Every ten years, an LPG tank has to be re-tested. There will be a date (usually stamped on a tag attached to the tank) showing when the tank was last tested.
Automotive Experts will need to remove the tank from the vehicle and have a licensed LPG cylinder tester inspect the tank for damage and check the valves in the tank for safety and operation. The safety valve is always changed during a tank test as its function is to vent gas out of the tank if the pressure ever happens to build up to high. It is very important to change the safety valve as their correct operation cannot be guaranteed after ten years.
Your car is not roadworthy if your gas tank has passed its expiry date.