Paper presented at SCHC Spring 2009 Meeting Houston, Texas .docx

Paper presented at SCHC Spring 2009 Meeting
Houston, Texas
April 7-8, 2009
1
Hazards of “Static Accumulating” Flammable Liquids
James Reppermund (Presenter)
Consulting Engineer, Howell, NJ
and
Laurence G. Britton PhD CEng CPhys
Consulting Scientist, Charleston WV
Abstract
In June 2008, The U.S Chemical Safety Board issued its report
2007-06-I-KS on its
investigation of an explosion and fire at Barton Solvents in
Valley Center, Kansas. The
recommendations issued with this report included the need for
improved MSDS
communication about the hazards associated with a particular

class of flammable
liquids. This paper attempts to explain what happened, why it
happened and offers
suggestions as to what statements can be added to future MSDSs
for these products to
alert the MSDS readers of the unusual hazards of these products
Introduction
In June 2008 the U.S. Chemical Safety Board (CSB) issued Case
Study No. 2007-06-I-
KS describing a tank explosion at Barton Solvents. The
recommendations included
improved communications for MSDS preparers. In brief:
1. Warn of liquids that are both “static accumulators” and can
form ignitable vapor-
air mixtures inside storage tanks
2. Warn that bonding and grounding may not be enough
3. Give specific examples of additional precautions needed.
4. Include conductivity testing data so that companies can apply
published guidance

Houston, Texas
April 7-8, 2009
2
What Happened at Barton Solvents
In July of 2007, Barton Solvents experienced a catastrophic fire
at its Valley Center,
Kansas facility. The fire destroyed the tank farm and caused the
evacuation of
approximately 6000 local residents. This incident occurred
during multi-stage unloading
of a multi-compartmented tank truck of VM&P (Varnish
Makers’ and Painters’) naphtha,
a NFPA Class IB flammable liquid.
The most likely cause of ignition was considered to be a spark
caused by a loose
connection between the metal float and the grounded metal tape
in the storage tank’s
level gauge system. An analysis showed that the float might
briefly attain a high voltage

during multi-stage loading of the tank and spark to the grounded
tape. However, it could
not be ruled out that ignition might have been caused by a non-
spark static discharge
from the liquid itself. A possible location for such a static
discharge (brush discharge)
was from the liquid to the side of the float.
The particular grade of VM&P naphtha involved (flash point
58ºF) is one of a
comparatively small number of commercial hydrocarbon
products that has both a low
conductivity and a vapor pressure that provides a persistent,
easily ignitable vapor-air
mixture close to the liquid surface in closed vessels or
containers. This is where ignition
must occur in cases where static discharges are produced by the
charged liquid itself.
In the Barton Solvents case, the ungrounded component of the
float gauge was also
located close to the liquid surface and the liquid was loaded at
77ºF (about 20ºF above its
flash point). The most easily ignitable vapor-air mixture
typically occurs about half way

between the Lower Flammable Limit (LFL) and the Upper
Flammable Limit (UFL). This
condition can be exhibited by many NFPA Class IB liquids and
(at higher ambient
temperatures) by many Class IC liquids. Some Class IB liquids,
such as most gasolines,
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April 7-8, 2009
3
quickly exceed the UFL close to the liquid surface as a tank is
filled, owing to the
presence of volatile “light end” components. However, many
pure liquids such as
toluene, and hydrocarbon mixtures such as some VM&P
naphthas, lack volatile “light
ends” and do not exceed the UFL during tank filling. They are
therefore more prone to
ignite by static discharges. Toluene, for example, maintains its
most easily ignitable
vapor-air mixture near the liquid surface throughout tank filling
in the high-70s

