Static Electricity 2000 Edition

77–
18
STATIC ELECTRICITY
2000 Edition
7.3 Generation and Dissipation of Charge in Liquids.
7.3.1* Charge Generation.
Charge separation occurs when
liquids flow through pipes, hoses, and filters, when splashing
occurs during transfer operations, or when liquids are stirred
or agitated. The greater the area of the interface between the
liquid and surfaces and the higher the flow velocity, the
greater the rate of charging. The charges become mixed with
the liquid and are carried to receiving vessels where they can
accumulate. The charge is often characterized by its bulk
charge density and its flow as a streaming current to the vessel.
(See Figure 7.3.1.)
FIGURE 7.3.1
Examples of charge generation in liquids. 
(Walmsley, 1992, p. 33.)
7.3.2* Charge Relaxation.
Static electric charge on a liquid
in a grounded conductive container will dissipate at a rate that
depends on the conductivity of the liquid. For liquids with
conductivity of 1 picosiemens per meter (1 pS/m) or greater,
charge relaxation proceeds by exponential, or ohmic, decay,
as described for semiconductive materials in 4.2.4. For liquids
with conductivity less than 1 pS/m, relaxation occurs more
rapidly than would be predicted by the exponential decay
model. (See 4.2.5.) According to the Bustin relationship (see
A.7.3.2), when low viscosity liquids (less than 30 
× 10
-6
m
2
/sec)
are charged, relaxation proceeds by hyperbolic decay. How-
ever, for these same liquids, the exponential decay constant
gives a conservative estimate for the relaxation time.
7.3.3* Factors Affecting Liquid Charging.
In grounded sys-
tems, the conductivity of the liquid phase has the most effect
on the accumulation of charge in the liquid or on materials
suspended in it. A liquid is considered nonconductive (charge
accumulating) if its conductivity is below 50 pS/m, assuming a
dielectric constant of 2. (See A.7.3.3 for a detailed discussion of
this subject.) Appendix B lists values of conductivity for typical
liquids. What is important is that charge should decay from
the liquid fast enough to avoid ignition hazards. The accept-
able conductivity in any particular application can be larger or
smaller than this range, depending on flow rate and process-
ing conditions.
Conductive liquids, defined as having conductivities
greater than 10
4
pS/m, do not pose a hazard due to static elec-
tric charge accumulation in typical hydrocarbon and chemical
processing and handling operations. Liquids having conduc-
tivities of 50 pS/m to 10
4
pS/m are considered semiconduc-
tive by this recommended practice.
The charging characteristics of many industrial liquids,