In central supply systems using gas cylinders, each header shall have a ________ valve to prevent backflow and to permit service to the header.

• A. Relief
• B. Bypass
• C. Ball
• D. Check

Answer: (D)

In central gas supply systems, preventing backflow into the cylinders or upstream lines is essential for safety and system integrity. A check valve is built to do just that: it allows gas to flow toward the header but automatically closes if downstream pressure pushes backward, stopping any reverse flow. This one-way action helps protect other cylinders and regulators and keeps the header available for service without risking gas migrating backward. The other valves don’t provide this automatic backflow protection: a relief valve vents excess pressure, a bypass valve redirects flow around equipment, and a ball valve is a general shut-off without inherently preventing reverse flow.