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avelocity of sound (length/time)
Aarea (length2) or Helmholtz free energy (energy/mole); or process component availability; or arrhenius reaction rate pre-exponential constant (time–1)
Attank cross sectional area (length2)
ΔAchange in Helmoltz free energy (energy/mole)
Badiabatic reactor temperature increase (dimensionless)
Cmass concentration (mass/volume) or capacitance (Farads)
C0discharge coefficient (unitless), or concentration at the source (mass/volume)
C1concentration at a specified time (mass/volume)
Cmconcentration of dense gas (volume fraction)
Cpheat capacity at constant pressure (energy/mass deg)
Cppmconcentration in parts per million by volume
Cvheat capacity at constant volume (energy/mass deg)
Cventdeflagration vent constant (pressure1/2)
Cxconcentration at location x downwind from the source (mass/volume)
<C>average or mean mass concentration (mass/volume)
ddiameter (length)
dpparticle diameter (length)
dfdiameter of flare stack (length)
Ddiffusion coefficient (area/time)
Dccharacteristic source dimension for continuous releases of dense gases (length)
Dicharacteristic source dimension for instantaneous releases of dense gas (length)
D0reference diffusion coefficient (area/time)
Dmmolecular diffusivity (area/time)
Dtidtotal integrated dose due to a passing puff of vapor (mass time/volume)
Eaactivation energy (energy/mole)
ERPGemergency response planning guideline (see Table 5-6)
EEGLemergency exposure guidance levels (see Section 5.5)
fFanning friction factor (unitless) or frequency (1/time)
f(t)failure density function
fvmass fraction of vapor (unitless)
Ffrictional fluid flow loss term (energy mass) or force or environment factor
FARfatal accident rate (fatalities/108 hours)
FEVforced expired volume (liters/sec)
FVCforced vital capacity (liters)
ggravitational acceleration (length/time2)
gcgravitational constant (mass length/force time2)
goinitial cloud buoyancy factor (length/time2)
gxbuoyancy factor at location x (length/time2)
GGibbs free energy (energy/mole) or mass flux (mass/area time)
GTmass flux during relief (mass/area time)
ΔGchange in Gibbs free energy (energy/mole)
hspecific enthalpy (energy/mass)
hLfluid level above leak in tank (length)
h0Linitial fluid level above leak in tank (length)
hsleak height above ground level (length)
Henthalpy (energy/mole) or height (length)
Hfflare height (length)
Hreffective release height in plume model (length)
ΔHchange in enthalpy (energy/mole)
ΔHcheat of combustion (energy/mass)
ΔHrrelease height correction given by Equation 5-65
ΔHventhalpy of vaporization (energy/mass)
Isound intensity (decibels)
IDpipe internal diameter (length)
IDLHimmediately dangerous to life and health (see Section 5.5)
I0reference sound intensity (decibels)
Isstreaming current (amps)
ISOCin-service oxygen concentration (volume percent oxygen)
jnumber of inerting purge cycles (unitless)
Jelectrical work (energy)
knon-ideal mixing factor for ventilation (unitless), or reaction rate (concentration1-m/time)
k1, k2constants in probit a equations
ksthermal conductivity of soil (energy/length time deg)
Kmass transfer coefficient (length/time)
Kbbackpressure correction for relief sizing (unitless)
Kfexcess head loss for fluid flow (dimensionless)
Ki, Kconstants in excess head loss, given by Equation 4-38
KGexplosion constant for vapors (length pressure/time)
Kjeddy diffusivity in x, y or z direction (area/time)
Kpoverpressure correction for relief sizing (unitless)
KStexplosion constant for dusts (length pressure/time)
KVviscosity correction for relief sizing (unitless)
K0reference mass transfer coefficient (length/time)
K*constant eddy diffusivity (area/time)
LELlower explosion limit (volume %)
LFL = LELlower flammability limit (volume %)
LOClimiting oxygen concentration (volume percent oxygen)
LOLlower flammable limit in pure oxygen (volume %)
mfmass fraction
m0total mass contained in reactor vessel (mass)
mLRmass of limiting reactant in Equation (8-34) (mass)
mTtotal mass of reacting mixture in Equation (8-34) (mass)
mTNTmass of TNT
mvmass of vapor
Mmolecular weight (mass/mole)
M0reference molecular weight (mass/mole)
MaMach number (unitless)
MOC, MSOCMinimum oxygen concentration or maximum safe oxygen concentration. See LOC
MTBCmean time between coincidence (time)
MTBFmean time between failure (time)
nnumber of moles or, reaction order
OSFCout of service fuel concentration (volume percent fuel)
ppartial pressure (force/area)
pdnumber of dangerous process episodes
psscaled overpressure for explosions (unitless)
Ptotal pressure or probability
Pbbackpressure for relief sizing (psig)
PELpermissable exposure level (see Section 2.8)
PFDprobability of failure on demand
Pggauge pressure (force/area)
Pmaxmaximum pressure for relief sizing (psig)
Psset pressure for relief sizing (psig)
Psatsaturation vapor pressure
qheat (energy/mass) or heat intensity (energy/area time)
qfheat intensity of flare (energy/time area)
qgheat flux from ground (energy/area time)
qsspecific energy release rate at set pressure during reactor relief (energy/mass)
Qheat (energy) or electrical charge (coulombs)
Qmmass discharge rate (mass/time)
instantaneous mass release (mass)
Qvventilation rate (volume/time)
rradius (length)
Relectrical resistance (ohms) or reliability
Sachs scaled distance, defined by Equation 6-29 (unitless)
Rdrelease duration for heavy gas releases (time)
RHIreaction hazard index defined by Equation 14-1
rfvessel filling rate (time–1)
Rgideal gas constant (pressure volume/mole deg)
ReReynolds number (unitless)
Sentropy (energy/mole deg) or stress (force/area)
Smmaterial strength (force/area)
SPEGLshort term public exposure guideline (see Section 5.5)
tdpositive phase duration of a blast (time)
teemptying time
tptime to form a puff of vapor
tvvessel wall thickness (length)
twworker shift time
Δtvventing time for reactor relief
Ttemperature (deg)
Tdmaterial decomposition temperature (deg)
Titime interval
TLVthreshold limit value (ppm or mg/m3 by volume)
Tmmaximum temperature during reactor relief (deg)
Tssaturation temperature at set pressure during reactor relief (deg)
TWAtime weighted average (ppm or mg/m3 by volume)
TXDtoxic dispersion method (see Section 5.5)
uvelocity (length/time)
uddropout velocity of a particle (length/time)
average velocity (length/time)
<u>mean or average velocity (length/time)
Uinternal energy (energy/mole) or overall heat transfer coefficient (energy/area deg time) or process component unavailability
UELupper explosion limit (volume %)
UFL = UELupper flammability limit (volume %)
UOLupper flammable limit in pure oxygen (volume %)
vspecific volume (volume/mass)
vfspecific volume of liquid (volume/mass)
vgspecific volume of vapor (volume/mass)
vfgspecific volume change with liquid vaporization (volume/mass)
Vtotal volume or electrical potential (volts)
Vccontainer volume
Wwidth (length)
Weexpansion work (energy)
Wsshaft work (energy)
xmole fraction or Cartesian coordinate (length), or reactor conversion (dimensionless), or distance from the source (length)
xtis the distance from the source to the transition (length),
xvis the virtual distance (length), and
xnbis the distance used in the neutrally buoyant model to compute the concentration downwind of the transition. (length)
Xfdistance from flare at grade (length)
ymole fraction of vapor (unitless) or Cartesian coordinate (length)
Yprobit variable (unitless)
YGgas expansion factor (unitless)
zheight above datum (length) or Cartesian coordinate (length) or compressibility (unitless)
zescaled distance for explosions (length/mass1/3)

