If you have an older propane tank that remains free of rust, you can rest assured that the tank cylinder will pass inspection. However, if the tank does not have an OPD or overfill protection device equipped fill valve, the tank is useless and will not pass inspection, regardless of the tank's condition. Some people simply turn in the old tanks and purchase new, OPD-compliant propane tanks. You can avoid this cost by replacing the old valve with an OPD-compliant valve to bring your current propane tank up to inspection standards.
Utilize any propane left in the tank as you normally would. Do not open the valve and expel the propane into the air. Once completely empty, turn the valve handle clockwise to shut off the valve.
Position the tank in a vice or use some other method to immobilize it. You can place the tank on its side on the ground and have a friend put his weight on the tank to keep it from moving.
Heat the soft-compound weld around the threads between the cylinder tank and the cylinder valve. Do not use a blowtorch, Two or three minutes with a heat gun will soften the weld.
Open the valve on the tank to being the inside of the tank to equal atmospheric pressure as you have outside the tank.
Place the jaws of a pipe wrench around the valve body and then slide a 2-foot-long steel pipe onto the handle. The pipe acts as a "cheater bar" and gives you more leverage.
Turn the valve counterclockwise to remove it from the threaded bung on the cylinder. Discard the old valve.
Inspect the inside of the tank with a flashlight for rust. Even though the outside of the tank looks rust-free, you might find rust in the inside. If you locate rust, the tank is not safe, so discard it and purchase a new one. If you do not see any rust, add 2 or 3 ounces of anhydrous methyl alcohol, which you can purchase at most places that sell paint. This will soak up any moisture in the tank.
Wrap yellow natural gas-approved Teflon tape around the threads on the OPD valve.
Thread the valve into the bung on the top of the cylinder and then tighten it with the pipe wrench and metal pipe.
Tell the person filling the tank for the first time after valve replacement that the tank requires an air purge and a leak test. All licensed fill stations can take care of this for you.