The principle of self-operated control valve: rely on the pressure of the medium flowing through the valve itself, the temperature as an energy-driven valve to work automatically, no external power supply and secondary instrument. This self-regulating valve is the use of the valve feedback signal output (pressure, pressure, temperature) through the signal pipe to the actuator drive valve to change the valve opening, to regulate the pressure, flow, temperature purposes. This valve is divided into direct and indirect effects of two. Direct action, also known as spring-loaded, the structure of elastic elements such as: pieces of spring, bellows, bellows-style temperature package, the use of elastic force and feedback signal balance principle. Indirect-acting control valve, an increase of a pilot (pilot valve) It plays a role in amplifying the feedback signal and then through the implementing agencies, driving the main valve disc movement to achieve the purpose of changing the valve opening. In the case of a pressure regulator, the feedback signal is the outlet pressure of the valve and is introduced into the actuator via the signal line. In the case of a flow control valve, an orifice (or other resistance device) at the outlet of the valve removes the pressure difference signal from both ends of the orifice and introduces it to the actuator. If it is a thermostatic valve, there is a temperature sensor (or warm bag) at the outlet of the valve to drive the actuator through the thermal expansion and contraction of the medium in the temperature sensor. Control Valves Regulators play a very important role in the automation of modern plants, whose production depends on the correct distribution and control of the flowing liquids and gases. These controls, whether energy exchange, pressure reduction or simple vessel loading, require some final control element to accomplish. The final control element can be considered a "physical" self-control. The final control element performs the necessary power amplification between the low energy level of the regulator and the high energy level required to perform the control of the flowing fluid. Control valves are the most widely used type of final control element. Other final control elements include dosing pumps, trim and louvered baffles (a variation of a butterfly valve), variable pitch fan blades, current regulators, and motor positioners other than valves. Despite the widespread use of control valves, there is probably no maintenance effort on the other units in the control system. In many systems, the regulating valve is subjected to more severe operating conditions such as temperature, pressure, corrosion and contamination than other components, however, it must operate satisfactorily with minimal maintenance when it controls the flow of process fluid the amount. Regulating valve in the pipeline from the role of variable resistance. It changes the turbulence of the process fluid or provides a pressure drop in the case of laminar flow, which is caused by changing valve resistance or "friction." This pressure reduction process is commonly referred to as "throttling." For gases, it is close to an isothermal adiabatic condition, with deviations depending on the degree of gas imperfections (Joule-Thomson effect). In the case of liquids, the pressure is consumed by turbulent flow or viscous friction, both of which convert pressure into heat, resulting in a slight increase in temperature. Common control loop includes three main parts, the first part is a sensitive element, it is usually a transmitter. It is a device that can be used to measure the parameters of the process being tuned, such as pressure, level or temperature. The output of the transmitter is sent to an adjustment meter-regulator which determines and measures the deviation between the setpoint or expected value and the actual value of the process parameter and sends the correction signal one by one to the final control element - valve. The valve changed the fluid flow, the process parameters to achieve the desired value. In the pneumatic control system, the pneumatic signal output by the regulator can directly drive the spring-diaphragm actuator or the piston actuator to actuate the valve. In this case, the energy required to determine the valve position is supplied by compressed air which should be dried in an outdoor unit to prevent freezing and should be decontaminated and filtered. When a pneumatic control valve and electric regulator supporting the use of electrical gas valve positioner or electric one gas converter. Compressed air supply system can be used for the same pneumatic adjustment system to consider. In the regulation of the terminology, the control valve has both static and dynamic characteristics, so it affects the success or failure of the entire control loop. The static characteristic or gain term is the flow characteristics of the valve and depends on the size of the valve, the combination of the valve plug and seat, the type of actuator, the positioner, the pressure before and after the valve, and the nature of the fluid.
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