Consequence and risk: two different questions

Consequence analysis asks: if this release happens, how far could the effects reach and how severe would they be? Quantitative risk assessment (QRA) adds the question: how often could that happen? Combining consequence and frequency across many scenarios gives a measure of risk to people, buildings and the community that can be compared with criteria and used to rank options.

Consequence analysis

We model the physical effects of loss-of-containment events, from the release itself through dispersion to the harmful outcome:

Phenomenon	What is modeled	Typical harm measure
Source term	Release rate, flashing, aerosol, pool formation and evaporation	Rate and duration
Dispersion	Dense and neutral gas dispersion under representative weather	Concentration vs. distance
Toxic exposure	Concentration and duration at receptors	Endpoints such as ERPG or AEGL values; probit models
Fires	Jet fires, pool fires and flash fires	Thermal radiation (heat flux)
Explosions	Vapor cloud explosions, BLEVEs and vessel bursts	Overpressure and impulse; fragments

Results are presented as effect zones and contours, together with the assumptions that drive them, so decision-makers can see both the answer and its sensitivity.

Quantitative risk assessment

A QRA combines many scenarios — each with a frequency and a set of possible outcomes — into risk measures such as:

Location-specific individual risk, often shown as contours on a site plan.
Individual risk to specific worker groups, accounting for occupancy.
Societal risk, typically shown as an F-N curve of frequency versus number of fatalities.
Risk ranking that identifies which scenarios and equipment contribute most.

QRA is most valuable for layout and siting decisions, comparing design alternatives, demonstrating risk reduction for major capital decisions and supporting land-use planning discussions.

Facility siting and occupied buildings

OSHA PSM requires PHAs to address facility siting. For buildings near process units, the API recommended practices provide the most widely used framework:

API RP 752 — permanent buildings
API RP 753 — portable buildings, such as trailers
API RP 756 — tents

We evaluate building occupancy, explosion, fire and toxic hazards using consequence-based or risk-based approaches, and recommend practical options: relocation, upgrading or restricting occupancy, or reducing the hazard at the source.

Where results are used

PHA & LOPA

Consistent severity ranking based on modeled effect distances rather than judgment alone.

EPA RMP

Worst-case and alternative release scenarios for off-site consequence analysis.

Emergency planning

Shelter-in-place, evacuation and detector placement decisions.

Projects

Plot plan layout, spacing and building design for new units and expansions.

Typical deliverables

Scenario selection basis and modeling assumptions
Consequence results: effect distances, contours and tables
QRA risk contours, F-N curves and contributor ranking
Building siting evaluation with recommendations
Executive summary for decision-makers

Frequently asked questions

Which modeling software do you use?

We select tools appropriate to the scope and your corporate standards, and document the models and inputs so results can be reviewed and reproduced. Ask us about compatibility with software you already use.

Is the EPA worst-case scenario the same as a realistic release?

No. The RMP worst-case scenario is defined by regulation with prescribed assumptions; it is useful for planning and communication but is not intended to predict the most likely event. Alternative release scenarios and site-specific modeling give a more realistic picture.

Do I need a full QRA for building siting?

Not always. Many siting studies can be completed with a consequence-based approach. A risk-based approach becomes useful when consequence results would otherwise drive costly modifications.

Consequence Analysis, QRA & Facility Siting