Fahrenheit and many static ignitions of toluene have been
reported.
What is Static Accumulation?
The liquids that are the object of the CSB recommendations are
low conductivity liquids,
also known as Static Accumulating liquids.
The defining characteristic of electrical conductivity is how
quickly electrical charge
moves over the surface of a material or through the body of a
material. When electrical
charges can move easily, the material is defined as a conductor.
When electrical charges
move very slowly, the material is defined as a non-conductor or
an insulator. Solid
materials can be classified both by volume resistivity and
surface resistivity, since the
movement of electrical charges across a solid surface is distinct
from movement through
the bulk material. Since liquids have electrical charges
distributed throughout the body of
the liquid, they are classified only by volume resistivity. It is
customary to use the inverse

of resistivity, conductivity, to electrostatically classify liquids.
The units of volume
conductivity are Siemens per meter. One Siemens is the
conductance of a material in
which an electric current of one ampere is produced by an
electrical potential of one volt.
The Siemens is the SI equivalent of the “Mho” (which, in turn,
is an inverse Ohm).
Low conductivity liquids (also called non-conductive liquids or
insulating liquids) have a
high resistance to the flow of electrons and will retain
significant electrical charge for
seconds or even minutes. Virtually all refined, petroleum-based
hydrocarbon products
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April 7-8, 2009
4
have low conductivity and are considered to be “static
accumulating”. Many of these also
create a persistent, easily ignitable vapor-air mixture close to
the liquid surface in closed

vessels or containers filled at ordinary ambient temperatures.
Examples include the
following Class IB flammable liquids:
Benzene Cyclohexane
Heptane VM&P Naphtha
Hexane Toluene
Various non-petroleum based liquids such as simple ethers,
carbon disulfide and
hexamethyldisilazane fall into this category as well.
Why is Static Accumulation a Hazard?
Low conductivity liquids, also called non-conductive liquids,
have a high resistance to
the flow of electrons and will retain an electrical charge for
significant lengths of time.
As with insulating solids such as plastics, once these materials
are charged, they will
remain charged even when in contact with grounded metal
surfaces. Since the electrical
charges are unable to move quickly to ground, they can build up

or accumulate in the
liquid receiver (tank, container, etc) provided there is a
continuous source of charging.
This is why low conductivity liquids are also called Static
Accumulating Liquids.
When a static accumulating liquid becomes charged, it can
cause ungrounded conductors
that are in contact with the liquid or near to the liquid to
become charged. If the charged,
ungrounded conductor becomes grounded, there can be a spark.
If the spark has sufficient
energy and if the spark occurs in an ignitable vapor-air mixture,
the result will be a fire.
Other hazards are more insidious and less obvious. When a
charged, static accumulating
liquid is pumped into a tank the surface voltage on the liquid in
the tank increases as
Houston, Texas
April 7-8, 2009
5

liquid level builds. The voltage can in some cases be
sufficiently high for static
discharges to occur from the liquid itself, even if the tank is
properly bonded and
grounded. Discharges known as “brush discharges” often occur
to grounded projections
above the liquid surface such as the ends of filling pipes (which
is one reason a slow start
is often used until dip pipes are submerged). Static discharges
may also in some cases
form “streamers” at the tank wall that travel across the liquid
surface. Both types of static
discharges may ignite flammable vapors in air under the right
conditions, which might
only occur once during years of operation.
To avoid ignition various precautions are required to limit the
accumulation of charge.
These are given in codes of practice and include such measures
as restriction of flow
velocity, depending on the size of tank, filling pipe diameter
and other conditions.
How do You Determine the Potential for Static Accumulation?

