Properties, hazards and behaviour of gases and vapours
Properties, hazards and behaviour of gases and vapours
This domain tests whether you can predict where a gas will go, how it will hurt you, and what that means for detector placement and test sequence.
Physical properties that drive hazard
- Vapour density (air = 1): methane 0.55 and hydrogen 0.07 rise and collect at high points; propane 1.5, butane 2.0, H2S 1.19 and CO2 1.53 sink into pits, drains and low areas. Carbon monoxide (0.97) mixes almost evenly.
- Flammable range: defined by LEL and UEL in %v/v in air. Methane 5–15%, propane 2.1–9.5%, hydrogen 4–75%. Outside the range there is no flame propagation, but a rich atmosphere can become flammable on dilution.
- Flashpoint and autoignition temperature: a liquid below its flashpoint does not form a flammable vapour/air mixture at that temperature; AIT is the surface temperature that ignites it without a spark (methane about 580 °C, hydrogen about 560 °C).
- Toxicity and asphyxiation: CO and H2S are chemical asphyxiants; nitrogen and argon are simple asphyxiants; CO2 is both. A gas can be non-flammable and still lethal.
- Behaviour: diffusion, stratification, migration along ducts and drains, and accumulation in confined spaces mean one reading at one level is never enough.
Worked example
A pit contains sludge that is releasing methane and hydrogen sulphide. A meter held at the pit top shows 0% LEL and 0 ppm H2S. Reasoning: methane is lighter than air and may be at the top, but H2S is heavier and will sit at the bottom; sludge disturbance releases both. You must test top, middle and bottom with a pumped probe before entry, and continue monitoring.
Traps that cost marks
Assuming a gas with density near 1 cannot stratify; treating CO2 only as an asphyxiant and ignoring its toxicity; quoting LEL without specifying the gas and the basis (%v/v in air); forgetting that temperature and pressure change the flammable range.