Atmospheric Unintended Release Analysis
How far can a release reach?
Describe a leak of flammable or toxic material. AURA works out the release rate, the cloud or pool it forms, and the distances to fire, explosion and toxic effects.
From a leak to a hazard distance
A study is a project. Each step feeds the next, and every page shows the equations and assumptions it used.
Project and site
Place the site on the map. Add the storage tanks, pressure vessels, spheres and pipes with what they hold, a pure chemical or a mixture, and the weather cases you plan against: 2F, 5D, the library or your own.
Scenario
Pick the equipment and the outcomes to assess. Release through a hole, at a set rate, or by catastrophic rupture of the whole inventory, with a choice of equation of state.
Source term
Release rate and duration, flashing, droplet size, and the part that rains out to form a pool against the part that stays airborne.
Pool and cloud
Pool spread inside or outside the bund and its evaporation; plume, puff or dense-gas dispersion, worked out for every weather case of the scenario.
Fire, explosion, toxic
Jet, pool, tank and flash fire; vapour cloud explosion; BLEVE and fireball; toxic exposure indoors and outdoors. Link scenarios to follow a fire to the vessel it threatens.
Map, summary and report
Hazard zones drawn on the map and saved as KML, GeoJSON or PDF; every distance in one table; a printable report and CSV. Issue a revision to fix the results on record.
What it models
Each event has a published screening method, checked against reference cases on the Verification page. Properties come from the chemical library through a choice of six equations of state, for pure fluids and mixtures.
Release
- Gas, liquid or two-phase leak
- Orifice flow, choked or sub-sonic; flash, droplet size and rain-out (De Vaull and King, Lautkaski); catastrophic rupture.
- Pressure vessel and pipe
- Gas blowdown, adiabatic or isothermal; pipe flow with friction; Wilson model for a long gas pipeline; pumped liquid lines.
- Draining tank
- Level, rate and mass against time for vertical, horizontal and spherical vessels.
- Liquid pool
- Spreading to the ground's minimum depth or the bund; evaporating or boiling, with heat from the ground, the air and the sun.
Cloud
- Vapour cloud
- Gaussian plume and puff with surface roughness (McMullen / Hosker or Briggs); Britter-McQuaid for dense gas. Distances to LFL and UFL and the flammable mass.
- Toxic exposure
- Toxic load and probit, indoors and outdoors for a building's air-change rate. Distances to IDLH, ERPG, AEGL and to 1, 10 and 50 % fatality.
Fire
- Jet fire
- Chamberlain conical flame, Johnson for horizontal gas jets, or point source with API 521 flame length. Distances to 37.5 down to 1.6 kW/m².
- Pool and tank fire
- Solid flame with tilt. Radiation on the next tank, cooling water, burn-out and boilover time.
- Flash fire
- Reach, width, area and duration of the burning cloud.
Explosion
- Vapour cloud explosion
- TNT equivalence, TNO Multi-Energy (strength chosen or by GAMES) and Baker-Strehlow-Tang, from curves taken from their sources. Distances to 70 down to 2 kPa.
- BLEVE
- Fireball size, duration and dose; blast from the vapour and the flashing liquid; fragment range.
Questions it answers
How far could this leak reach, and does it get to the control room?
Put a number on a hazard raised in a HAZID or HAZOP, so the action can be closed, kept or escalated.
Is this tank far enough from its neighbour, the fence and the muster point?
Radiation and overpressure distances for layout and separation.
Which area do we evacuate for a chlorine leak at night with light wind?
Distances to toxic limits and fatality levels for the weather cases you plan against.
How much cooling water does the next tank need?
Radiation on a neighbouring tank, cooling water flow, burn-out and boilover time.
Can this jet fire cause the LPG bullet beside it to fail?
Link a fire to the vessel it threatens and assess the BLEVE that could follow.
Which of these forty scenarios deserve detailed modelling?
Run them all, rank them by hazard distance, and send only the governing ones for detailed work.
What it is not for
- Use it to screen and to check, not as the final basis for design, siting or emergency plans.
- It gives consequences, not risk. It has no leak frequencies, ignition probabilities or population data.
- It assumes flat, open ground. Buildings, terrain and congestion are not modelled, except as a class or a flame speed in the explosion methods.
- Some library values and curves are starting values marked "verify". Check them against your own references first.