow characteristics of the regulating valve include linear characteristics, equal percentage characteristics and parabolic characteristics. The meanings of the three magnetic flux characteristics are as follows:
(1) Equal percentage characteristic (logarithm)
The relative stroke of the equal percentage characteristic has a non-linear relationship with the relative flow. At each travel point, the flow change caused by the unit travel change is proportional to the flow at that point, and the percentage of flow change is equal. Therefore, it has the advantages of small flow rate and small flow change. When the flow rate is large, the flow rate changes greatly, that is, it has the same adjustment accuracy under different opening degrees.
(2) Linear characteristic (linear)
The relative travel of the linear characteristic is linear with the relative flow. The flow change caused by the unit stroke change is constant. When the flow rate is large, the relative flow value does not change much; when the flow rate is small, the relative flow value changes greatly.
(3) Parabolic characteristics
Flow is proportional to both sides of the stroke, with an intermediate characteristic of linear and equal percentage characteristics.
From the analysis of the above three characteristics, it can be seen that the adjustment performance of the equal percentage characteristic is good, the adjustment is stable, and the adjustment performance is good. However, the parabolic characteristic is superior to the linear characteristic in terms of regulation performance. Depending on the requirements of the application, any flow characteristic can be selected.