In the interlocked reaction of gasoline hydrodesulfurization, why is the emergency pressure relief valve set to FC (fail-closed)?

Whether it should be FC or FO depends on the process requirements.

I made a mistake; it should indeed be FO

The emergency pressure relief valve should be closed when it’s working properly, right? In that case, FC makes no sense; it must be a mistake – it should be FO instead

FO: If there’s no instrument air, the pressure relief valves will open fully, leading to material leakage, won’t that happen? So what is the fate of the safety valve? If the instrument air supply for the entire unit is lost, but the pressure relief valve is equipped with an air storage tank, then air available in that tank can be used to open the valve and allow pressure to be released for a while, thereby reducing the pressure in the equipment and preventing the safety valve from activating. Even in the worst case, if all the tank air is used up and the FC valve closes while the pressure in the equipment continues to rise, then the safety valve should activate; it’s time for it to do its job. The pressure relief valves are set to FO: If the instrument air supply for the entire system is lost, all pressure relief valves will open and vent the gas to the flare stack, which does ensure safety. But what does your boss think about this? The safety valve didn’t activate – why did your gauge start working first? Can I hold back the adjustments on my process control screen to prevent the safety valve from activating? For a device (system) equipped with both a pressure relief valve and a safety valve, FC can be considered; in this case, economic losses are taken into account. @jiaguoyun

The original text of the gasoline hydrogenation process instructions states: “The emergency pressure relief valve is designated as FC; the instrumentation department shall provide backup purge air facilities to ensure at least three strokes of the emergency pressure relief valve or 30 minutes of operation time.” Shouldn’t emergency pressure relief valves be of the fail-open (F.O) type? If the medium is flammable and explosive, then there’s no problem. This issue is related to the specific production conditions and production safety.

During the HAZOP phase, one of the topics is pressure relief protection, and the pressure relief valves FC/FO are a subject of discussion. Because the safety valve triggers at the last stage, and considering economic losses, FC is used more frequently. If FO is used, it indicates that other protection methods are \"unreliable\"; pressure relief must come first. In the case of SIL 3 systems, two FO pressure relief valves may be required – since other methods are unreliable, the hope lies in the pressure relief valves. Although the system loses pressure, the presence of a gas storage tank allows the valves to be opened to release pressure; if it’s possible to withstand that pressure, there’s no need to use FO for release. @jiaguoyun