Properties, Combustion and Behaviour of Natural Gas and LPG in Commercial and Industrial Settings
This domain checks that you can predict how natural gas and LPG behave in a plant room, and use gas properties to calculate gas rates, air requirements and combustion products.
Key properties
- Natural gas (methane-rich): relative density about 0.6 (lighter than air), gross calorific value around 38.9 MJ/m³, flammable range roughly 5–15% by volume in air.
- Propane: relative density about 1.55 (heavier than air), gross calorific value about 95 MJ/m³ as vapour, flammable range roughly 2.1–10.1%.
- Butane: relative density about 2.0, gross calorific value about 121 MJ/m³ as vapour.
- Liquid LPG expands to roughly 270 volumes of vapour per volume of liquid (propane), so a small liquid leak becomes a large flammable cloud.
Combustion
Methane needs about 9.5 m³ of air per m³ of gas for complete combustion; propane needs about 24 m³. Maximum CO₂ is about 11.7% for natural gas and 13.8% for propane. Incomplete combustion produces CO, and the CO/CO₂ ratio is a better indicator than CO concentration alone.
Because LPG vapour is heavier than air it collects in basements, pits, drains and low plant-room corners; natural gas rises and collects at high level. This drives detector placement, ventilation design and emergency procedure.
Worked example
A 100 kW output natural-gas boiler runs at 90% gross efficiency. Input = 100/0.90 = 111 kW = 400 MJ/h. Gas rate = 400/38.9 = 10.3 m³/h. If the meter reads 12 m³/h, the appliance is over-gassed: check injector sizing, governor pressure and gas family.
Traps
Mixing net and gross calorific values; sizing LPG pipework from m³/h of vapour while the meter or tank data is in kg/h; assuming LPG vapour disperses upwards; using CO ppm alone while ignoring dilution.
Exam focus
Density and buoyancy, calorific values, stoichiometric air, CO/CO₂ interpretation and gas-rate conversion.