Static Electricity 2000 Edition

Table A.3.1.5 Examples of Capacitance of Various Items
Item
Capacitance
(pF)
Tank car
1000
Automobile
500
Person
100 to 300
Oil/solvent drum
10 to 100
Metal scoop
10 to 20
Needle electrode
1
Dust particle
10
−7

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APPENDIX A
77
–37
2000 Edition
necessary to reach equilibrium. The fraction of the total pres-
sure exerted by a vapor determines the composition of the
vapor–air mixture. Thus, when the total pressure is reduced, as
could be the case at high altitudes, the vapor concentration in
air increases. Because flash points are reported at a pressure of
1 atm (760 mm Hg or 101.3 kPa), an ambient pressure less than
this value will lower the actual effective flash point. The flash
point correction given in ASTM E 502 is expressed as follows:
where:
FP
corr
= corrected flash point
F = observed flash point (
°F)
C = observed flash point (
°C)
A = ambient barometric pressure (kPa)
B = ambient barometric pressure (mm Hg)
Effect of Low Concentration of Volatiles. Small concentrations
of volatile components in a liquid mixture can accumulate in
the vapor space of a container over time, which can reduce the
flash point to a temperature below the reported value. This
effect can result from off-gassing, chemical reaction, or some
other mechanism. An example is bitumen. Similarly, if a tank
truck is not cleaned after delivering gasoline and a higher
flash point liquid, such as kerosene or diesel fuel, is trans-
ferred to it, the residual gasoline will create an ignitible atmo-
sphere both in the truck’s tank and possibly in the receiving
tank as well. Solids that contain more than 0.2 wt percent flam-
mable solvent need to be evaluated for their potential to form
ignitible vapor in containers.
Liquid Mist. If a liquid is dispersed in air in the form of a
mist, it could be possible to propagate a flame through the
mist, even at temperatures below the liquid’s flash point. In
such cases, the mist droplets behave like dust particles. The
flash point of the liquid is irrelevant when determining com-
bustibility of mists. Even at very low liquid temperatures, fro-
zen liquid droplets can burn in this manner. Ease of ignition
and rate of combustion both increase as the droplet size of the
mist decreases. Depending on the volatility of the liquid, drop-
lets with a diameter less than 20 to 40 
µm typically vaporize
and ignite ahead of a flame front, and their overall combus-
tion behavior is similar to that of a vapor. Because mists are
usually produced by some form of shear process and these
same processes also generate static electricity, it is good prac-
tice to avoid splashing and other procedures that generate
mist inside equipment.
A.7.2.2
Operating a process at less than the LFL is often safer
than operating above the UFL, particularly for tanks and other
large vessels. Even if liquid in a tank rapidly generates suffi-
cient vapor for operation above the UFL, flammable mixtures
can still be present at tank openings, such as sampling ports,
and the flammable range could be traversed inside the tank
during start-up or some other operating condition. Often, the
atmosphere in the vessel can be inerted, as described in
NFPA 69, Standard on Explosion Prevention Systems. This tech-
nique reduces the oxygen concentration below that required
to sustain combustion. Inerting might not be effective near
tank openings, especially in cases where additions of solids can
entrain air. Also, for storage tanks, the inert gas supply should
be capable of compensating for changes in temperature or in-
breathing of air during tank emptying.