- Working Principle
Compressed air as a power source, cylinder as an actuator, and with the help of electric valve positioner, torque converter, solenoid valves, holding valves and other accessories to drive the valve to achieve switching or proportional adjustment. The control signal of the automatic control system is used to complete the adjustment of pipeline media: flow, pressure, temperature and other process parameters. Characterized by simple control, fast response time and intrinsic safety, no additional explosion-proof measures are required.
Pneumatic control valve usually consists of pneumatic actuator and control valve connection, installation and commissioning. Pneumatic actuators can be divided into single-acting and double-acting. Single-acting actuators have a reset spring, while double-acting actuators do not have a reset spring. When the valve loses its home position or suddenly fails, the single-acting actuator can automatically return to the initially set open or closed state.
Pneumatic control valves are categorized into air-opening and air-closing types according to the form of action, the so-called normally open and normally closed types. Pneumatic control valve pneumatic opening or closing is usually through the positive and negative action of the actuator and the valve state structure of different assembly methods.
- Mode of action
Air open type (normally closed type) means that when the air pressure on the diaphragm head is increased, the valve moves in the direction of increased openness. When the upper limit of the input air pressure is reached, the valve opens completely. Conversely, when air pressure decreases, the valve moves in the direction of closure, and when there is no air input, the valve closes completely. Gu usually refers to air-open control valves as fail-closed valves.
The air-closed (normally open) type operates in the opposite direction from the air-open type. When air pressure increases, the valve moves in the closed direction; when air pressure decreases or is not present, the valve moves in the open direction or until it is fully open. Gu usually refers to gas shut-off control valves as fail-open valves.
- According to the safety of process production to consider the choice of open and closed gas. Is it safe for a control valve to be in the closed or open position when the gas supply is cut off?
For example, for combustion control in a heating furnace, a regulating valve is installed on the gas line and the fuel supply is controlled based on the temperature of the furnace or the temperature of the heating material at the furnace outlet. In this case, it is safer to choose a valve that opens for air because it is more appropriate to close the valve than to open it completely once the air supply stops. If the air supply is interrupted and the fuel valve is fully open, there is a danger of overheating. Another example is a heat exchange unit cooled by cooling water. Hot material is cooled by heat exchange with cooling water in a heat exchanger. A regulating valve is mounted on the cooling water pipe. The temperature of the material after heat exchange is used to control the amount of cooling water. When the air supply is interrupted, the regulating valve should be in the open position for greater safety and an air shut-off (i.e. FO) regulating valve should be used.