December 27, 2024
There are two main options for pneumatic control valves, air-open and air-closed, and the following is a detailed description of them:
Air - to - Open (A - TO for short)
1, the working principle: when the input signal (usually air pressure signal) increases, the opening degree of the control valve increases; when the signal decreases, the opening degree decreases. For example, in a pneumatic control valve system with compressed air as the signal source, as the air pressure gradually rises from 0, the spool of the control valve gradually opens to increase the fluid flow.
2、Application Scenarios
Occasions with high safety requirements: In many chemical production processes, when the control system malfunctions (such as interruption of the air source, loss of signals, etc.), it is necessary to ensure that the fluid is shut down or below the safe flow rate. For example, a gas-open control valve is installed on a pipeline that delivers steam to a reactor. Under normal conditions, the steam flow is regulated based on a temperature control signal. In the event of a control system malfunction, the steam regulating valve closes to prevent the kettle from becoming dangerously hot.
Systems that require positive flow rate adjustment: In some cases where the flow rate needs to be increased gradually according to process requirements, such as in urban water supply systems, the air-opening regulating valve on the control water supply pipeline is gradually opened to meet the demand for water as the water consumption increases.
Air - to - Close (A - TC for short)
1, the working principle: the opposite of the air open type, when the input signal increases, the opening of the control valve decreases; when the signal decreases, the opening increases. For example, in the same pneumatic control valve system, as the air pressure rises, the spool of the control valve is gradually closed instead.
2、Application Scenarios
Occasions to ensure continuous supply: In some processes, it is necessary to ensure the continuous supply of fluid, even if the control system fails. For example, gas shut-off regulating valves are used on pipelines that deliver fuel gas to combustion furnaces. When the control system fails, such as signal interruption or gas source interruption, the regulating valve automatically opens to ensure the supply of fuel gas and prevent the combustion furnace from going out, because the combustion furnace goes out and then re-ignites may bring about explosions and other safety hazards.
Systems requiring reverse flow regulation: In some special processes, the flow rate needs to be regulated in reverse according to the signal. For example, in a cooling system, when the temperature rises, the flow of the cooling medium needs to be reduced. In this case, a gas shut-off control valve is used, so that as the temperature signal rises (the corresponding air pressure signal may also rise), the control valve closes and reduces the flow of the cooling medium.
When selecting the mode of action of the control valve, the following factors need to be taken into account:
Process requirements
Production safety: This is the primary consideration. For some of the fluid may be out of control and cause safety accidents (such as explosions, poisoning, over-temperature, etc.) of the process, the need to choose the mode of action according to the safety status of the valve when the accident occurs. For example, for toxic and hazardous gases in the pipeline, the valve should be closed as much as possible in the event of failure, it may be the choice of gas-open control valve.
Product quality: In some of the strict requirements of product quality in the production process, such as pharmaceutical, food processing and other industries, the role of the control valve to be able to ensure that the process parameters (such as flow, pressure, temperature, etc.) of the precise control to ensure product quality stability. For example, in the pharmaceutical process of the dosage link, according to the formula need to accurately control the flow of various raw materials, the role of the valve to adapt to the requirements of this precise control.
