Electric Valve Pneumatic Valve Control valves characteristics and selection method

Regulating valve is a commonly used actuator in automatic control system, which is used to adjust the flow of controlled object. Correct choice of regulating valve, is to adjust the quality assurance system control. On the composition of the control valve, flow characteristics were described in detail, and gives the control valve selection method and should pay attention to the problem. Control valve consists of two main components: body components and actuator components (or actuator system), divided into four series: single-seat series of Control valves, two-seat series of Control valves, sleeve series of Control valves and self-reliance series of Control valves.  Variations of the four valve types lead to many different serviceable configurations, each with its own particular application, characteristics, advantages and disadvantages.  Although some control valves have a wider range of applications than others, they are not suitable for all applications to build the best solution for increased performance and reduced cost.

Regulating valve is a commonly used actuator in automatic control system, which is the main control element of automatic control, which directly controls the delivery volume of the measured medium. Control valve by the implementing agencies and regulatory agencies, to accept the control signal of the regulator or computer, used to change the flow of the controlled medium, to be transferred parameters to maintain the required range, so as to achieve process control automation. In the field of automatic control, whether the control process is steady or not depends directly on whether the control valve can act accurately, so that the process control can be reflected as the precise changes of the material energy and the flow rate. So, according to different needs to choose a different control valve. Select the appropriate control valve is the main problem of pipeline design, but also to ensure the regulation of system security and smooth operation of the key.
In the automatic control of modern factories, the regulating valve plays a very important role, the production of these factories depends on the correct distribution and control of flowing media.  These controls, whether energy exchange, pressure reduction or simple container feeding, require some final control element. The regulating valve acts as a variable resistance in the pipeline.

Laminar flow provides a pressure drop caused by a change in valve resistance or "friction".  This pressure reduction process is often referred to as "throttling".  For gases, it is close to an isothermal adiabatic state, and the deviation depends on the non-ideal degree of the gas (joule-Thomson effect).  In the case of liquids, pressure is consumed by turbulent flow or viscous friction, both of which convert pressure into heat energy, resulting in a slight increase in temperature.

The common control loop consists of three main parts. The first part is the sensor, which is usually a transmitter.  It is a device that can be used to measure the process parameters to be adjusted, such as pressure, level or temperature.  The output of the transmitter is sent to the regulating instrument, the regulator, which determines and measures the deviation between the given value or expected value and the actual value of the process parameters, and one by one sends the correction signal to the final control element, the regulator.  The valve changes the flow rate of the fluid to achieve the desired process parameters.  

Control valve belongs to the control valve series, the main role is to adjust the medium pressure, flow, temperature and other parameters, is the process loop in the final control components.  Control valve, also known as the control valve, by accepting the control signal output from the control unit.

The regulator: Fluid flow is changed by means of dynamic operation.  Regulating valve is generally composed of actuator and valve.  If according to the power of the actuator, the regulating valve can be divided into pneumatic control valve, electric control valve, hydraulic control valve three, in addition, according to its function and characteristics of points, linear characteristics, such as percentage characteristics and parabolic characteristics of three.  

The valve type: There are many kinds of valve body types, commonly used valve body types are straight single seat, straight double seat, Angle, diaphragm, small flow, three, eccentric rotation, butterfly, sleeve, spherical, etc..  In the specific selection, the following considerations can be made: (1) spool shape and structure: Mainly according to the selected flow characteristics and unbalanced force and other factors to consider (2) wear resistance: When the fluid medium is a suspension containing a high concentration of abrasive particles, the internal material of the valve should be hard.  (3) Corrosion resistance: Because the medium is corrosive, try to choose simple valve structure. (4) temperature and pressure of the medium:  When the temperature and pressure of the medium is high and changes greatly, the valve core and valve seat material should be selected by temperature and pressure change small valve, when the temperature ≥250℃ should be added to the radiator.  (5) Prevent flash and cavitation: Flash and cavitation occur only in liquid media.  In the actual production process, flash and cavitation will form vibration and noise, shorten the service life of the valve, so in the selection of valve should prevent the valve flash and cavitation.  

Control valve actuator: In order to make the regulator work normally, the actuator with sufficient output force to produce Control valve (14 pieces) to ensure high seal and valve opening. For double-acting pneumatic, hydraulic and electric actuators, there is generally no reset spring.  The size of the force has nothing to do with its running direction, so the key to choose the actuator is to make clear the maximum output force and the rotational torque of the motor.  For the single role of pneumatic actuator, the output force is related to the opening of the valve, the force appearing on the regulating valve will also affect the movement characteristics, so it is required to establish force balance in the opening range of the whole regulating valve. After determining the output force of the actuator, select the corresponding actuator according to the requirements of the process environment.  For the site explosion-proof requirements, pneumatic actuators should be selected.  From the aspect of energy saving, electric actuator should be selected as far as possible.  If the adjustment precision is high, hydraulic actuator can be selected.  Such as power plant transparent machine speed regulation, oil refinery catalytic reactor temperature regulation and control.

The mode of action of the regulating valve is only in the selection of pneumatic actuator, its mode of action through the positive and negative action of the actuator and the positive and negative action of the valve combination.  Combination form has 4 kinds namely positive (gas close type), positive and negative (gas open type), anyway (gas open type), inverse (gas close type), through the four combinations formed by the way of regulating valve gas open and gas off two kinds.

For the choice of regulating valve action mode, mainly from three aspects: a) process production safety;  B) Characteristics of the medium;  C) Ensure product quality and minimize economic loss.

Kinds of classification: Regulating valve according to stroke characteristics can be divided into: straight stroke and Angle stroke.  Straight stroke includes: single-seat valve, two-seat valve, sleeve valve, cage valve, Angle valve, three-way valve, diaphragm valve;  Angle stroke includes: butterfly valve, ball valve, eccentric rotary valve, full function ultra light regulating valve. Regulating valve according to the driving mode can be divided into: manual regulating valve, pneumatic regulating valve, electric regulating valve and hydraulic regulating valve, namely to compressed air as power source of pneumatic valve, electric as power source of electric valve, to liquid medium (such as oil, etc.) pressure as power hydraulic regulating valve; According to the regulation form can be divided into: regulation type, cut off type, cut off type; According to the flow characteristics can be divided into: linear, logarithmic type (percentage), parabola, fast.