Gas Fundamentals and Properties of Gases
Domain 1 - Gas fundamentals and properties of gases
This domain tests whether you can predict how natural gas and propane behave before, during and after combustion: composition, density, heating value, flammable range, gas laws and pressure measurement. These are the numbers behind orifice sizing, input verification, leak response and combustion troubleshooting.
Composition and density
- Natural gas is mostly methane (typically 90–96%), with nitrogen, CO2 and traces of heavier hydrocarbons. Specific gravity is about 0.60 (air = 1.0), so a natural gas leak rises and dissipates readily outdoors but can stratify at ceiling level indoors.
- Propane (C3H8), commercial grade HD-5, has a vapour specific gravity of about 1.52, so leaks sink into basements, pits, drains and excavations. Vapour density is roughly 1.9 kg/m³ at 15 °C against air at 1.2 kg/m³.
- Liquid propane expands roughly 270 times when it vaporises, which is why cylinders and tanks are filled to about 80% to leave vapour space.
Heating value and combustion
- Natural gas is about 37.3 MJ/m³ (1,000–1,050 Btu/ft³); propane vapour is roughly 2,500 Btu/ft³, with about 93,000 Btu per US gallon of liquid.
- Stoichiometric combustion needs about 10 volumes of air per volume of natural gas and about 24 per volume of propane. Real burners run lean (excess air) to avoid CO.
- Flammable range is about 4–15% gas in air for natural gas and about 2–10% for propane. Below the lower limit the mixture is too lean to light; above the upper limit it is too rich.
Gas laws and pressure
Use absolute pressure (psig + 14.7) and absolute temperature (°F + 460) in P1V1/T1 = P2V2/T2. Pressures are quoted in inches of water column: 1 in w.c. = 0.249 kPa; 1 psi = 27.7 in w.c. = 6.9 kPa. Typical natural gas supply is 7 in w.c. with 3.5 in w.c. manifold; propane supply is 11 in w.c. with 10 in w.c. manifold.
Worked example
A meter with a 0.05 m³ test dial completes one revolution in 45 s. Rate = 0.05 × (3600 ÷ 45) = 4 m³/h. At 37.3 MJ/m³ the input is 4 × 37.3 = 149.2 MJ/h ≈ 41 kW (about 141,000 Btu/h). If the rating plate says 35 kW, the appliance is overfiring by roughly 17%, so check orifice size and manifold pressure before assuming the meter is wrong.
Traps
Confusing specific gravity with density (SG has no units); using gauge instead of absolute pressure in the gas law; assuming a heavy cylinder means the vapour is heavy; and mixing up Btu per cubic foot with Btu per pound or per gallon.