Glossary
Terms, symbols and units used across the TekRisk technical documentation.
A
- Atmospheric stability classA–F
- Pasquill–Gifford category (A very unstable … F very stable) describing turbulent mixing. Stable classes give longer, narrower clouds.
- Atmospheric transmissivitytau_atm
- Fraction of the thermal radiation not absorbed by water vapour and CO₂ along the path. τ = 2.02·(p_a·X)^{-0.09}, with p_a the water vapour partial pressure.CCPS (2000), eq. 2.2.42 See in the documentation →
- Average individual riskIR_av[yr⁻¹]
- Individual risk averaged over the exposed population: Σ(f·N_fatal)/N_exposed. Summarises the risk to the people actually present. See in the documentation →
B
- BLEVE
- Boiling Liquid Expanding Vapour Explosion: catastrophic failure of a vessel holding a liquid above its atmospheric boiling point. The flash vaporisation drives a blast and, if the fuel ignites, a fireball.CCPS (2000), §2.2.3 See in the documentation →
C
- CCPS
- Center for Chemical Process Safety (AIChE). Its Guidelines for Consequence Analysis and for Chemical Process Quantitative Risk Analysis are the main references of the models. See in the documentation →
D
- Domino effect
- Escalation in which the consequences of one event (radiation, overpressure, fragments) cause the failure of neighbouring equipment and a secondary release. See in the documentation →
F
- F-N curve
- Cumulative plot of the frequency F of accidents causing N or more fatalities versus N, on log-log axes, compared with acceptability lines. See in the documentation →
- Fatality probabilityP_f
- Probability that a person exposed to the effect at a point dies, obtained from the probit or from a threshold (flash fire).
- FireballD_max[m]
- Rising, roughly spherical mass of burning fuel produced by the ignition of a BLEVE release. Its diameter, duration and height are correlated with the released mass.CCPS (2000), eq. 2.2.32-2.2.34 See in the documentation →
- Flash fire
- Rapid, non-explosive combustion of a flammable vapour cloud. Fatality is assumed inside the LFL (or ½·LFL) envelope; overpressure is negligible. See in the documentation →
H
- Heat of combustionDelta H_c[J/kg]
- Energy released per unit mass of fuel burnt completely. Hydrocarbons: about 40–50 MJ/kg.
I
- Impulsei_s[Pa·s]
- Time integral of the overpressure over the positive phase of the blast wave. Needed by TNO impulse-based damage probits. See in the documentation →
- Individual risk (IR)IR[yr⁻¹]
- Frequency (per year) that a hypothetical person permanently located at a point suffers a fatal effect from all scenarios of the facility. See in the documentation →
- ISA atmospherep(z)[Pa]
- International Standard Atmosphere: reference model giving pressure and density as a function of altitude (p₀ = 101 325 Pa at sea level).
- Iso-risk contour
- Line joining points with the same individual risk (e.g. 10⁻⁶ yr⁻¹). Used to compare against land-use planning criteria. See in the documentation →
J
- Jet fire
- Turbulent diffusion flame fed by a pressurised release of flammable gas or two-phase fluid. Flame length and radiation depend on discharge rate and momentum. See in the documentation →
L
- LFLLFL[% vol]
- Lower flammability limit: minimum fuel concentration in air that sustains a flame. Below it the mixture is too lean to burn. See in the documentation →
P
- Peak side-on overpressureP_s[kPa]
- Maximum pressure rise above ambient of the blast wave at a point. Drives structural damage and injury probits. See in the documentation →
- Point-source model
- Radiation model that concentrates the flame power in a point and spreads it over a sphere: q = τ·f_s·Q/(4πX²). Valid far from the flame. See in the documentation →
- Pool fire
- Fire over a pool of flammable liquid on the ground. Burning rate, flame height and surface emissive power set the thermal radiation field. See in the documentation →
- ProbitY
- Probability unit: a normally distributed variable Y = a + b·ln(dose) that maps a physical dose (thermal, overpressure, toxic) to a probability of effect. Y = 5 corresponds to 50 %.TNO Green Book (CPR 16E) See in the documentation →
- Purple Book
- Guidelines for quantitative risk assessment (CPR 18E). Defines release scenarios, failure frequencies and risk calculation procedures. See in the documentation →
R
- Radiative fractionf_s
- Fraction of the combustion energy emitted as thermal radiation. Depends on fuel and, for fireballs, on vessel burst pressure (0.2–0.4).Roberts (1982)
- Receiver
- Point or population at which consequences are evaluated: a person, a building, an ignition point or a piece of equipment.
S
- Scenario frequencyf[yr⁻¹]
- Expected number of occurrences per year of a release scenario. Comes from historical data or from a fault tree.
- Societal risk
- Relationship between the frequency of an accident and the number of people it affects, usually presented as an F-N curve. See in the documentation →
- Solid-flame model
- Radiation model that treats the flame as a solid emitter with surface emissive power E and a geometric view factor: q = τ·E·F. See in the documentation →
- Surface emissive power (SEP)E[kW/m²]
- Radiant power emitted per unit area of the flame surface. Used by solid-flame models together with the view factor and transmissivity. See in the documentation →
T
- Thermal doseD[(kW/m²)^{4/3}·s]
- Radiation exposure integrated over time, D = t·q^{4/3}, with q in kW/m² and t in s. The dose, not the flux alone, drives burn and fatality probits. See in the documentation →
- Thermal radiation fluxq[kW/m²]
- Radiant heat received per unit area at the receiver. Typical thresholds: 1.6 kW/m² (no discomfort), 5 kW/m² (pain in 20 s), 12.5 kW/m² (wood ignition), 37.5 kW/m² (equipment damage).
- Time to failure (TTF)ttf[s]
- Estimated time for a vessel exposed to thermal radiation to fail. Cozzani correlations distinguish atmospheric and pressurised vessels.Cozzani et al. (2005)
- TNO Yellow Book
- Methods for the calculation of physical effects (CPR 14E). Source of many correlations for fires, explosions and dispersion. See in the documentation →
- TNT equivalentW_TNT[kg]
- Mass of TNT that would produce the same blast as the vapour cloud, W_TNT = η·M·ΔH_c/ΔH_TNT, with η the explosion yield factor. See in the documentation →
U
- UFLUFL[% vol]
- Upper flammability limit: maximum fuel concentration in air that sustains a flame. Above it the mixture is too rich to burn.
V
- Vapour cloud explosion (VCE)
- Explosion of a flammable vapour cloud in a congested or confined area, producing damaging overpressure. Modelled with TNT-equivalent or multi-energy methods. See in the documentation →
- View factorF
- Geometric fraction of the radiation leaving the flame surface that reaches the receiver. Depends on flame shape, distance and receiver orientation.
W
- Wind rose
- Distribution of wind direction frequency in 16 sectors. Weights the directional consequences when building risk contours.