It’s also about FO\FC
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Regarding the on/off status of the anti-surge valve in the main fan unit that was discussed today, as we all know it operates in FO mode: the valve remains open when there is no power supply and also when the gas supply is interrupted. But has anyone thought about what would happen to this valve if, despite neither a power outage nor a disruption in the gas supply, the control current signal suddenly fails? If 4 mA corresponds to a closed valve, that means the valve stays closed when the control signal is lost; in that case it can no longer be called FO mode. On the other hand, if 4 mA corresponds to an open valve, then during normal operation, the loss of the control signal would cause the valve to stay open, resulting in a shutdown. Is it really necessary for a shutdown to occur just because of the loss of a single control signal? . . . From this issue, we should properly consider the definition of an accident condition, and whether the loss of control signals needs to be regarded as a condition for an emergency stop. . Is what I said too messy? It’s my first time posting, so I hope everyone can take their time to read it and discuss this tricky issue once they understand it. . Thank you
I think your view is incorrect; it is the process engineers who determine whether a valve is malfunctioning and needs to be turned on or off, not the instrument technicians. Why is it necessary to have a valve fail-open/fail-close function? I think it is to protect the production system. If a valve is set to FO, it means that the process requires the valve to remain open in case of a failure; with the valve open, the impact on the process is not significant, allowing enough time to take action. If the system stops operating when it is in the open position, it indicates that the fault location of the valve has been set incorrectly.
The FO valve, also known as an air shut-off valve, has its valve in the open position at 4mA; therefore, in the event of a signal failure, the valve remains open. Whether it is a problem with the air supply, the power supply, or the signal, the control valve will stay in the open position, which meets the design requirements.
The brother upstairs is absolutely right; in fact, there’s another concept that you’ve confused. To achieve the security control method you want, all you need to do is change the way the valve operates. This can be done by modifying the DCS system, or by changing the actuator – so that 4 mA corresponds to the valve being open, and 20 mA corresponds to the valve being closed
Your questions are becoming increasingly unclear to me. I think the location of a valve failure and the direction of action of the valve are not the same thing; they shouldn’t be confused with each other.
Could you explain in detail the relationship between the \"location of the valve failure\" and the \"forward and reverse operating mode of the valve\"?
The fault position of a valve is the safe position where the valve is when a fault occurs, that is, the position set by the safety interlock In the structure of rotary valves, the linear extension of the actuator’s push rod moves the ball or valve disc toward the closed position, which is referred to as direct action ； The linear extension of the actuator rod moves the ball or valve disc toward the open position, which is known as reaction.
Gas open, gas close, and FO FC are two distinct concepts. From a process perspective, gas open and gas close are determined first, after which the forward or reverse action of the actuator is decided to create a negative feedback loop in the control system, determining the behavior of the valve in cases of power or gas supply failure. FO FC refers to the interlock action that takes place in an emergency situation; it means that an interlock is triggered when something goes wrong in a certain section or due to faults in the valve itself (such as signal issues, loose wires, or a stuck valve core, which leads to abnormalities in pressure, temperature, or flow rate), according to my understanding!
FO/FC refers to the valve switching state when the valve is in a faulty condition; this can be caused by issues with the air supply or electrical signals.