Which gas is used to purge lines during maintenance?

• A. Oxygen
• B. Argon
• C. Helium
• D. Nitrogen

Answer: (D)

Purging lines during maintenance is about displacing air and any vapors with an inert, dry gas to prevent ignition and moisture-related problems when the line is opened or tested. Nitrogen is the best choice because it’s inert, dry, and inexpensive, and it won’t react with fuels or hydraulic fluids. It effectively pushes out oxygen and moisture, reducing fire risk and corrosion. Other gases either react with contents (oxygen), or are less practical due to cost or availability (argon is inert but more expensive and not as practical for routine line purging; helium is inert but costly and not typically used for purge). So, using dry nitrogen provides a safe, effective purge for maintenance lines.