If you do install in-line valves, this is the safest way to purge the lines before opening them up to install the valves: Light a burner on the stove, then while the burner is lit, go out to the tank and shut off the propane at the tank. The lit burner will go out by itself when it burns up the remaining proane in the line. This reduces the amount of propane in the lines to the barest minimum before you crack open the line to install the valve. Then, don't forget to shut off the burner before you turn the propane back on at the tank.
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