Where is the relief valve on a propane tank positioned?

• A. Vapor space
• B. On the valve stem
• C. At the top of the tank
• D. Along the liquid line

Answer: (A)

The safety relief device on a propane cylinder is designed to vent gas when the tank heats up and the pressure in the vapor space increases. Since propane is stored mainly as a liquid with a small amount of vapor above it, the pressure rise occurs in that vapor space. Venting the vapor keeps the liquid inside the tank and prevents a dangerous liquid spray. Therefore the relief valve must be connected to and venting from the vapor space, typically mounted on the cylinder head as part of the pressure-relief assembly. Mounting it on the valve stem or along the liquid line wouldn’t provide proper protection and could release liquid or fail to relieve pressure correctly. Hence, the relief valve is positioned in the vapor space.