The propensity of a liquid to accumulate static electricity can
quickly be determined by
measuring the liquid’s electrical conductivity. Instruments are
commercially available
that can quickly and (relatively) inexpensively make this
measurement. The instrument
selected to make these measurements must be capable of
measurements in the pico
Siemens (pS) range. One pico Siemens is equal to 1 x 10-12
Siemens. Typical laboratory
conductivity meters only measure in the micro Siemens range,
roughly 6 orders of
magnitude larger than what is needed. While there is no longer
a single ASTM Standard
Test Method that is applicable to the testing of all liquids,
including high conductivity
liquids such as alcohols, ASTM D2624 Standard Test Methods
for Electrical
Conductivity of Aviation and Distillate Fuels addresses the
conductivity range up to 2000
pS/m, which includes the low conductivity liquids discussed in
this paper. However,
instruments are commercially available that measure
conductivities over very wide

ranges, based on other standardized test procedures. Hence it is
possible to determine the
conductivity of almost any liquid for MSDS reporting purposes.
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Liquids having a measured conductivity of 100 pS/m or less are
considered to be “Static
Accumulating” liquids.
Liquids having a measured conductivity greater than 100 pS/m
are considered to be either
“Semi-Conductive” liquids or “Conductive Liquids”. The
hazards of such liquids are
specific to the handling conditions and will be mentioned only
briefly in this paper.
The demarcation of 100 pS/m as given above is arguably
conservative for hydrocarbons
(at least in the case of tank filling), and the Petroleum Industry
in particular uses a lower

demarcation of 50 pS/m. However, the conductivity of “Static
Accumulating” liquids is
highly dependent on temperature and purity. The conductivity
of a liquid handled in a
chemical plant on a cold day might be only one-third of that
measured in the laboratory.
Also, at a given temperature, it is common for samples of the
“same” liquid to have quite
different conductivities depending on the source of the liquid. A
“pure” liquid such as n-
heptane has virtually no intrinsic conductivity and what is
measured is the effect of trace
contaminants. Different samples of n-heptane could have
conductivities varying by at
least two orders of magnitude. Another complication is that the
rate at which a charged
liquid loses its charge depends on its dielectric constant. This
typically ranges from about
2 for hydrocarbons to about 4 for other “Static Accumulating”
liquids such as simple
ethers. Hence, for general reporting purposes such as MSDS, the
higher demarcation of
100 pS/m should be used.

Hazards of Suspended Water Droplets & New Hypothesis for
“Water Slug”
Hazards
According to CSB, the Barton tank likely contained sediment
plus water. It was an air-
breathing tank so water (condensed from humid, ambient air)
would gradually
accumulate in the tank bottom over time. The Barton tank
volume (~15000 gallons)
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April 7-8, 2009
7
exceeded 50 cubic meters and places it in the “large” tank size
category defined in
CENELEC 50404.
It is well known that static charging can be greatly increased
when mixtures of oil and
water are pumped together, especially when subjected to high
shear such as during

passage through a component such as a partly closed valve that
produces small water
droplets having a large interfacial area relative to the
continuous oil phase. CENELEC
50404 (2003) warns of this hazard in Chapter 5.4.4.2.1 with
respect to “medium” tanks
(1-50 cubic meters) where it states: “For two-phase flow or if
water bottoms could be
stirred up in the tank, the filling velocity should be restricted to
1 m/s”. Here the usual
“two-phase flow” warning has been extended to address
suspension of water droplets
derived from water bottoms already in the receiving tank.
Most air-breathing tanks are likely to contain water bottoms that
could be stirred up
(provided water is insoluble in the lading) and the authors are
unaware of what (if any)
procedures are used to limit its accumulation. It is not
uncommon for a “side-bottom-
entry” fill pipe to double as the outlet pipe, in which case the
limiting factor would be
entrainment of settled water bottoms during tank emptying. The
inlet/outlet pipe is often

located close to the tank floor.
CENELEC’s “1 m/s” flow velocity restriction with respect to
water bottoms in “medium”
tanks (1-50 cubic meters) is not currently provided in other
codes such as NFPA 77.
Also, CENELEC does not apply the 1 m/s flow rate restriction
to tanks larger than 50
cubic meters, which are designated as “large” tanks. The reason
behind the selected
volume cut-off is likely based on the maximum capacity of
single compartment tank
trucks (<50 cubic meters and typically about 26 cubic meters).
The 50 cubic meter cut-
off allows ready differentiation between tank trucks and rail
cars, which have a larger
capacity (typically about 89 cubic meters for single
compartment cars). Greater flow
velocities are allowed for rail cars than for tank trucks. The
reader should refer to
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April 7-8, 2009

