| a | velocity of sound (length/time) |
| A | area (length2) or Helmholtz free energy (energy/mole); or process component availability; or arrhenius reaction rate pre-exponential constant (time–1) |
| At | tank cross sectional area (length2) |
| ΔA | change in Helmoltz free energy (energy/mole) |
| B | adiabatic reactor temperature increase (dimensionless) |
| C | mass concentration (mass/volume) or capacitance (Farads) |
| C0 | discharge coefficient (unitless), or concentration at the source (mass/volume) |
| C1 | concentration at a specified time (mass/volume) |
| Cm | concentration of dense gas (volume fraction) |
| Cp | heat capacity at constant pressure (energy/mass deg) |
| Cppm | concentration in parts per million by volume |
| Cv | heat capacity at constant volume (energy/mass deg) |
| Cvent | deflagration vent constant (pressure1/2) |
| Cx | concentration at location x downwind from the source (mass/volume) |
| <C> | average or mean mass concentration (mass/volume) |
| d | diameter (length) |
| dp | particle diameter (length) |
| df | diameter of flare stack (length) |
| D | diffusion coefficient (area/time) |
| Dc | characteristic source dimension for continuous releases of dense gases (length) |
| Di | characteristic source dimension for instantaneous releases of dense gas (length) |
| D0 | reference diffusion coefficient (area/time) |
| Dm | molecular diffusivity (area/time) |
| Dtid | total integrated dose due to a passing puff of vapor (mass time/volume) |
| Ea | activation energy (energy/mole) |
| ERPG | emergency response planning guideline (see Table 5-6) |
| EEGL | emergency exposure guidance levels (see Section 5.5) |
| f | Fanning friction factor (unitless) or frequency (1/time) |
| f(t) | failure density function |
| fv | mass fraction of vapor (unitless) |
| F | frictional fluid flow loss term (energy mass) or force or environment factor |
| FAR | fatal accident rate (fatalities/108 hours) |
| FEV | forced expired volume (liters/sec) |
| FVC | forced vital capacity (liters) |
| g | gravitational acceleration (length/time2) |
| gc | gravitational constant (mass length/force time2) |
| go | initial cloud buoyancy factor (length/time2) |
| gx | buoyancy factor at location x (length/time2) |
| G | Gibbs free energy (energy/mole) or mass flux (mass/area time) |
| GT | mass flux during relief (mass/area time) |
| ΔG | change in Gibbs free energy (energy/mole) |
| h | specific enthalpy (energy/mass) |
| hL | fluid level above leak in tank (length) |
| h0L | initial fluid level above leak in tank (length) |
| hs | leak height above ground level (length) |
| H | enthalpy (energy/mole) or height (length) |
| Hf | flare height (length) |
| Hr | effective release height in plume model (length) |
| ΔH | change in enthalpy (energy/mole) |
| ΔHc | heat of combustion (energy/mass) |
| ΔHr | release height correction given by Equation 5-65 |
| ΔHv | enthalpy of vaporization (energy/mass) |
| I | sound intensity (decibels) |
| ID | pipe internal diameter (length) |
| IDLH | immediately dangerous to life and health (see Section 5.5) |
| I0 | reference sound intensity (decibels) |
| Is | streaming current (amps) |
| ISOC | in-service oxygen concentration (volume percent oxygen) |
| j | number of inerting purge cycles (unitless) |
| J | electrical work (energy) |
| k | non-ideal mixing factor for ventilation (unitless), or reaction rate (concentration1-m/time) |
| k1, k2 | constants in probit a equations |
| ks | thermal conductivity of soil (energy/length time deg) |
| K | mass transfer coefficient (length/time) |
| Kb | backpressure correction for relief sizing (unitless) |
| Kf | excess head loss for fluid flow (dimensionless) |
| Ki, K∞ | constants in excess head loss, given by Equation 4-38 |
| KG | explosion constant for vapors (length pressure/time) |
| Kj | eddy diffusivity in x, y or z direction (area/time) |
| Kp | overpressure correction for relief sizing (unitless) |
| KSt | explosion constant for dusts (length pressure/time) |
| KV | viscosity correction for relief sizing (unitless) |
| K0 | reference mass transfer coefficient (length/time) |
| K* | constant eddy diffusivity (area/time) |
| L | length |
| LEL | lower explosion limit (volume %) |
| LFL = LEL | lower flammability limit (volume %) |
| LOC | limiting oxygen concentration (volume percent oxygen) |
