Regarding combustible gas installations
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The design specifications for chemical process control rooms state that air intake openings for air conditioning in the control room, doorways of outdoor doors, cable trenches, and openings in buildings where cable trays are located – wherever flammable gases may enter – must be provided. Are you referring to the area at risk of explosion? In outdoor doorways, if they are located in areas at risk of explosion, such structures are indeed installed. But how can there be openings in cable trenches and cable trays within buildings? Aren’t those all sealed off? Control rooms are generally located outside the explosion-hazardous areas; even if fresh air systems are in place, they are still outside such areas. Doesn’t that mean it’s impossible for gases to get in? If the reason is to draw in gases through exhaust vents, then since exhaust vents are installed in all areas of the plant, there’s no need to consider the possibility of flammable gases entering those areas. The controller is already located outside the explosion-hazardous zone, and the concentration of gases there is already very low; so how could exhaust vents allow gases to accumulate without spreading? I don’t quite understand the meaning of this clause
The original poster’s question is very professional; it’s clear that they pay close attention to the details of the regulations. I’d like to talk a bit from a practical engineering perspective: the key aspect in the standards regarding the possibility of flammable gases entering is the word \"possible\" – it doesn’t apply only to areas with explosion hazards. Although the control room is located outside the explosion hazard area, if there are openings such as air inlets or cable trenches, flammable gases can still spread in under extreme conditions (such as changes in wind direction, pipeline leaks, or pressure fluctuations). A low concentration does not mean absolute safety, especially since these gases can accumulate in enclosed spaces. Cable trenches and cable trays in the building area do indeed have seals, but many of these seals develop gaps due to construction work, aging, or settlement, so they are not completely airtight. The requirement to install detectors is essentially a way of taking into account the risk of leaks, and it constitutes preventive design. Regarding the installation of detectors at air intakes, it is not necessary to install them at all air intakes in a facility; rather, they should be installed in areas such as control rooms, where there are many people present and the consequences of a leak could be severe. If it is just an intake vent in a regular workshop, the risk level varies, and accordingly the regulatory requirements also differ. It is recommended that the original poster check the explanatory notes in the \"Design Code for Petrochemical Control Rooms\" (SH/T 3006-2012) or the \"Design Code for Chemical Industry Control Rooms\" (HG/T 20508-2014); these documents usually provide explanations regarding the situations in which combustible gases may enter such rooms. You can also consult the safety specialists at the design institute; they will determine, in the course of conducting HAZOP analyses, which openings require detectors to be installed.
Core principles for combustible gas systems: Select detectors based on the type of gas, determine the installation height according to its density; an emergency shut-off valve must be connected in series, and independent systems must not be connected to the fire alarm bus.