Propane is a widely used source of energy for heating homes, cooking outdoors, and fueling commercial operations. Because of its physical properties, proper storage is necessary to manage its behavior and ensure user safety. Understanding how this fuel is contained is the first step toward safe and efficient use.
The Physical State of Propane
Propane is classified as a liquefied petroleum gas (LPG) because it is stored as a liquid but used as a gas. While propane is naturally a gas at standard temperatures, it is compressed into a liquid for storage and transport. This phase change allows large amounts of fuel to be stored in relatively small containers, as the liquid occupies significantly less volume than the gaseous form.
The pressure inside the tank is directly related to the surrounding temperature. When the temperature increases, the liquid propane warms up, causing more of it to vaporize into gas, which increases the internal pressure. This means a tank sitting in the sun on a hot day will have a higher internal pressure than one stored in the shade.
To safely accommodate thermal expansion and the resulting pressure increase, containers are only filled to about 80% of their total capacity. This 20% reserve space is a mandated safety cushion, allowing the liquid to expand without dangerously stressing the tank walls. Propane expands in volume nearly 17 times more than water over the same temperature increase, making this reserve space essential.
Common Container Types
Propane containers fall into two primary categories, regulated by different standards based on size and intended use. Portable cylinders are typically regulated by the Department of Transportation (DOT) and are designed for transportation and temporary use. These cylinders are commonly used for outdoor grills, RVs, and smaller appliances, ranging from small disposable canisters to larger 20-pound barbecue tanks.
Fixed tanks, known as ASME (American Society of Mechanical Engineers) tanks, are built for permanent, stationary installations and are regulated by rigorous standards. These larger tanks are used for residential home heating, commercial applications, or to supply backup generators. ASME tanks are generally installed horizontally and can range from 120 gallons up to over 1,000 gallons in capacity.
Both container types are constructed from durable materials like steel or aluminum. DOT cylinders require periodic requalification testing to confirm their integrity for transport. In contrast, ASME tanks are designed to be permanently mounted and do not have the same recertification requirements.
Crucial Safety Guidelines
Propane containers must always be stored outdoors in a well-ventilated area, as the vaporized gas is heavier than air and can pool in low, enclosed spaces. Storing tanks inside a building, including a garage, basement, or shed, presents a significant hazard because a leak could lead to a dangerous buildup of gas. The tank must always be secured and kept in an upright position to ensure the pressure relief valve functions correctly and prevent liquid propane from escaping.
Tanks should be placed on a non-combustible, flat surface, such as concrete or gravel, and kept away from ignition sources. Direct sunlight increases internal pressure, so storing tanks in a shaded area is highly recommended. Proper distance requirements mandate that smaller tanks (under 125 gallons) be kept at least 3 to 5 feet away from buildings and ignition sources, with larger tanks requiring greater separation.
Regular maintenance is also an important part of safe storage. Portable cylinders have requalification dates stamped on them, indicating when they must be inspected and certified for continued use. For all containers, the area around the tank must be kept clear of debris, vegetation, and flammable materials to ensure clear access and prevent fire hazards.