The regulating valve is also called control valve, in the field of industrial automation Process Control, the final control elements of the process parameters, such as flow rate, pressure, temperature and liquid level, can be changed by means of the control signal of adjusting control unit output. It is generally composed of actuator and valve.

If according to the stroke characteristics, the regulating valve can be divided into straight stroke and angular stroke, according to its function and characteristic, it is divided into linear characteristic, equal percentage characteristic and parabola characteristic three kinds. Regulating valves are suitable for air, water, steam, various corrosive media, mud, oil and other media.

Control valve commonly used categories: pneumatic control valve, electric control valve, self-regulating valve, hydraulic control valve. The valve has many types of products, the structure is also varied, and is constantly updated and changed. Generally speaking, the valve can be used to match the pneumatic actuator or to the electric actuator or other actuator.

So how do we choose from these various control valve selection, valve body type selection Valve body selection is the most important part of the control valve selection, so in the specific choice, the following aspects can be considered:

First, from the control process of the media considerations

(1) The shape structure of the spool is mainly considered according to the flow characteristics and unbalance force chosen.

(2) corrosion resistance due to the corrosive medium, in the case of the adjustment function, as far as possible to choose a simple structure valve.

(3) When the medium temperature, pressure high and changes in large, should choose the valve core and seat materials by temperature, pressure changes small valve. (4) Abrasion resistance when the fluid medium is a high concentration of wear particles of the suspension, spool, seat junctions every time the shutdown will be seriously rubbed.

So the valve flow path to smooth, valve internal material to be hard. (5) Prevent Flash and cavitation flash and cavitation only in liquid medium.

In the actual production process, flash and cavitation affect not only the calculation of flow coefficient, but also the formation of vibration and noise, so that the service life of the valve is shortened, so in the selection of valves should prevent the valve from flashing and cavitation.

Second, from the aspect of the output force In order for the regulating valve to work properly, the executing agency should be able to produce sufficient output force to overcome various resistance, ensure the high seal and valve opening. For the dual-acting pneumatic, hydraulic, electric actuator, there is generally no reset spring. The magnitude of the force has nothing to do with its direction, so the key to selecting the actuator is to find out the maximum output force and the rotational torque of the motor.

For the single acting pneumatic actuator, the output force is related to the opening of the valve, the force on the regulating valve will also affect the movement characteristics, so it is required to establish the force balance in the entire control valve open range.

Third, from the role of the control valve mode of consideration The function of the regulating valve is only in the selection of pneumatic actuator, and its action is formed by the positive and negative action of the actuator and the combination of the valve. Combination of 4 kinds of form is positive (gas-type), positive and negative (gas open type), anyway (gas open type), reverse (gas-type), through the four combination of the formation of the regulating valve mode of action and gas open two kinds.

The choice of the function mode of the regulating valve is mainly considered from three aspects: a The safety of the process production, b the characteristic of the medium, the quality of the product and the lowest economic loss.

Four, from the control valve characteristics of the consideration The flow characteristic of the regulating valve refers to the relationship between the relative flow of the medium flowing through the valve and the displacement (relative opening of the valve), ideal flow characteristics are mainly linear, equal percentage (logarithmic), parabolic and fast-opening, such as 4 kinds, commonly used ideal flow characteristics only straight line, equal percentage (logarithmic), fast open three kinds.

Parabolic flow characteristics between the line and equal percentages, generally available as a percentage of characteristics to replace, and the fast-opening characteristics are mainly used in two-bit regulation and program control, so the selection of control valve characteristics is actually linear and equal percentage flow characteristics of the choice.

Five, from the diameter of the control valve to consider The selection and determination of the valve caliber is mainly based on the flow capacity of the valve, i.e. CV. In a variety of engineering instrumentation design and selection, the adjustment valve to the CV calculation, and provide control valve design instructions.