【Q&A Question 118】April 28, 2017

Air-open valves are mainly used for regulating the flow rate or pressure of ordinary materials. Air-close valves are used for gas transmission in sealing systems, especially for materials that need to be discharged urgently in case of power outages; their function is to prevent a sudden increase in pressure inside the equipment due to blocked passages, which could lead to accidents. It is also necessary to take into account the relevant requirements of the process!

Air open is also known as 【Fault Close】—FC, while air close is known as 【Fault Open】—FO. In fact, in addition to FC and FO, there is also FL, which stands for fault retention. Any device has the potential to fail, but in order to ensure the safety of the production system in such cases, the design requires that the device remain in a defined state when it fails. For example, in the case of non-toxic and harmless pressure storage tanks, the control valves used for pressure relief are generally required to have FO functionality – that is, when the valve loses its power source, it should remain fully open to ensure that the equipment does not experience overpressure. Thereby avoiding accidents. FL, or fault holding, also has many applications; for example, the water supply control valve in boilers features such a function, as both a lack of water in the boiler (dry burning) and an excess amount of water (excessive liquid level causing water to enter the steam system) are serious accidents. That’s why some boilers feature a position-maintaining design.

1. To meet the requirements for safe production under different process conditions; 2. The safety of the process equipment in certain accident scenarios; 3. Reducing losses in the process or power supply as well as ensuring product quality in accident situations; 4. Ensuring the safety of the media used

Whether to use a gas-on valve or a gas-off valve should be determined based on the medium involved in the specific process flow and production safety considerations. When the gas is available, it is usually said that \"it will operate when there is gas\", that is, \"FC\" (off in fault condition) ； The air valve is usually operated on the principle of \"closing when there is air\", that is, \"FO\" (open in fault condition)

The valve opens with wind; when there is no wind, it closes completely. Air shut-off valve: fully open when there is no air flow.

1. Process production safety； 2. Ensure product quality and reduce economic losses.

1. To meet the requirements for safe production under different process conditions; 2. To ensure the safety of the process equipment in certain accident scenarios; 3. To reduce losses in the process or power supply as well as to maintain product quality in accident situations; 4. To ensure the safety of the media used

Only by checking the condition of the valve core or examining the nameplate can it be determined accurately whether it is air-operated open or air-operated closed； For membrane control valves, air-open and air-close refer to whether they open or close when supplied with air ； There are two types of actuators for pneumatic diaphragm control valves: in one type, air enters from below, and the valve stem moves upward when air is supplied; this is known as \"direct-acting\". In the other type, air enters from above, and the valve stem moves downward when air is supplied; this is known as \"reverse-acting\"” ； The valve body structure of control valves is complex; when the valve is opened, the valve stem moves either upward or downward. Some control valves have a flange at the bottom, which is of the same size as the upper valve cover. By swapping the positions of these two flanges, the direction in which the valve can be opened or closed can be reversed (which is actually quite troublesome) ； Only when the actuator and the valve body work together can it be determined whether the valve operates on air open or air close principle. Among different types of control valves, the situation where the valve opens during air intake occurs in a little over half of them, but it accounts for a very large proportion in terms of the total number of valves in use. (Don’t let experience hold you back.) Double-acting and single-acting actuators exist only in cylinder-type actuators; diaphragm valves do not have a distinction between single-acting and double-acting (they are all single-acting). In a single-acting piston, there is a spring at one end of the piston, while the other end is connected to the air supply; the compression of the spring occurs when air is supplied, and the piston returns to its original position due to the spring’s force. The position of the piston is controlled by the pressure of the air. In the double-acting type, air is introduced from both ends of the piston, and the piston’s position is controlled by the pressure difference between those two ends. Cylinder valves are mostly controlled in two-position mode; continuous control must be achieved through a positioner. Cylinder valve outputs come in two types: angular stroke and linear stroke. For those with angular stroke, it is straightforward to determine whether they operate on air open or air close principle – one only needs to look at one end of the valve. Linear stroke valves can be compared to diaphragm valves.

Safety considerations: When the signal pressure is interrupted, it is necessary to ensure the safety of process equipment and production. If the valve is in the open position when the signal is interrupted, it is safest to ensure a continuous flow of fluid; in such cases, a air-operated shut-off valve should be used ； If the valve remains in the closed position after the signal pressure is lost, and it is not safest for fluid to pass through, then a air-actuated valve should be used.