8
CENELEC 50404 and NFPA 77 for specific recommendations.
It is important to
recognize the practicalities involved in “tank size” definitions.
Although the Barton tank
strictly exceeded 50 cubic meters capacity, the geometry of
vertical storage tanks results
in faster accumulation of liquid level and larger surface
voltages than would apply to a
rail car of equal capacity, all other factors being equal. It is
prudent to apply significant
latitude when considering what flow rate restrictions should be
applied to vertical storage
tanks.
The discussion has so far focused on hazards caused by
suspension of small water
droplets. We now propose a new hazard scenario in which large
“slugs” of water derived
from tank water bottoms pose a spark ignition hazard, even
where all tank components
are properly bonded and grounded. This hazard is different than
that of “increased static”
caused by suspension of small water droplets; it is potentially

far more severe and has not
previously been recognized.
If a tank has significant water bottoms and is side filled at the
bottom, as was the Barton
tank, it is possible that large water “slugs” will be launched into
the liquid and convected
to the surface. Electrostatic charging of water slugs may occur
via a variety of
mechanisms once they are adrift in the charged oil. Collision
with grounded tank
components and break-up of slugs, particularly in regions of
high electric fields, is in
many ways a more plausible spark ignition scenario than the
much-studied “supertanker
water washing” explosion scenario advanced in the early 1970s
(Britton 1999 pages 217-
218).
The hypothesized water slugs will at this point be “charged
ungrounded conductors” that
may spark to the tank wall. The minimum voltage for vapor
ignition via sparking is less
than about 10 kV, depending on the size of the slug. Such

voltages are commonly
exceeded when filling medium sized storage tanks. Hence, all
that is needed is for a slug
of sufficient size (capacitance) and voltage to attain the correct
trajectory through the
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April 7-8, 2009
9
liquid. Conversely, the minimum voltage for ignition via a
brush discharge is at least 25
kV (see later). This 25 kV threshold applies only to negative
charging with an optimized
electrode and vapor-air mixture above the liquid surface; in all
other cases the threshold
voltage is greater. It follows that increased static due to
suspended water droplets should
be far less of a hazard than the formation of large water slugs.
Note that the specific
gravities of some common Class IB liquids are not very
different from that of water;
while heptane has a specific gravity of about 0.7, that of typical
VM&P is about 0.8 and

toluene is about 0.9.
This “water slug” hypothesis might explain some atmospheric
tank explosions that did
not involve high flow velocities or other adverse conditions,
such as pumping oil-water
mixtures or the location of a microfilter close to the tank. If
large water slugs can float
around in a receiving tank, even slow filling velocities might
not exclude the possibility
of spark ignition; indeed, slower velocities might favor the
creation of larger slugs.
However, the use of decreased flow rates will reduce the liquid
voltage in the tank and
hence the ignition frequency.
As a practical matter, it would be helpful to gather information
on the accumulation of
water bottoms in air breathing storage tanks. If there is general
consensus that the
problem needs to be addressed, we hope that the matter will be
taken up by an
appropriate safety organization.

The “water slug” hypothesis is not currently recognized in
codes of practice. A suitable
warning statement would need to address water bottoms
directly, such as “Do not load
liquid into tank containing water bottoms that could be stirred
up”. As noted above, there
is no “safe” flow velocity associated with the hypothetical
scenario.
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April 7-8, 2009
10
What Can Be Done About Static Accumulation?
Static accumulating liquids can become charged by numerous
different operations
including but not limited to:
• Spraying

