Freezing and frosting of propane regulators is quite common and usually nothing to be concerned about provided everything is in working order and operating as it should. Frost can form on regulators connected to both propane cylinders and bulk (stationary) LP Gas tanks. If in doubt about the safety of your regulator, turn off the tank service valve and contact your propane company. Further reading will help propane users understand the reasoning and causes of a "freezing" regulator.
During normal operation propane regulators can become covered in frost, which may alarm some users. While this "freezing" of the regulator may be a symptom of an actual problem, it usually is a sign that outside humidity is at a level capable of producing condensation. The only difference is, the condensation forming on a regulator is frozen. As described, propane regulators act as a barrier between high tank pressures and delivery pressure as required by downstream appliances and/or equipment.
Once a propane appliance is actively in use, the liquid propane in a tank or cylinder begins to boil. The propane vapor, as boiled off the top of the liquid begins its journey downstream to the point at which it is used. Before making its way to the LP Gas system piping, it passes through the regulator where its pressure is reduced to a usable level. Keep in mind that the regulator will only deliver a constant pressure on the outlet side while inlet pressures can significantly vary. As the propane passes through the regulator, it expands (resulting in sub zero temperatures) and causes the regulator to gradually reach the extremely cold temperature of the propane vapor passing through it. Depending on the temperature and humidity of the surrounding air, the regulator will produce condensation, much like that of a frozen mug or glass taken out of a freezer.
This is why, under normal operation in hot and humid climates, the external surface of a regulator will freeze and appear to be frozen or frosted. The rate at which propane is being withdrawn from the tank or cylinder will also cause the container to display a visible frost line, which indicates the liquid level of the propane within the tank.
Although regulators can freeze under normal and "proper" operating conditions, there are times when regulators are freezing because of actual problems. One of the problematic issues causing a regulator to freeze is due to liquid propane entering and passing through the regulator. Liquid propane can produce an effect of extreme freezing when introduced abruptly into a regulator. There are two ways that liquid can be delivered through the tank (or cylinder) service valve: 1) If the container is overfilled or, 2) If the tank, usually a bottle, is not upright with the service valve communicating with the vapor space of the container. Both of these scenarios are possible and while avoidable, are not very common.
These freezing regulator problems both involve one thing; that one factor is liquid propane. For this reason, cylinders and tanks should always be located and positioned as designed for use so that not only liquid propane is kept out of the regulator but also, is kept out of the downstream appliances designed to work with propane vapor. Additionally, regulators that are frozen due to tank or cylinder overfilling pose the same problem as an improperly positioned container. Propane cylinders equipped with OPD valves are designed to prevent this problem but cylinders and tanks that are not equipped with OPD valves can be filled completely with liquid propane and result in liquid flowing through the service valve, into the regulator and downstream to appliances designed to work with vapor. Again, this is not common and is not probable, but it is possible. If you feel that your tank has been overfilled by looking at the gauge, opening the bleeder valve and seeing the frost on the regulator, contact your propane company after closing the container service valve.