Views: 181 Author: Site Editor Publish Time: 2021-04-06 Origin: Site
Pneumatic control valve is one of the industrial process control instruments widely used in petroleum, chemical, electric power, metallurgy and other industrial enterprises. It is usually a combined instrument formed by connecting pneumatic air actuator and regulating valve after installation and debugging. The actuator is the thrust component of the regulating valve, which generates the corresponding thrust according to the pressure of the control signal to push the regulating mechanism to move. The valve body is the regulating component of the pneumatic regulating valve, which directly contacts the regulating medium to regulate the flow of the fluid. Pneumatic valve bodies often include control ball valve, flow control ball valve and butterfly control valve.
Pneumatic actuators are mainly divided into pneumatic diaphragm actuator and pneumatic piston actuator.
According to the mode of action, it is divided into air-open type and air-close type.
According to the air-to-open type (FC), the regulating valve remains normally closed when the pneumatic gate valve actuator is not ventilated. With the increase of the control signal, the opening is gradually opened. Generally, K is used to represent the air opening.
According to the air-to-close type (FO), the regulating valve remains normally open when the pneumatic control valve actuator is not ventilated. With the increase of the control signal, it gradually closes, and B is generally used to represent the air-closed type.
The control system sends out a control signal of 4-20mA. After the valve positioner transforms into a pneumatic signal of 20-100KPa, or 40-200KPa, 80-240KPa, it enters the pneumatic film actuator and pushes the push rod. Because the push rod and the control valve stem are connected as one body, the control valve stem and the valve core are driven to complete the adjustment process of the medium in the pipeline. The feedback lever of the positioner is installed on the valve stem of the regulating valve. With the opening of the regulating valve, the feedback lever will give a feedback signal to the positioner when it rotates. When it is balanced with the input signal, the valve stops moving, and it has a positioning effect on the valve. When the control signal is 4mA, the valve position is zero. At 12mA, the valve stroke is 50%. At 20mA, the stroke is 100%. The control stroke corresponds to the control signal one by one for proportional adjustment.
Adjustment methods are divided into adjustment type and adjustment cut-off type. The former focuses on the adjustment process and does not require strict closure. The latter not only needs to adjust the process, but also maintain a good seal when closed to minimize leakage.
The single-seat control valve is mainly composed of the valve body and the valve cover. It consists of valve seat, valve core, valve stem, guide seat, packing, gland and other parts.
The biggest advantage of the single-seat control valve is its small leakage, because it has only one sealing line. Both the double-seat control valve and the sleeve control valve are two sealing lines. Taking into account the processing error and deformation and other reasons, the leakage is larger than the single-seat control valve. Because it has only one spool and the thrust of the medium acts on it, the imbalance force of the spool is relatively large, especially at high pressure and large diameter. This needs to be equipped with piston pneumatic mechanism and valve positioner to control. At this time, double-seat control valve and sleeve control valve can be used for better effect.
Pneumatic single seat control valve is the most commonly used type of pneumatic control valve. It uses compressed air as power, receives the pressure signal of the control system, adjusts the flow of the medium in the pipeline according to the signal change ratio, and realizes the automatic process of production control. It is composed of pneumatic actuator and single-seat regulating valve.