Greek Letters

αvelocity correction factor (unitless) or thermal diffusivity (area/time)
βthermal expansion coefficient (deg–1)
δdouble layer thickness (length)
εpipe roughness (length) or emissivity (unitless)
εrrelative dielectric constant (unitless)
ε0permittivity constant for free space (charge2/force length2)
ηexplosion efficiency (unitless)
Φnonideal filling factor (unitless), or phi-factor for calorimeter thermal inertia (dimensionless)
γheat capacity ratio (unitless)
γcconductivity (mho/cm)
Γdimensionless activation energy
χfunction defined by Equation 9-10
λfrequency of dangerous episodes
λdaverage frequency of dangerous episodes
μviscosity (mass/length/time) or mean value or failure rate (faults/time)
μVvapor viscosity (mass/length/time)
Ψoverall discharge coefficient used in Equation 10-15 (unitless)
ρdensity (mass/volume)
ρLliquid density (mass/volume)
ρrefreference density for specific gravity (mass/volume)
ρVvapor density (mass/volume)
ρxdensity at distance x downwind from source (mass/volume)
σstandard deviation (unitless)
σx, σy, σzdispersion coefficient (length)
τrelaxation time, or dimensionless reaction time
τiinspection period for unrevealed failures
τ0operation period for a process component
τrperiod required to repair a component
τuperiod of unavailability for unrevealed failures
ζzeta potential (volts)


fformation or liquid
gvapor or gas
Hhigher pressure
iinitiating event
Llower pressure
sset pressure
oinitial or reference


stochastic or random variable

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