Consequence Endpoint for a Sour Water Vessel Near an Occupied Building
A 25 m3 horizontal vessel contains sour water with 12,000 ppm H2S at 8 bar. The nearest occupied building is 110 m downwind of the prevailing wind direction and houses 15 people. The site has gas detection, a deluge system, and an emergency response team with a 10-minute response. A proposed project would increase throughput by 20% and raise the vessel inventory to 30 m3. The current consequence analysis credits gas detection and isolation to reduce the toxic endpoint.
1. Which consequence types should be evaluated, and which is likely to govern?
Read the worked answer
Evaluate both toxic and flammable consequences, but the toxic endpoint is likely to govern. At 12,000 ppm, the gas is about 1.2 vol% H2S, below its approximate lower flammable limit of 4 vol%, so a released H2S cloud by itself may not be immediately flammable. However, if hydrocarbon is co-released, a flammable cloud or flash fire must also be assessed.
The toxic endpoint is severe: 12,000 ppm is far above acute exposure thresholds, and the occupied building is 110 m downwind in the prevailing direction. The analysis should model a credible release size and duration and compare the downwind toxic endpoint distance with the 110 m separation. Population and occupancy at the building should be included in the consequence estimate.
2. Should gas detection and isolation be credited in the consequence analysis?
Read the worked answer
Credit only if the detection and isolation system is reliable, maintained, tested, and documented, and if the credited response time is realistic. Detection can shorten release duration and reduce the toxic dose, but it does not prevent the initial release or the near-field concentration. A 10-minute emergency response time is not the same as automatic isolation; do not credit manual response unless procedures and drills demonstrate it.
For risk ranking, be careful not to double-count mitigation: if detection and isolation are credited in consequence, do not also credit the same safeguards as independent probability reductions without justification. A conservative assessment should show the unmitigated consequence alongside any credited mitigation so decision-makers can see the difference.
3. How should the proposed 20% inventory increase affect the consequence and risk result?
Read the worked answer
The inventory increase from 25 m3 to 30 m3 is a 20% increase in source volume. For a toxic release, that can increase release duration and the downwind extent of the toxic endpoint, especially if the release rate is not limited by the hole size. The consequence analysis must be rerun with the new inventory rather than scaling the old distance linearly.
If the toxic endpoint already reaches the occupied building at 110 m, the increase may extend the duration or concentration at that location and could push the item across a risk threshold. The assessment should also consider whether the project introduces additional flammable inventory. Risk reduction options include inventory reduction, relocation or hardening of the occupied building, improved automatic isolation, and water-wash or scrubber reliability improvements.