FO, FC, FL – can you tell them apart?

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Pneumatic valves are one of the devices widely used in various industries in recent years; they are driven by compressed air, which enables the opening and closing of the valve. Pneumatic actuators can easily perform rapid linear reciprocating motions. They feature a simple structure and are easy to maintain. Additionally, they can be used in various harsh working environments, such as those requiring explosion protection, or environments with high dust levels or humidity. They are primarily applied in industries like oil and gas, chemical processing, power generation, textile printing and dyeing, mining, and pharmaceuticals. Based on their mode of operation, pneumatic valves can generally be classified into air-open type and air-close type. The choice between gas on and gas 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 gas supply is cut off? The Air to Open type means that as the air pressure at the diaphragm head increases, the valve moves in a direction that increases its opening degree; when the maximum input air pressure is reached, the valve is in its fully open state. Conversely, as the air pressure decreases, the valve moves in the direction of closure, and 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 mechanism, 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 (gas in the cylinder is exhausted). FLO – Air supply lost; valve holds its position but tends to open, thus remaining in the open position (gas in the cylinder is 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 – 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 but the solenoid valve does not lose power, the valve retains its current position ； 2. Whether or not the air supply is lost and the solenoid valve loses power, the valve remains in the open position. Pneumatic valves utilize output signals to achieve functions such as closing, opening, and regulating the flow; they have a relatively fast opening and closing speed, and are often used for rapid on/off switching. They can also be used for regulating flow, and by combining them with different accessories, various control methods can be achieved.