Static Electricity 2000 Edition

times for some typical liquids.)
Nonconductive Liquids. Liquids that have relaxation time
constants greater than 0.36 seconds (equivalent to a conduc-
tivity of less than 50 pS/m for typical hydrocarbons having
dielectric constants of about 2) are considered nonconduc-
tive. Examples include purified toluene and most low-sulfur
diesel oils. They are highly susceptible to variation due to trace
contamination. Corona and brush discharges, rather than
spark discharges, are observed from charged nonconductive
liquids. Because only partial discharge is possible, induction
charging from highly charged plastic containers is not a signif-
icant hazard. Nonconductive liquids are most prone to accu-
mulate charge in grounded metallic containers. For the
purposes of this recommended practice, the criterion of 50
pS/m is not iron-clad; the dielectric constant also plays a role.
For example, the dielectric constant of ethyl ether is 4.6 versus
2.3 for benzene. Therefore, the relaxation time constant for
ethyl ether at a conductivity of 100 pS/m is approximately the
same as that for benzene at a conductivity of 50 pS/m. It is the
relaxation time constant, not the conductivity alone, that
determines the rate of loss of charge. 
Semiconductive Liquids. Liquids that have relaxation time
constants ranging from 0.36 sec down to 0.002 sec (equivalent
to a conductivity range between 50 and 10
4
pS/m for typical
hydrocarbons having dielectric constants of about 2) are con-
sidered conductive. Examples include crude oil and butyl ace-
tate. They tend not to accumulate charge, except where
charging rates are extremely high or where they are effectively
isolated from ground, such as when flowing through a rubber
hose or end-of-line “polishing” filters. Spark discharges are
possible from the more conductive of these liquids.
Conductive Liquids. Liquids that have relaxation time con-
stants less than 0.002 sec (equivalent to a conductivity greater
than 10
4
pS/m for typical hydrocarbons having dielectric con-
stants of about 2) are considered highly conductive. These liq-
uids tend not to accumulate charge except where handling
conditions isolate them from ground. Such conditions
include complete isolation in the form of a droplet suspended
in air, partial isolation by suspension in another liquid, and
containment in a plastic or other highly resistive container.
Conductive liquids are most prone to induction charging by
plastic containers and are sufficiently conductive to lose much
of the induced charge in the form of a spark.
Changes in Conductivity Caused by Solidification. Liquids can
undergo a sudden and dramatic decrease in conductivity at
their freezing points, which in some cases can cause unex-
pected static electricity hazards. For example, the conductivity
of biphenyl decreases by 4 orders of magnitude between the
liquid phase (above 69
°C) and the solid phase. A static electric
ignition was reported when biphenyl at 120
°C was loaded into
a tank containing a thick layer of solid biphenyl from a previ-
ous operation.
Normally, hot biphenyl is conductive enough to rapidly dis-
sipate charge when loaded into a grounded metal tank. But
due to the presence of the thick, insulating layer of solid
biphenyl, charge was able to accumulate and a brush dis-
charge occurred from the liquid surface to the fill pipe.
A.7.4.1
See Britton, Avoiding Static Ignition Hazards in Chemical
Operations, for additional information.
Various theoretical and empirical models have been
derived expressing either charge density or charging current
in terms of flow characteristics, such as pipe diameter and flow
velocity. Liquid dielectric and physical properties appear in
more complex models. For turbulent flow of a nonconductive
liquid through a pipe under conditions where the residence
time is long compared with the relaxation time, the charging
current, I