Issues regarding the installation of hydrogen fluoride gas detectors
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Regarding the issue of the installation height for hydrogen fluoride detectors, the drawings provided by the design institute are based on the assumption that the molecular weight of hydrogen fluoride is lower than that of air; hydrogen fluoride is indeed designed to be lighter than air. Is this approach correct? Are there any relevant regulatory guidelines to guide this?
In accordance with GB/T 50493-2019 \"Design Code for Detection and Alarm of Flammable and Toxic Gases in Petrochemical Industries\", to determine whether a leaking gas is heavier than air, the ratio of the molecular weight of the leaking gas to that of ambient air should be used as a reference, and the following criteria shall apply: a) When the ratio is greater than or equal to 1.2, the leaking gas is heavier than air; b) When the ratio is greater than or equal to 1.0 but less than 1.2, the leaking gas is slightly heavier than air; c) When the ratio is between 0.8 and 1.0, the leaking gas is slightly lighter than air; d) When the ratio is less than or equal to 0.8, the leaking gas is lighter than air. When detecting flammable or toxic gases that are heavier than air, the installation height of the detector should be between 0.3 and 0.6 meters above the ground or floor ； When detecting flammable gases or toxic gases that are lighter than air, the installation height of the detector should be within 2.0 m above the source of emission. When detecting flammable gases or toxic gases that are slightly heavier than air, the installation height of the detector should be between 0.5 and 1.0 m below the source of emission ； When detecting flammable gases or toxic gases that are slightly lighter than air, the installation height of the detector should be 0.5 to 1.0 m above the source of emission. The installation height of an environmental oxygen detector should be between 1.5 and 2.0 meters above the ground or floor.
It’s fine to consider it based on molecular weight. You may have considered that hydrogen fluoride forms polymers whose density is greater than that of air, but in the event of a leak, both monomers and polymers are present; thus there are substances with a density lower than that of air as well as those with a higher density. In such cases, detection can be effective whether the device is installed above or below the source of release. By using the standard molecular weight as a reference parameter, it is appropriate to install the device above the source of release.