FO, FC, FL – can you tell them apart?

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This post was last edited by shijiazhuang on 2023-3-17 19:09. Can you distinguish between FO, FC, and FL? Pneumatic valves are one of the devices widely used in various industries in recent years; they are driven by compressed air to open and close the valve. Pneumatic actuators can simply achieve rapid linear reciprocating motion; they have a simple structure and are easy to maintain. They can also be used in various harsh working environments, such as those with explosion-proof requirements, high dust levels, or high humidity. They are primarily used in industries such as petroleum, natural gas, chemicals, power generation, textiles, mining, and pharmaceuticals. Pneumatic valves can generally be classified into air-open type and air-close type according to their operating mode. The choice between air on and air off is determined from the perspective of safety in the production process. Is it safer for the valve to be in the closed position or the open position when the air supply is cut off? The Air to Open type operates such that as the air pressure at the diaphragm head increases, the valve opens further; when the maximum input air pressure is reached, the valve is in its fully open position. Conversely, as the air pressure decreases, the valve moves in the direction of closure; when no air is supplied, the valve closes completely. Therefore, sometimes air-operated valves are also referred to as fail-to-close (FC) valves. The operating direction of the air-to-close type is exactly the opposite of that of the air-to-open type. As air pressure increases, the valve moves in the direction of closure ； When the air pressure decreases or is absent, the valve moves in the opening direction until it is fully open. Therefore, it is sometimes also referred to as Fail to Open FO. During use, the common failure locations are (FO, FC, FL); in the context of valve failure to close/open, a failure refers to the valve’s behavior when there is an issue with the air supply. Regarding the failure positions of pneumatic valves, there are mainly several scenarios: 1. In the case of interlock actions involving the pneumatic valve assembly, the valve can be in one of the following positions: FC – Air supply lost; valve is in the closed position. FO – Air supply lost; valve is in the open position. FL – Air supply lost; valve remains in its current position and stays there. FLC – Air supply lost; valve holds its position but tends to close, thus remaining in the closed position (the gas in the cylinder has been exhausted). FLO – Air supply lost; valve holds its position but tends to open, thus remaining in the open position (the gas in the cylinder has been exhausted). 2. When control valves or on/off valves are involved in the interlock actions, the valve can be in one of the following positions: FC – Air supply lost or solenoid valve loses power; valve is in the closed position. FO – Air supply lost or solenoid valve loses power; valve is in the open position. AFL/EFC – 1. Air supply lost but the solenoid valve does not lose power; valve holds its position ； 2. Whether or not the air supply is lost, if the solenoid valve loses power, the valve remains in the closed position. AFL/EFO – 1: If the air supply is lost, the solenoid valve does not lose power, and the valve retains its current position ； 2. Whether or not the air supply is lost, if the solenoid valve loses power, the valve remains in the open position. Pneumatic valves use output signals to achieve functions such as closing, opening, and regulating the flow; their speed of operation is relatively fast, making them suitable for rapid on/off switching. They can also be used for regulating flow, and by using different accessories, various control methods can be implemented.