Gas Characteristics
What you must know
Natural gas (mainly methane) properties: relative density ~0.6 (lighter than air, rises and vents high), calorific value ~38–39 MJ/m³, Wobbe number, and the fact it is odourised with mercaptan so leaks are detectable. LPG (propane/butane) is heavier than air (propane RD ~1.5), pools low, and is stored as a liquid under pressure - hence different ventilation and leak-detection logic. Know complete vs incomplete combustion products: CO₂ and water vs CO and water. Understand flammability limits (lower and upper explosive limits), ignition temperature, and the significance of the gas group and appliance category (e.g. natural gas only, LPG only, dual).
Traps in questions
- Assuming LPG behaves like natural gas for ventilation - LPG sinks, so low-level ventilation and no low-level ignition sources matter.
- Confusing relative density with vapour pressure or calorific value.
- Forgetting that CO is colourless and odourless - the smell of gas is the added odorant, not CO.
- Mixing up lower and upper flammability limits (below LEL = too lean to burn; above UEL = too rich).
Worked example / rule of thumb
Rule of thumb: Natural gas rises (RD < 1); LPG falls (RD > 1). If a question describes gas accumulating at floor level or in a basement, think LPG; if it vents at high level, think natural gas. For leak response, ventilate at the correct level and remove ignition sources.
Key exam behaviours
Match the fuel to its physical behaviour before choosing a ventilation or detection answer. Read for the appliance category - using the wrong gas is a classic ID scenario.