Cylinders should be positioned in customer vehicles so that the __________ is in communication with the vapor space.

• A. Main valve
• B. Gauge port
• C. Pressure relief valve
• D. Handle

Answer: (C)

Propane cylinders rely on a safety device that automatically vents gas if pressure gets too high. That relief valve must be connected to the vapor space inside the cylinder—the space where the propane is in gas form. When the cylinder heats up in a vehicle, liquid propane can flash into gas and raise pressure in that vapor space. If the pressure relief valve isn’t connected to or exposed to that vapor space, there’s nowhere for the excess gas to escape, increasing the risk of a dangerous overpressure situation. The main valve controls normal gas flow, the gauge port is for reading pressure, and the handle is for operating the valve—none of those provide automatic overpressure relief.