G3 - Gas Technician Level 3 Properties of Fuel Gases Questions and Answers 1 — Questions and Answers
Question 1: A gas technician is investigating a reported gas odor in the basement of a residence. Which property of propane (LPG) is most critical to consider in this scenario?
- It has a high heating value of approximately 2,500 BTU/ft³.
- It is colorless and odorless in its natural state.
- Its specific gravity is greater than air, causing it to accumulate in low areas. (Correct answer)
- Its ignition temperature is higher than that of natural gas.
Correct answer: Its specific gravity is greater than air, causing it to accumulate in low areas.
Propane has a specific gravity of approximately 1.52, making it heavier than air (specific gravity of 1.0). This means that if a leak occurs, the propane gas will sink and collect in low-lying areas like basements, creating a significant safety hazard. Natural gas, with a specific gravity of about 0.6, is lighter than air and would dissipate upwards.
Question 2: Which of the following statements accurately compares the heating value of natural gas and propane?
- Natural gas and propane have nearly identical heating values per cubic foot.
- Natural gas has a higher heating value per cubic foot than propane.
- Propane has a heating value of approximately 1,000 BTU per cubic foot.
- Propane has a significantly higher heating value per cubic foot than natural gas. (Correct answer)
Correct answer: Propane has a significantly higher heating value per cubic foot than natural gas.
Propane has a much higher energy content by volume compared to natural gas. A cubic foot of propane yields approximately 2,500 BTUs, whereas a cubic foot of natural gas yields about 1,000-1,030 BTUs. This difference is crucial for appliance orifice sizing and system design.
Question 3: Incomplete combustion of a hydrocarbon fuel, such as natural gas or propane, occurs due to insufficient oxygen. What is the primary toxic gas produced as a result?
- Carbon Dioxide (CO2)
- Carbon Monoxide (CO) (Correct answer)
- Sulfur Dioxide (SO2)
- Nitrogen Dioxide (NO2)
Correct answer: Carbon Monoxide (CO)
When there is not enough oxygen for complete combustion, the burning process is incomplete and produces carbon monoxide (CO), a colorless, odorless, and highly toxic gas. Complete combustion ideally produces carbon dioxide (CO2) and water vapor.
Question 4: The 'flammable range' or 'explosive limits' of a fuel gas refers to the concentration of gas in a gas-air mixture that will support combustion. What defines the Lower Explosive Limit (LEL)?
- The maximum concentration of gas in air that will burn.
- The ideal concentration of gas in air for perfect combustion.
- The minimum concentration of gas in air that will burn. (Correct answer)
- The temperature at which the gas will auto-ignite.
Correct answer: The minimum concentration of gas in air that will burn.
The Lower Explosive Limit (LEL), also known as the Lower Flammability Limit (LFL), is the lowest concentration (percentage) of a gas or vapor in the air that is capable of producing a flash of fire when an ignition source is present. A mixture below the LEL is considered 'too lean' to burn.
Question 5: A technician is converting a furnace from natural gas to propane. What fundamental property difference requires the technician to change the orifices?
- Propane is lighter than air.
- Propane has a lower ignition temperature.
- Propane has a higher heating value and requires a different air-to-gas ratio. (Correct answer)
- Natural gas is more corrosive to brass components.
Correct answer: Propane has a higher heating value and requires a different air-to-gas ratio.
Propane has a higher heating value (approx. 2,500 BTU/ft³) than natural gas (approx. 1,000 BTU/ft³). To deliver the correct amount of energy and ensure complete combustion, a smaller orifice is needed for propane to restrict the higher-energy fuel flow. Propane also requires a different air-to-gas ratio for combustion (approx. 24:1) compared to natural gas (approx. 10:1).
Question 6: What is the specific gravity (or relative density) of a gas a measure of?
- The energy content of the gas per cubic foot.
- The pressure at which the gas liquefies.
- The ratio of the gas's density to the density of air. (Correct answer)
- The auto-ignition temperature of the gas.
Correct answer: The ratio of the gas's density to the density of air.
Specific gravity, also known as relative density, for a gas is a dimensionless quantity that compares the density of that gas to the density of a reference material, which for gases is typically air. Air is assigned a specific gravity of 1.0. Gases with a specific gravity less than 1.0 are lighter than air, and those with a value greater than 1.0 are heavier than air.
G3 - Gas Technician Level 3 Properties of Fuel Gases Questions and Answers 1
A gas technician is investigating a reported gas odor in the basement of a residence.
Which property of propane (LPG) is most critical to consider in this scenario?