In a cryogenic liquid container system, the gas produced by evaporation from the secondary header shall enter the supply system upstream of which component?

• A. not required
• B. upstream
• C. upstream of the filter
• D. downstream

Answer: (B)

The main idea here is ensuring the gas that boils off from the secondary header is clean before it travels through the rest of the supply system. Boil-off gas can carry particulates, moisture, and other contaminants. Introducing that gas into the supply line upstream of the filter means it must pass through the filtration element as it continues downstream, so contaminants are removed before reaching downstream components. If the gas were injected downstream of the filter, it would bypass filtration and could introduce impurities into the system, risking contamination, valve and regulator issues, or degraded gas quality. We know filtration is a standard step in maintaining purity, so the correct placement is upstream of the filter to ensure the gas is filtered before continuing through the system.