human-source ignition exists. See Table
1 for the lowest minimum ignition energy for selected combustible liquids.
Static-accumulating liquids (i.e., those
with low electrical conductivity) are typically non-polar aliphatic and aromatic
hydrocarbons; however, there are exceptions, such as carbon disulfide. When
liquids are transferred, a static charge
accumulates in the liquid receiver and
the liquid surface voltage increases as
liquid levels rise. The resulting charge, if
not dissipated, may produce a spark.
Control Measures
As a result of numerous recent accidents
such as the Barton Solvents fire, The CSB
has recommended increased awareness of
static electrical hazards in handling nonconductive flammable liquids. (Further
details can be found in the CSB case
studies mentioned in the sidebar on page
34.) Personal protective equipment (such
as cotton garments, static-free zippers
and conductive footwear) and a host of
work practices (such as the use of static
discharge stations and static safe floors
and mats) are specifically designed to
minimize static buildup on personnel.
The best solution for preventing static
hazards is to dissipate the electrical charge
through bonding/grounding, use of static
collectors, anti-static additives, substitution of material and humidification.
Evaluating the buildup of static electricity can be accomplished by using a
direct-reading instrument that measures
in kilovolts (kV). Consult an electrical
engineer for proper measuring techniques and measurement analysis. This
type of meter can indicate where and
how static is generated, its magnitude
and its polarity. Currently, only one
static meter is approved by the European
Atmosphères Explosibles (ATEX) for testing in hazardous atmospheres. This instrument, manufactured by UK-based
Fraser Anti-Static Techniques, can measure static electricity on the surface of a
material at a distance of 100 millimeters.
After installation of engineering controls, a static meter can also be used to
determine the effectiveness of the abatement action.
Liquid conductivity can also be meas-
ured using specialized instruments that
measure units of conductivity in siemens
(the SI equivalent to an mho or inverse
ohm) per meter. The units of measure-
ment for liquid conductivity are reported
in picosiemens (1 x 10-12 siemens) per
meter (pS/m). Generally, liquids can be
divided into three classes according to
their conductivity: high (>100 pS/m),
medium (50–100 pS/m) and low (<50
pS/m). Because low-conductivity liquids
are not capable of dissipating the static
charge, static buildup can arise even if
the receiver is buried in the ground.
Liquids with conductivity below 50 pS/m,
called “static accumulators,” are at the
highest risk for static buildup hazards.
Some material safety data sheets
(MSDSs) include static hazard information. However, the CSB, after reviewing
many MSDSs of the most widely used
nonconductive flammable liquids, stated
in 2007 that although static charge hazards were noted, none warned specifically that these liquids can be ignited in
storage tanks. The CSB recommended
that six major oil and chemical industry
Charged Situations | FEATURE
associations ask their member companies
to improve the warnings and information on the MSDSs of flammable liquids
because these materials can accumulate
static electricity. Specifically, the CSB requested that these MSDSs contain the
warning “static accumulators,” conductivity testing data and specific examples
of additional precautions to be observed,
and that they reference relevant consensus guidance (e.g., American National
Standards Institute standard, American
Petroleum Institute Recommended Practices and National Fire Protection standards). Other recommended warnings
would include, “This liquid may accumulate static electricity even when transferred into properly grounded
containers” and “Bonding and grounding
may be insufficient to remove static
electricity.” When small amounts of
water are present in some liquids, the
static electricity may be significantly increased and lower liquid flow velocities
should be maintained. This warning
should also be listed on the MSDS. In
addition, the CSB suggested that companies that handle these liquids obtain
more information not found on MSDSs.
There are several ways to prevent or
reduce static discharge:
Bonding and grounding. An object is
properly grounded when it is connected
to ground through adequate wiring,
which eliminates the charge potential
between the materials and the ground.
Bonding will eliminate a difference in
potential between materials but will not
eliminate a difference in potential between these materials and earth. Bonding must be used in conjunction with
grounding to reduce the potential for
static discharge. Common mistakes in
grounding include:
• inadequate depth of rods or stakes
• improper rod or stake material
• inadequately sized wire
• improper or corroded connectors
• connection to nongrounded piping
systems and building support beams
Remember that bonding and ground-
ing does not eliminate the surface
charge—the buildup of charge on surfaces
generated when two objects come into
contact—created when liquid is dispensed.
Consult manufacturers’ recommendations
