What are air-on and air-off valves?
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What are air-on and air-off valves, and how can this principle be understood?
Air-open valve: It opens only when the compressed air pressure is above a certain value; it closes completely when the pressure falls below that value. Air-close valve: It closes only when the compressed air pressure is above a certain value; it opens completely when the pressure falls below that value. In short, an air-open valve closes completely when the air supply is stopped, while an air-close valve opens completely under the same conditions.
It is determined by the mode of operation of the self-regulating valve. An air-open valve is one in which the self-regulating valve is opened using instrument air; if the supply of instrument air is interrupted, the valve will automatically close completely. An air-close valve, on the other hand, is one in which the self-regulating valve is closed using instrument air, and if the supply of instrument air is interrupted, the valve will automatically open completely. For these two types of valves, they cannot function properly if the instrument air pressure is below the rated pressure of the self-regulating valve; the choice of valve is made based on the process requirements.
Gas control valve: A control valve whose flow cross-sectional area increases as the signal pressure rises. Air shut-off valve: A control valve whose flow cross-sectional area decreases as the signal pressure increases. In simple terms, a valve that opens when pressure is applied is called a pressure-operated valve, while one that closes when pressure is applied is called a pressure-released valve. In terms of operation on a computer screen, for air-open valves, the greater the output signal applied, the more the valve opens; whereas for air-close valves, the greater the output signal, the more the valve closes.
Air open valves and air close valves have the same effect on production under normal conditions. The difference is that in an emergency, when the gas supply is interrupted, the gas-on valve automatically shuts off completely ； Air shut-off valve: when the air supply is interrupted, the valve opens fully automatically. On the DCS screen, parameter settings have positive and negative effects; during debugging, the parameters are adjusted so that the valve position displays 100 when the valve is fully open, and 0 when it is fully closed.
A valve that opens when there is air (signal pressure) is called a pneumatically operated valve; once the signal is interrupted, the valve returns to its original state (closed). A valve that can be closed only when there is air pressure (signal pressure) is called a pneumatic shut-off valve; once the signal is interrupted, the valve returns to its original state (open). The selection of air-operated open or air-operated closed control valves is primarily determined by the requirements of the production process and safety considerations ； The principle is that when the signal pressure is interrupted, the safety of process equipment and production must be ensured. If high safety is required when the valve is in the fully open position, a air-operated open valve should be chosen; otherwise, an air-operated closed valve should be selected.
An open-air valve closes completely when there is a power loss or when the air supply is cut off, while a closed-air valve opens completely under the same conditions
“Gas control valve: A control valve whose flow cross-sectional area increases as the signal pressure rises. Air shut-off valve: A control valve whose flow cross-sectional area decreases as the signal pressure increases. ” It’s not entirely correct; the opening degree of the valve changes in response to the signal. When the valve opens more as the signal value increases, it is considered a direct-acting valve; otherwise, it is called an inverse-acting valve. The relationship between the valve opening degree and the signal value is determined based on process requirements and is not fixed. Air-open valves are denoted by FC; on flow diagrams, the direction of the actuator is indicated by ↓. Air-close valves are indicated by FO on flow diagrams, with the direction of the valve actuator shown as ↑.
Got it. Thank you all for your answers, :loveliness:
In simple terms, a gas-operated valve opens when there is gas available, and it closes automatically when there is no gas. A pressure-operated valve remains closed as long as there is instrument air; it closes automatically when that air is unavailable. For example, in the event of a power outage or a lack of instrument air, the pressure-operated valve will open automatically, thereby preventing accidents
Air-operated valve: It requires compressed air to open, and it closes when the air supply is cut off. Air shut-off valve: It requires compressed air to close; once the air supply is cut off, the valve opens.