Thanks for the logical reason, noted and won't do again thanks!Actually just saw this post - should have reacted earlier. Very dangerous to lie a cylinder flat ! Never ever do this.
The Pressure Relief Valve is incorporated into the main gas valve on the bottle.
The reason is that you immerse the pressure relief valve in liquid if a full bottle is laying flat (propane in the bottle is liquid).
If the pressure inside increases (fire, heat, impact) the pressure relief valve releases some of the gas to relieve the pressure.
When the bottle is full and lying flat, it can't work and the bottle can explode if pressure were to accidentally increase.