| LOL | lower flammable limit in pure oxygen (volume %) |
| m | mass |
| mf | mass fraction |
| m0 | total mass contained in reactor vessel (mass) |
| mLR | mass of limiting reactant in Equation (8-34) (mass) |
| mT | total mass of reacting mixture in Equation (8-34) (mass) |
| mTNT | mass of TNT |
| mv | mass of vapor |
| M | molecular weight (mass/mole) |
| M0 | reference molecular weight (mass/mole) |
| Ma | Mach number (unitless) |
| MOC, MSOC | Minimum oxygen concentration or maximum safe oxygen concentration. See LOC |
| MTBC | mean time between coincidence (time) |
| MTBF | mean time between failure (time) |
| n | number of moles or, reaction order |
| OSFC | out of service fuel concentration (volume percent fuel) |
| p | partial pressure (force/area) |
| pd | number of dangerous process episodes |
| ps | scaled overpressure for explosions (unitless) |
| P | total pressure or probability |
| Pb | backpressure for relief sizing (psig) |
| PEL | permissable exposure level (see Section 2.8) |
| PFD | probability of failure on demand |
| Pg | gauge pressure (force/area) |
| Pmax | maximum pressure for relief sizing (psig) |
| Ps | set pressure for relief sizing (psig) |
| Psat | saturation vapor pressure |
| q | heat (energy/mass) or heat intensity (energy/area time) |
| qf | heat intensity of flare (energy/time area) |
| qg | heat flux from ground (energy/area time) |
| qs | specific energy release rate at set pressure during reactor relief (energy/mass) |
| Q | heat (energy) or electrical charge (coulombs) |
| Qm | mass discharge rate (mass/time) |
 | instantaneous mass release (mass) |
| Qv | ventilation rate (volume/time) |
| r | radius (length) |
| R | electrical resistance (ohms) or reliability |
 | Sachs scaled distance, defined by Equation 6-29 (unitless) |
| Rd | release duration for heavy gas releases (time) |
| RHI | reaction hazard index defined by Equation 14-1 |
| rf | vessel filling rate (time–1) |
| Rg | ideal gas constant (pressure volume/mole deg) |
| Re | Reynolds number (unitless) |
| S | entropy (energy/mole deg) or stress (force/area) |
| Sm | material strength (force/area) |
| SPEGL | short term public exposure guideline (see Section 5.5) |
| t | time |
| td | positive phase duration of a blast (time) |
| te | emptying time |
| tp | time to form a puff of vapor |
| tv | vessel wall thickness (length) |
| tw | worker shift time |
| Δtv | venting time for reactor relief |
| T | temperature (deg) |
| Td | material decomposition temperature (deg) |
| Ti | time interval |
| TLV | threshold limit value (ppm or mg/m3 by volume) |
| Tm | maximum temperature during reactor relief (deg) |
| Ts | saturation temperature at set pressure during reactor relief (deg) |
| TWA | time weighted average (ppm or mg/m3 by volume) |
| TXD | toxic dispersion method (see Section 5.5) |
| u | velocity (length/time) |
| ud | dropout velocity of a particle (length/time) |
 | average velocity (length/time) |
| <u> | mean or average velocity (length/time) |
| U | internal energy (energy/mole) or overall heat transfer coefficient (energy/area deg time) or process component unavailability |
| UEL | upper explosion limit (volume %) |
| UFL = UEL | upper flammability limit (volume %) |
| UOL | upper flammable limit in pure oxygen (volume %) |
| v | specific volume (volume/mass) |
| vf | specific volume of liquid (volume/mass) |
| vg | specific volume of vapor (volume/mass) |
| vfg | specific volume change with liquid vaporization (volume/mass) |
| V | total volume or electrical potential (volts) |
| Vc | container volume |
| W | width (length) |
| We | expansion work (energy) |
| Ws | shaft work (energy) |
| x | mole fraction or Cartesian coordinate (length), or reactor conversion (dimensionless), or distance from the source (length) |
| xt | is the distance from the source to the transition (length), |
| xv | is the virtual distance (length), and |
| xnb | is the distance used in the neutrally buoyant model to compute the concentration downwind of the transition. (length) |
| Xf | distance from flare at grade (length) |
| y | mole fraction of vapor (unitless) or Cartesian coordinate (length) |
| Y | probit variable (unitless) |
| YG | gas expansion factor (unitless) |
| z | height above datum (length) or Cartesian coordinate (length) or compressibility (unitless) |
| ze | scaled distance for explosions (length/mass1/3) |
