What is H2S dispersion?
Hydrogen sulphide dispersion assessment predicts the downwind concentration of H2S released from sour service equipment, so that exposure to personnel can be evaluated against toxic thresholds. Because H2S is acutely toxic at concentrations far below its flammable limit, the toxic footprint almost always governs over the flammable one.
Why H2S is treated separately
- It is denser than air, so releases slump and travel at ground level — exactly where people are.
- It deadens the sense of smell at higher concentrations, so odour is not a reliable warning.
- It is acutely lethal in the low hundreds of ppm, orders of magnitude below its 4 vol% LFL.
- Even small sour gas inventories can produce large toxic footprints.
Exposure thresholds commonly applied
Assessments typically reference occupational exposure limits for routine work, and emergency response planning guidelines (ERPG) or acute exposure guideline levels (AEGL) for release scenarios. The threshold set must be stated in the study basis, since different frameworks give materially different distances.
For fatality estimation in a QRA a probit relationship is used, combining concentration and exposure duration into a dose. This makes cloud persistence and escape time as important as peak concentration.
Modelling considerations
The release usually starts as a high-momentum jet, then transitions to dense-gas behaviour once momentum decays. Modelling the whole release as passive under-predicts the ground-level distance. Low wind speed and stable atmospheric conditions produce the longest footprints and are normally the governing case.
Results feed muster point location, escape route design, H2S detector layout, wind sock placement and breathing apparatus requirements.
Frequently asked questions
At what concentration is H2S dangerous?
Effects begin at very low ppm levels, olfactory paralysis occurs in the tens to low hundreds of ppm, and short exposure in the several-hundred ppm range can be rapidly fatal. Project-specific thresholds should come from the applicable ERPG, AEGL or regulatory framework rather than from rules of thumb.
Does H2S sink or rise?
H2S is heavier than air and tends to accumulate in low points, trenches, pits and enclosed spaces, especially in still conditions.
Which weather case governs for toxic assessment?
Stable, low-wind conditions typically give the longest toxic distances because dilution is slowest and the cloud remains near ground level.
Calculators for this topic
- Gas Dispersion — Atmospheric gas dispersion calculator
- Gas Discharge — Gas discharge rate calculator for pressurised releases
- Gas Detection Mapping — Gas detection coverage mapping