Because the safety relief valve performs such a vital role in protecting the propane tank, it should be maintained and repaired by licensed propane professionals only.
Do not ever look into a relief valve or stand over it. Tanks must be empty before relief valves can be replaced. Safety relief valves need to be replaced if they “pop off”, meaning that the valve has opened. A protective cover should be placed over the relief valve to ensure proper operation. Suppose the valve has debris in the upper mechanism of the relief valve and it opens. In that case, the debris may cause the valve to remain open, resulting in a potentially dangerous situation and/or excessive loss of propane. A simple protective relief valve cover can be made from a plastic bag fastened around the external portion of the valve with a rubber band.
If you encounter a relief valve that seems “hissing” (opened to relieve pressure), do not tap or mess with it. If the tank has been recently filled in hot weather and the face gauge reads over 80 or 85%, spray some water from a hose on the tank. This may cool it down and reduce the expansion of the liquid in the tank. Call your propane company immediately and inform them of the situation.
Industrial Propane Cylinders & Tanks FACT SHEET.pdf