Gas on and gas off

FO is the abbreviation for fail open, which means that the control valve opens in an emergency situation； FO refers to air-off, while FC refers to air-on.

The air-operated valve remains open when there is air in the diaphragm head. A pneumatic shut-off valve closes when there is air at the diaphragm head. FO means the air supply valve is open, while FC means the air supply has been cut off and the valve is closed.

With gas supply, it operates in fail-close mode; when there is no gas supply, it shuts off. (Electric valves operate in fail-open mode – they open when there is no power supply – but electric valves generally do not have this function and are instead used for maintaining a set position.) With gas supply cut off, it operates in fail-open mode; when there is no gas supply, it opens. (Electric valves behave in the opposite way.) The meanings of \"accident\" and \"fault\" are clear from their literal translations in Chinese. Choosing between FC and FO is a decision based on safety considerations. For example, in a reactor that is heated by steam, a control valve is used to regulate the steam flow. If there is a problem with the air supply to this control valve, it needs to shut off to prevent an increase in temperature that could lead to an explosion. In such cases, an FC control valve should be used. I hope you understand this explanation

Open when there is air: it opens when air is present； ==FC Gas shut-off: shuts off when gas is present ； ==FO FO: Open without air ； ==Air shut-off FC: shuts off when no air is present ； ==Gas turned on

I have a vague memory; I studied it further*.

Try searching on forums~ Refer to the following posts: http://bbs.hcbbs.com/viewthread.php?tid=330518&highlight=%C6%F8%BF%AA http://bbs.hcbbs.com/viewthread.php?tid=193368&highlight=%C6%F8%BF%AA

It’s all explained in detail on the FO FC floor; now I’ll explain the relationship between the forward and reverse actions of the regulator. First, it’s necessary to understand what the forward action of a regulator is – in simple terms, as the positive deviation increases, the output current also increases； The reaction is that as the positive deviation increases, the output current decreases. For example, if the control valve is set to the \"open\" mode, denoted as FC, it controls the liquid level in the tower: when the measured value increases from 50% to 60%, the positive deviation grows. If a positive-acting regulator is used, its output current increases as the deviation becomes larger, which in turn increases the current supplied to the valve positioner of the control valve. (Generally, valve positioners operate in a direct-acting manner; that is, when 4MA is output, the air pressure is 0.02 MPA, and when 20MA is output, the corresponding air pressure is 0.1 MPA). As the current increases, the air pressure also increases, and since it is an air-operated valve, the valve gradually opens. Another example: should the control valve be set to a shut-off mode, i.e., FO, or should it be used to control the liquid level in the tower? When the measured value increases from 50% to 60%, the positive deviation grows. If a positive-acting regulator is used, its output current increases as the deviation becomes larger, which in turn increases the current supplied to the valve positioner of the control valve. (Typically, valve positioners operate in a direct-acting manner: when 4MA is output, the air pressure is 0.02 MPA; when 20MA is output, the corresponding air pressure is 0.1 MPA). As the current increases, the air pressure also increases, and due to the fact that it is an air-operated valve, the valve gradually closes. Is it clear now? If you choose the regulator to be reactive, the above situation is exactly the opposite.

Faults and accidents are not the same concept. A fault refers to an incident in which mechanical equipment or systems lose their specified functions during use due to certain reasons; this concept includes both faults that have been identified and potential faults that have not yet been detected. An accident is also a type of failure; it occurs when equipment is damaged abnormally, resulting in shutdown or reduced performance, with the duration of the shutdown and the economic losses exceeding specified limits.

FO=FAIL OPEN = air-actuated valve that closes; FC=FAIL CLOSE= air-actuated valve that opens. FO and FC are determined by process engineers from the perspective of ensuring the safety of equipment and personnel, and are not related to instrument technicians. The positive or negative action of the regulator should be chosen based on the requirements of the process control. In most cases, FO corresponds to positive action, while FC corresponds to negative action; however, this is just a general rule, and the specific type of regulator required should be determined according to the actual operating conditions.

Opening: The control valve opens when the pneumatic signal increases. Air shutdown: The control valve closes when the pneumatic signal increases. FC: The valve closes in case of a fault. FO: The valve opens in case of a fault. The position of the valve in the event of a failure must be considered from the perspective of ensuring the safety of personnel and equipment.

The control loop must be a negative feedback loop!

In the absence of a gas source, electric control valves can be used. There are two types of electric control valves: 1. Operation mode (electrically open type, electrically closed type). The operation mode of control-type electric actuators is generally electrically open type (taking 4–20mA control as an example, the electrically open type means that a 4mA signal corresponds to the valve being closed, while 20mA corresponds to the valve being open); the other type is electrically closed type (also using 4–20mA control as an example, the electrically closed type means that a 4mA signal corresponds to the valve being open, while 20mA corresponds to the valve being closed). In general, the working mode must be specified when selecting a model. 2. Signal loss protection: Signal loss protection refers to the mechanism by which, in the event that a control signal is lost due to faults in the lines or similar issues, the electric actuator operates the control valve to a predetermined protective position. The common protective positions are full open, full closed, or maintaining the current position.

The answers for the first 10 floors are already very accurate. It can be turned off for everyone to learn from*.

Regarding FO/FC, everyone’s explanations were very thorough； I’d like to add some thoughts on the orientation of regulators – front, back, left, and right. Modern DCS systems allow the setting of air-on and air-off modes to be configured on their AO cards, with the determination of whether air is on or off being handled internally by the DCS. So it becomes simpler for us to determine whether it is direct or reverse action: if the positive deviation increases, the control valve needs to be opened more, which indicates direct action; otherwise, it is reverse action.

The selection of air shut-off and air open valves is based on considerations of process safety

This issue puzzled me for some time; later, through research, it was found that FO and FC represent the fault condition valve positions in process design. What we refer to as air-operated or air-actuated when designing instruments actually denotes the operating state of the valve’s actuator when it is supplied with air or not. In simpler terms, it simply means that the actuator is designed to be of the air-operated or air-actuated type, and this has nothing to do with the actual state of the valve! ! !

A valve that closes when there is a pressure signal and opens when there is no pressure signal is of the air-operated closed type. Conversely, it is of the air-open type. Selection requirements: Primarily based on safety requirements in the manufacturing process. When the signal pressure is interrupted, the safety of the equipment and operators must be ensured. If the hazard is low when the valve is in the open position, a air-operated shut-off type should be chosen, so that in the event of a failure in the air supply system and a disruption of air supply, the valve can open automatically to ensure safety. Conversely, if the valve is less hazardous when closed, a normally open valve should be selected.

When choosing between air-open and air-close valves, the decision is first made based on your process requirements. Then, determine whether the valve needs to be opened or closed based on the required adjustment change, in order to select the regulator

\"Gas open\" refers to the valve opening in the event of a gas supply interruption, while \"Gas close\" means the valve closing in such an incident