A propane tank is hardly a device that comes to mind when you think of ice forming. However, the unique safety aspects of a propane tank promote the accumulation of ice or frost around the tank. Although this comes from a safety effect, you need to safely remove the frost that forms so that the propane tank can be utilized properly.
Propane tanks have a safety feature called the Overflow Protection Device or OPD. The OPD minimizes small amounts of propane from trickling out of the tank when the tank is in use. However, through the laws of physics and the chemistry of propane, frost tends to form around the OPD.
The Venturi Effect is a physics property where a gas passing through a small opening tends to cool. This is because of both the pressure differences between the outside environment and the tank, and the narrow opening flow for the gas. As more propane exits out the opening, the tank may begin showing signs of frost.
Although the phenomenon might seem like a neat physics experiment, the Venturi Effect on a propane tank tends to reduce the quality of the propane gas inside. Therefore, it is in your best interests to remove the frost as soon as possible. One of the best solutions promoted by the Artist Resource on Fire, an art collective that uses fire-based tools to create art, is to submerge the propane tank partially in a bucket of warm water.
A matter that may complicate the frosting issue is an overfilled propane tank. When tanks are overfilled, the liquid propane is close to the OPD valve. This region is where the pressure between the outside and the inside of the tank meet. When liquid propane hits this threshold, frost may form within the valve itself. The only real solution is to bleed the propane tank to let some propane out. This would require the help of a professional.