Liquid propane gas valves are designed to release the propane. The overfill protection device valve releases a flow of propane when hooked up to a hose end connection and opened. Safety relief valves release propane if a dangerous buildup of pressure occurs inside the tank.
The protection valve is designed so that it does not release a flow of gas unless it is hooked up to a hose end connection. If a user turns the flywheel without hooking up a hose, or hooking it up too loosely, nothing happens. This is a safety feature that prevents propane gas from being released into the air.
LP gas tanks contain either an internal pressure relief valve or an external relief valve. External valves are more common on older propane tanks.
A strong spring holds the relief valve closed. The pressure inside the tank must remain less than the pressure of the spring. If the pressure inside the tank increases to that of the spring, it releases the valve. The relief valve then opens, releasing excess propane gas with a hissing sound. This hiss alerts users to the presence of gas. However, if the internal pressure rises dramatically, the valve opens with a loud pop and a whoosh of propane gas. Either way, once the internal pressure of the tank is stabilized, the valve closes again.