The regulating valve is used to regulate the flow, pressure and liquid level of the medium. According to the signal of the regulating position, the opening of the valve can be automatically controlled so as to achieve the adjustment of the medium flow, pressure and liquid level. Regulating valves are divided into electric control valves, pneumatic control valves and hydraulic control valves.

The regulating valve consists of an electric actuator or a pneumatic actuator and a regulating valve. Regulating valves are usually divided into two types: through single-seat regulating valve and through double-seat regulating valve. The latter has the characteristics of large flow capacity, small unbalance and stable operation, so it is usually especially suitable for large flow, high pressure drop and less leakage.

Flow capacity Cv is one of the main parameters to select the control valve. The flow capacity of the control valve is defined as: when the control valve is fully open, the pressure difference between the two ends of the valve is 0.1 MPa, and the fluid density is 1 g/cm3, the flow rate per hour of the control valve is called flow capacity, also known as flow coefficient, expressed in Cv, expressed in t/h, and the CV value of the liquid is calculated in the following formula. The nominal diameter DN of the control valve can be determined by looking up the table of the Cv value of the circulation capacity.

The flow characteristic of the control valve is the relationship between the relative flow rate of the medium flowing through the control 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 parabolic. The significance of the three fluence characteristics is as follows:

(1) The relative stroke of equal percentage characteristic (logarithm) and equal percentage characteristic has no linear relationship with the relative flow. The change of flow caused by the change of unit stroke at each point of the journey is 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 change of flow rate is small, and when the flow rate is large, the change of flow rate is large, that is to say, it has the same adjustment precision in different openings.

(2) The relative stroke of linear characteristic (linear) linear characteristic has a linear relationship with the relative flow rate. The change of flow rate caused by the change of unit stroke is constant. When the flow is large, the relative value of flow changes little, and when the flow is small, the relative value of flow changes greatly.

(3) The parabolic characteristic flow varies in proportion to the two sides of the stroke, and generally 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, and its regulation is stable and its regulation performance is good. And the parabolic characteristic is better than the linear characteristic in regulating performance. According to the requirements of different occasions, we can select any of the flow characteristics.