What Causes a Propane Tank to Frost Up?

Do you ever wonder what causes the frost on the side of your propane tank? The answer to this question is not that simple. To accurately answer it requires the understanding of some physical science behind propane.

Propane is also known as liquefied petroleum gas. As the name implies, it is a liquid before it is a vapor, or gas. Water boils at 212 degrees Fahrenheit and freezes at 32 degrees at sea level or normal atmospheric pressure. Propane, however, boils at -44 degrees, turning from a liquid to a gas, or vaporizing.

Inside the tank, the liquid propane is under pressure, which prevents it from boiling. When you open the tank valve to begin using the propane, the pressure drops and causes the liquid to vaporize. As it does this, it draws heat from the surrounding air through the metal wall of the tank, making the metal instantly cold. Humidity (water vapor) in the outside air condenses and freezes on the cold surface of the metal; that's the frost you see on the tank. The frost accumulates at the liquid line, or the level of liquid inside the tank.

Pressure and temperature rise and fall together. The greater the flow of the vapor through the tank valve, the more rapidly the tank pressure drops, creating a drop in temperature and cooling the outside of the tank. The humid air will freeze or frost due to the extreme lower temperatures at the liquid level inside the propane tank. So, keeping your tank warm and filled in colder climates will keep the pressure up, helping to eliminate problems, especially during extreme cold weather.

The first issue concerning frost is how rapidly the propane is leaving the tank. When propane leaves the tank too rapidly, it causes the temperature in the tank to drop, which can cause frost on the tank at the level of the liquid, where it is changing from a liquid to a vapor.

If you have ever used a can of compressed air to clean your computer keyboard and have seen frost build up on the side of the can, then you may understand the concept. Much like the compressed air, this frost is directly related to the rate at which the vapor is leaving the tank, so even on a warm day, you can see the frost if the propane is leaving the tank too quickly.

Frost on a propane tank is not dangerous or abnormal, but it is something about which you should be conscious.

Frost can also build up if the propane tank is overfilled or if it is not in the position for which the tank is designed. Most propane tanks are designed to be in the upright position and should be transported in the upright position as well. For example, a barbecue tank should always be vertical with the valve on the top. Consider a whole-house tank in the horizontal position and note that the valve is centered on the top of the tank. The manufacturer has designed the tank for best use of the liquefied petroleum gas. So be sure to position the tank in the way that it is designed.

Propane tanks manufactured after April 1, 2002 are equipped with a special valve that prevents the overfilling of the tank. In addition, all tanks are certified for safety and are stamped with a date code that indicates the date of manufacture or a recertification date. Tanks should be filled and used only when their certification life has not expired. For example, a barbecue-style propane bottle can be filled for 12 years after the manufacture date or 5, 7, or 12 years after it is recertified (based on the type of bottle and recertification). Propane tanks must be filled at appropriate facilities by personnel who are properly trained in filling procedure.