Newer propane tanks come equipped with an overfill protection valve that prevents the propane canister from being overfilled. The valve works using a float inside the tank. When the level of liquid propane reaches the limit, the float rises and the valve closes. This prevents more propane from being added to the tank. Since April 2002, propane tanks without OPD valves cannot be refilled. It is possible to convert a tank to the new valve, but you will need to have the tank re-certified before it can be refilled.
Items you will need
Ratcheting straps or rope
Blowtorch or heat gun
Crowsfoot wrench
Flashlight
Gas rated Teflon sealant tape
Methanol
Empty the propane tank completely. Make sure there is no propane left inside. Open the valve and let the pressure equalize.
Tie the tank down to a stationary object so that it cannot rotate. Stout trees work well. You can use ratcheting straps or heavy rope.
Close the tank valve as tight as you can. Then use a blowtorch or heat gun to soften the thread sealant around the old valve. Once the metal is warmed, turn off the blowtorch and wait a few seconds.
Grip the valve with a crowsfoot wrench and rotate the valve. You may need to use a lot of force on older tanks.
Shine a flashlight into the tank and inspect the interior once the old valve is out. If you spot any rust, the tank should be refurbished or discarded.
Add a small amount of methanol to the tank and swirl it around. This will remove any moisture in the tank.
Wrap the new valve with gas rated yellow, green or blue Teflon tape. The tape acts as a sealant.
Screw the new valve into place by hand. When it gets too hard to turn, use the wrench to realign the valve with the opening on the top of the tank.
Tell the operator that the tank needs to be leak tested and re-certified when you go to refill it. They will test the valve and cylinder and give you a certification.