Static Electricity 2000 Edition

A.3.1.5 Capacitance.
The property of a system of conductors
and nonconductors that permits the storage of electrically sep-
arated charges when potential differences exist between the
conductors. For a given potential difference, the higher the
capacitance, the greater the amount of charge that can be
stored. Quantitatively, it is the ratio of the charge on one of
the conductors of a capacitor (there being an equal and oppo-
site charge on the other conductor) to the potential differ-
ence between the conductors. The unit of capacitance is the
farad. Because the farad is so large a quantity, capacitance is
usually reported in microfarads (
µF) or picofarads (pF).
1 farad = 10
6
microfarads = 10
12
picofarads. (See Table
A.3.1.5 for examples.)
Capacitance is the constant of proportionality between the
charge and potential difference for a system of conductive
bodies.
A.4.3.3.4
MIEs can be determined for pure materials and
their mixtures. The actual ignition energy could be higher
than the MIE by an order of magnitude or more if the mixture
varies significantly from the most easily ignited concentration.
For hazard evaluation, the MIE should be considered as the
worst case.
A.4.3.8
See Britton, Avoiding Static Ignition Hazards in Chemical
Operations, for additional information.
A.6.6.2.2
See ANSI Z41, American National Standard for Personal
Protection — Protective Footwear.
A.6.6.3
See Britton, Avoiding Static Ignition Hazards in Chemical
Operations, for additional information.
A.6.6.4.2
See Britton, Avoiding Static Ignition Hazards in Chem-
ical Operations, for additional information.
A.6.6.4.3
See NFPA 53, Recommended Practice on Materials,
Equipment, and Systems Used in Oxygen-Enriched Atmospheres.
A.7.2.1
Class I flammable liquids [i.e., those that have flash
points of less than 100
°F (37.8°C)] can form ignitible vapor–
air mixtures under most ambient conditions. Class II and Class
III combustible liquids, which have flash points of 100
°F
(37.8
°C) or greater, typically require some degree of preheat-
ing before they evolve enough vapor to form an ignitible mix-
ture. Certain liquids of low fire hazard, such as solvent
formulations consisting of mostly water, might be classed as
combustible liquids, yet they can generate ignitible vapor–air
mixtures in closed containers at less than 100
°F (37.8°C). Sim-
ilarly, certain liquids that do not have a flash point could be
capable of generating an ignitible vapor–air mixture as a
result of degassing or slow decomposition, especially where
the vapor space is small compared with the liquid volume.
Errors in Flash Point Testing. The reported flash point of a liq-
uid might not represent the minimum temperature at which a
pool of liquid will form an ignitible atmosphere. Typical
closed-cup test methods involve downward flame propagation,
which is more difficult than upward propagation, and the
region where the test flame is introduced is normally fuel-lean
relative to the liquid surface. Also, the volume of the test appa-
ratus is too small to allow flame propagation of certain flam-
mable vapors such as halogenated hydrocarbons. Limitations
of flash point test methods are discussed in ASTM E 502, Stan-
dard Test Method for Selection and Use of ASTM Standards for the
Determination of Flash Point of Chemicals by Closed Cup Methods. In
most cases, closed-cup flash points are lower than open-cup
values.
Safety Margin for Use of Flash Point. The temperature of inter-
est in determining the hazard is the temperature at the
exposed liquid surface, not that of the bulk liquid, because
vapor is in equilibrium with the liquid at the surface. However,
the surface temperature is difficult to determine in many
instances. While the surface temperature should be consid-
ered to the extent possible, most hazard evaluations are, by
necessity and practicality, based on bulk temperature. There-
fore, a safety factor should be applied where the hazard is
assessed using the flash point. For pure liquids in containers,
the vapor should be considered potentially ignitible if the liq-
uid temperature is within 4
°C of the reported flash point. For
mixtures whose composition is less certain, such as hydrocar-
bon mixtures, the safety factor should be at least 9
°C. Where
combinations of adverse effects are identified, the safety fac-
tors might have to be increased accordingly.
Effect of Bulk Liquid Temperature. The surface temperature of
a quiescent liquid in a tank can significantly exceed the tem-
perature of the bulk liquid, due to heat transfer from the
unwet upper walls of the tank, which in some cases could be
heated by sunlight to as much as 140
°F. Because vapor–liquid
equilibrium is established at the vapor–liquid interface, this
higher surface temperature can result in a vapor concentra-
tion that is elevated compared to the concentration based on
the bulk liquid temperature. This elevated concentration
means that vapor in the tank could be ignitible even if the bulk
liquid temperature is less than the reported flash point, which
can be a significant hazard during sampling. Vapor vented
from large storage tanks could be at a concentration that is
only 30 percent to 50 percent of theoretical saturation, based
on bulk liquid temperature. This vapor also could be a signif-
icant hazard, if tank vapor is assumed to be above the UFL.
Effect of Ambient Pressure. The vapor pressure above a liquid
depends only on the temperature at the surface and the time