Regarding the selection of fault closed (FC) or fault open (FO) for control valves
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This post was last edited by ouhanshu on 2017-7-14 at 13:56. While discussing the selection of control valves with an engineer, we talked about this issue; he said that in factories, fault-closed (FC) control valves are generally chosen, and only in a few processes with special requirements is fault-open (FO) used. But recently, while working on diagrams, I analyzed the situation and it seems that faulty valves are used in most cases; therefore, I think his statement isn’t entirely accurate. The selection of control valves should be based on the process requirements. He also provided diagrams for the fault-on and fault-off states. As shown in FO, the flow guide valve is located on the side of the valve body that faces the flow direction FC; the flow guide valves are at both ends of the valve body. I hope those who are familiar with this can offer some suggestions
The diagram is not visible. Generally, for heat supply and material transfer, FC is used; for pressure relief in high-pressure towers and cooling water, FO is used. The choice depends mainly on process safety considerations – whichever method ensures better safety is used
In fact, it is related to the level of risk in the upstream and downstream environments in which the material is located. For example, the main fuel gas for a heating furnace is definitely equipped with an emergency shut-off valve: the valve opens when air is supplied, and it closes once the air supply is interrupted. Personally, from the perspective of processing conditions, which one should be used for safety purposes?~~~
There is no dominant view. The fault-off and fault-on modes of control valves are determined based on production safety principles; when there are no safety concerns, the principles of energy and material savings should be followed ； It is determined by the properties of the medium during the production process (such as high viscosity, tendency to crystallize, etc.).
Generally, fault-off modes are more common~~fault-on modes are less frequent~~There are also some cases where fault retention is required~~As for the number of fault switches in a particular process, it needs to be determined based on considerations such as safety, the process itself, and the equipment~~It’s important to understand the reasons behind this~~
I don’t know that much about it; I could only proceed by following the usual methods.
Just consider two words: safety. Choose the state that is safe, the one to switch to in case of a failure. It is safe when turned on; choose FO for fault condition ； For safe shutdown, choose fault shutdown FC ； If the operating conditions remain unchanged, choose FL.
The accident mode principle is: reduce temperature and pressure, and avoid external transfer; therefore, recirculation is enabled for normal faults, disabled for external transfer faults, disabled for fuel faults, and enabled for faults involving the material being heated.
For example, materials A and B are burned in reactor C, which is equipped with an emergency discharge valve. For safety reasons, the discharge valve of reactor C should be set to operate in emergency mode; however, the valves for materials A and B should be set to operate in shutdown mode, as feeding should be stopped after an accident, and the emergency discharge valve needs to be activated at that point. In short, everything should be arranged in a way that ensures safety – this is just my personal understanding. If you have any suggestions, please let me know.
The key factor is the role of the control valve in the pipeline, that is, its functional purpose. Some valves are used to control the flow rate and pressure at the outlet, but to ensure continuous material transfer, it is necessary to maintain operation even in case of failures, which is what is referred to as fail-open operation. If a material shortage occurs downstream and affects production, it is necessary to fail the mold opening. Failure is exactly the opposite.