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)

A_t

tank cross sectional area (length²)

ΔA

change in Helmoltz free energy (energy/mole)

AEGL

acute exposure guidance level

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)

C_p

heat capacity at constant pressure (energy/mass deg)

C_ppm

concentration in parts per million by volume

C_V

heat capacity at constant volume (energy/mass deg)

C_vent

deflagration vent constant (pressure^1/2)

Cx

concentration at location x downwind from the source (mass/volume)

〈C〉

average or mean mass concentration (mass/volume)

d

diameter (length)

d_p

particle diameter (length)

d_f

diameter of flare stack (length)

D

diffusion coefficient (area/time), or diameter (length)

D₀

reference diffusion coefficient (area/time)

D_m

molecular diffusivity (area/time)

D_tid

total integrated dose due to a passing puff of vapor (mass time/volume)

E_a

activation energy (energy/mole)

ERPG

emergency response planning guideline (see Table 5-4)

EEGL

emergency exposure guidance levels (see Section 5-5)

f

Fanning friction factor (unitless) or frequency (1/time)

f(t)

failure density function

f_v

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)

g_c

gravitational constant (mass length/force time²)

G

Gibbs free energy (energy/mole) or mass flux (mass/area time)

G_T

mass flux during relief (mass/area time)

ΔG

change in Gibbs free energy (energy/mole)

h

specific enthalpy (energy/mass), or height (length)

h_L

fluid level above leak in tank (length)

hL

initial fluid level above leak in tank (length)

h_s

leak height above ground level (length)

H

enthalpy (energy/mole) or height (length)

H_f

flare height (length)

H_r

effective release height in plume model (length)

ΔH

change in enthalpy (energy/mole)

ΔH_c

heat of combustion (energy/mass)

ΔH_v

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)

I_s

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 (concentration^1-m/time)

k₁, k₂

constants in probit a equations

k_s

thermal conductivity of soil (energy/length time deg)

K

mass transfer coefficient (length/time)

K_b

backpressure correction for relief sizing (unitless)

K_f

excess head loss for fluid flow (dimensionless)

K_i, K_∞

constants in excess head loss, given by Equation 4-38

K_G

explosion constant for vapors (length pressure/time)

K_j

eddy diffusivity in x, y, or z direction (area/time)

K_P

overpressure correction for relief sizing (unitless)

K_St

explosion constant for dusts (length pressure/time)

K_V

viscosity correction for relief sizing (unitless)

K₀

reference mass transfer coefficient (length/time)

L

length

LEL

lower explosion limit (volume % fuel in air)

LFL = LEL

lower flammability limit (volume % fuel in air)

LOC

limiting oxygen concentration (volume percent oxygen)

LOL

lower flammable limit in pure oxygen (volume % fuel in oxygen)

m

mass

mf

mass fraction

m₀

total mass contained in reactor vessel (mass)

m_LR

mass of limiting reactant in Equation (8-36) (mass)

m_TNT

mass of TNT

m_v

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)

p_d

number of dangerous process episodes

p_s

scaled overpressure for explosions (unitless)

P

total pressure or probability

P_b

backpressure for relief sizing (psig)

PEL

permissable exposure level (see Section 2-8)

PFD

probability of failure on demand

P_g

gauge pressure (force/area)

P_max

maximum pressure for relief sizing (psig)

P_s

set pressure for relief sizing (psig)

P^sat

saturation vapor pressure

q

heat (energy/mass) or heat intensity (energy/area time)

q_f

heat intensity of flare (energy/time area)

q_g

heat flux from ground (energy/area time)

q_s

specific energy release rate at set pressure during reactor relief
(energy/mass)

Q

heat (energy) or electrical charge (coulombs)

Q_m

mass discharge rate (mass/time)

Qm*

instantaneous mass release (mass)

Q_v

ventilation rate (volume/time)

r

radius (length)

R

electrical resistance (ohms) or reliability

R−

Sachs scaled distance, defined by Equation 6-31 (unitless)

R_d

release duration for heavy gas releases (time)

r_f

vessel filling rate (time⁻¹)

R_g

ideal gas constant (pressure volume/mole deg)

Re

Reynolds number (unitless)

S

entropy (energy/mole deg) or stress (force/area)

S_m

material strength (force/area)

SPEGL

short term public exposure guideline (see Section 5-5)

t

time

t_d

positive phase duration of a blast (time)

t_e

emptying time

t_p

time to form a puff of vapor

t_v

vessel wall thickness (length)

t_w

worker shift time

Δt_v

venting time for reactor relief

T

temperature (deg)

T_i

time interval

TLV

threshold limit value (ppm or mg/m³ by volume)

T_m

maximum temperature during reactor relief (deg)

TMEF

target mitigated event frequency (1/year)

TQ

threshold quantity (mass)

T_s

saturation temperature at set pressure during reactor relief (deg)

TWA

time weighted average (ppm or mg/m³ by volume)

u

velocity (length/time)

u_d

dropout velocity of a particle (length/time)

ū

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 % fuel in air)

UFL = UEL

upper flammability limit (volume % fuel in air)

UOL

upper flammable limit in pure oxygen (volume % fuel in oxygen)

v

specific volume (volume/mass)

v_f

specific volume of liquid (volume/mass)

v_g

specific volume of vapor (volume/mass)

v_fg

specific volume change with liquid vaporization (volume/mass)

V

total volume or electrical potential (volts)

V_c

container volume

W

width (length)

W_e

expansion work (energy)

W_s

shaft work (energy)

x

mole fraction or Cartesian coordinate (length), or reactor conversion (dimensionless), or distance from the source (length)

X_f

distance from flare at grade (length)

y

mole fraction of vapor (unitless) or Cartesian coordinate (length)

Y

probit variable (unitless)

Y_G

gas expansion factor (unitless)

z

height above datum (length) or Cartesian coordinate (length) or compressibility (unitless)

z_e

scaled distance for explosions (length/mass^1/3)

α

velocity correction factor (unitless) or thermal diffusivity (area/time)

β

thermal expansion coefficient (deg⁻¹)

δ

double layer thickness (length)

ε

pipe roughness (length) or emissivity (unitless)

ε_r

relative dielectric constant (unitless)

ε₀

permittivity constant for free space (charge²/force length²)

η

explosion efficiency (unitless)

Φ

nonideal filling factor (unitless), or phi-factor for calorimeter thermal inertia (dimensionless)

γ

heat capacity ratio (unitless)

γ_c

conductivity (mho/cm)

Γ

dimensionless activation energy

χ

function defined by Equation 10-10

λ

frequency of dangerous episodes

λ_d

average frequency of dangerous episodes

μ

viscosity (mass/length/time), or mean value, or failure rate (faults/time)

μ_V

vapor viscosity (mass/length/time)

ψ

overall discharge coefficient used in Equation 10-14 (unitless)

ρ

density (mass/volume)

ρ_L

liquid density (mass/volume)

ρ_ref

reference density for specific gravity (mass/volume)

ρ_V

vapor density (mass/volume)

σ

standard deviation (unitless)

σ_x, σ_y, σ_z

dispersion coefficient (length)

τ

relaxation time, or dimensionless reaction time

τ_i

inspection period for unrevealed failures

τ₀

operation period for a process component

τ_r

period required to repair a component

τ_u

period of unavailability for unrevealed failures

ζ

zeta potential (volts)

a

ambient

ad

adiabatic

c

combustion

f

formation or liquid

g

vapor or gas

H

higher pressure

i

initiating event

j

purges

L

lower pressure

m

maximum

s

set pressure

o

initial or reference

°

standard

′

stochastic or random variable