In a cryogenic system, the gas produced by evaporation from the secondary header enters the supply system upstream of the final line regulator. Which is the correct description of its relation to the final regulator?

• A. downstream of the final regulator
• B. upstream
• C. upstream of the filter
• D. downstream of the filter

Answer: (B)

Understanding flow direction is key here: in a gas supply line, the final line regulator is the last pressure-reducing valve before the distribution network that delivers gas to devices. If gas enters the system before that regulator, it is described as upstream of the regulator.

So, gas produced by evaporation from the secondary header feeds the supply upstream of the final regulator, meaning it is before the regulator and has not yet been reduced to the final delivery pressure. The regulator’s job then is to take this higher-pressure upstream gas and drop it to the safe, stable downstream pressure used in the pipeline.