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Propane Fuel Basics

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Also known as liquefied petroleum gas (LPG) or propane autogas, propane is a clean-burning alternative fuel that's been used for decades to power light-, medium-, and heavy-duty propane vehicles.

Propane is a three-carbon alkane gas (C3H8). It is stored under pressure inside a tank as a colorless, odorless liquid. As pressure is released, the liquid propane vaporizes and turns into gas that is used in combustion. An odorant, ethyl mercaptan, is added for leak detection. 

Propane has a high octane rating, making it an excellent choice for spark-ignited internal combustion engines. If spilled or released from a vehicle, it presents no threat to soil, surface water, or groundwater. Propane is produced as a byproduct of natural gas processing and crude oil refining. It accounts for about 2% of the energy used in the United States. Of that, less than 3% is used for transportation. Its main uses include home and water heating, cooking and refrigerating food, clothes drying, and powering farm and industrial equipment. The chemical industry also uses propane as a raw material for making plastics and other compounds.

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Propane as an Alternative Fuel

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Interest in propane as an alternative transportation fuel stems from its domestic availability, high-energy density, clean-burning qualities, and relatively low cost. It is the world's third most common transportation fuel, behind gasoline and diesel, and is considered an alternative fuel under the Energy Policy Act of 1992.

Propane used in vehicles is specified as HD-5 propane and is a mixture of propane with smaller amounts of other gases. According to the Gas Processors Association's HD-5 specification for propane, it must consist of at least 90% propane, no more than 5% propylene, and 5% other gases, primarily butane and butylene. 

For vehicle fueling, the quick-release "Type K15" dispenser connector is required to be installed on all new vehicles beginning January 1, 2020, per National Fire Protection Association Code 58. This connector allows for one-handed fueling and does not require the use of personal protective equipment such as gloves and face shield (which are required for the older style connector).

Propane is stored onboard a vehicle in a tank pressurized to about 150 pounds per square inch—about twice the pressure of an inflated truck tire. Under this pressure, propane becomes a liquid with an energy density 270 times greater than its gaseous form. Propane has a higher octane rating than gasoline, so it can be used with higher engine compression ratios and is more resistant to engine knocking. However, it has a lower British thermal unit rating than gasoline, so it takes more fuel by volume to drive the same distance.

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Properties and Characteristics

 

A Material Safety Data Sheet (MSDS) is available from propane suppliers or distributors and must be available and accessible to all employees at the workplace. The MSDS provides important information on propane including physical properties, health effects, first aid, safety precautions, and personal protective equipment (PPE). This program will discuss information from the MSDS that relates to your job of dispensing propane safely. Propane is either a liquid or a gas depending on the amount of pressure it is stored under. To keep propane as a liquid above its normal boiling point, it must be stored and transported in pressure-tight containers. Liquid propane stored in containers at ambient temperatures will boil off and become a vapor that occupies empty space in the container. This vapor is what is used in customer appliances and equipment. Like water, liquid propane will expand when heated. However, liquid propane will increase in volume nearly 17 times greater than water will. To allow for this expansion, propane containers are filled typically to only 80% of their capacity. If liquid propane is released into the air, the lack of pressure quickly causes it to vaporize and expand to 270 times its original volume. Therefore, liquid propane leaks can be more hazardous than vapor leaks. Propane is non-toxic, but its vapor is still dangerous to inhale because it displaces oxygen. Since propane vapor is 1-1/2 times heavier than air, propane released in a confined space may initially remain in low-lying areas. However, if there is sufficient air movement, especially outdoors, the vapor will quickly dissipate in the air. When released into the atmosphere, liquid propane has a refrigerating effect that makes everything it touches extremely cold. This means if it comes in contact with your skin, it can cause third-degree or deep-freeze burns. For this reason, you should wear gloves or other PPE resistant to propane when filling containers. Your employer may require additional safety equipment, depending on your specific responsibilities. For more information on PPE see the resources section (Module 10). Every time propane is released, there is potential for hazard. Three ingredients are needed to start and sustain combustion—propane, oxygen, and an ignition source. All three ingredients must be present for combustion to occur, and the ignition source must provide enough heat to the propaneoxygen mixture to raise the temperature of propane to its ignition point. In order to minimize possible ignition sources that could lead to combustion, customers should be restricted from the immediate area around the liquid propane transfer areas.