• Air entrainment
• High velocity flow or agitation
• Two phase flow or mixing
• Settling of an entrained solids or immiscible second phase
• Passing through a micro-filter
Static accumulation in low conductivity liquids cannot be
prevented although it can be
reduced by reduced velocity and the addition of conductive
liquids or of conductivity
enhancing materials.
Part-per-million levels of antistatic additive can be used to raise
the conductivity of a
nonconductive liquid to above 100 pS/m depending on the needs
of the customer. This
eliminates the need to use the phrase “Static Accumulating
Liquid”. However, static
accumulation cannot be prevented under all conditions.
The phrase “Static Accumulating Liquid” (or “Static
Accumulator”) must be confined to
those liquids that may accumulate hazardous levels of static

charge when pumped into
properly grounded metal tanks or containers. The purpose of the
warning is to identify
those liquids that may accumulate sufficient surface voltage for
a so-called “brush
discharge” to occur. This is generally associated with liquid
conductivities less than 100
pS/m and usually much less than this value. However, as
discussed above, conductivity is
not constant and for communication purposes, a safety factor is
needed to account for
batch-to-batch variation plus the effect of low ambient
temperatures.
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April 7-8, 2009
11
Codes of practice on “Static Electricity” warn of precautions
needed to fill tanks safely,
such as limiting the flow velocity and using dip pipes. The
codes also warn of special
hazards such as nonconductive tanks, plastic lined tanks,

entrainment of air or water,
passage through static generators such as micro-filters and
partly blocked strainers, and
suspension of water bottoms in a tank. The user should follow
these recommendations.
However, the recommendations are not consistent in different
codes and also vary with
the type and size of the tank. It is impossible to summarize all
of these on a MSDS and
reference should be made to codes such as NFPA 77 and
CENELEC CLC/TR 50404
(which has far more explicit information on tank filling
precautions). In the next year it is
expected that a new IEC document will be issued that will
update and replace the
CENELEC document.
It must be emphasized that various common process operations
such as two phase mixing
and spraying can accumulate static electricity at much higher
conductivities than 100
pS/m. For various mixing operations, it is common practice to
increase the liquid
conductivity to several thousand pS/m to avoid static problems,

such as by adding a
suitable conductive liquid to a nonconductive hydrocarbon.
Even alcohols and ketones,
which typically have conductivities of 1 million pS/m or more,
can accumulate hazardous
static on ungrounded spray nozzles such as in painting
applications.
Even where all other precautions are taken, an ungrounded
person may be the cause of a
static spark, independent of any electrical properties of the
liquid. Hence general warning
statements about static ignition should be given separately
along with boilerplate
warnings about open flames and the like.
What Should be on the MSDS to warn of Static Accumulation
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April 7-8, 2009
12

1. Include a representative “standard” conductivity
measurement (25ºC) on the
MSDS for all static accumulating liquids and identify them as
both “static
accumulating liquid” and “low conductivity liquid (non-
conductive liquid)”.
Warn that the value may change with temperature and purity,
including how the
liquid is stored and handled.
2. Include where possible a representative conductivity value
for all liquids, so that
various code practices can be applied. The conductivity should
at a minimum be
given for liquids with conductivity up to 2000 pS/m, which is
within the
capabilities of various commercial instruments and includes the
demarcation of
1000 pS/m used by the Petroleum Industry for “high
conductivity liquid”. Since a
higher demarcation of 10,000 pS/m is widely used in the
Chemical Industry
(especially for operations such as liquid-solid mixing), it would
be prudent to
have a 20,000 pS/m capability, as available in some commercial

instruments.
Ideally, the instrument should be capable of wide range
determination from less
than 1 pS/m to about 10,000,000 pS/m, so conductivities can be
given not only for
“static accumulating liquids” but also for commonly used
conductive solvents
such as many esters, alcohols and ketones. Note that some
liquids have
intermediate conductivity (between 50 and 1000 pS/m, or
between 100 and
10,000 pS/m, depending on the code of practice referred to) and
are described as
“medium conductivity” or “semi conductive”. These require
special consideration
in various Codes of Practice. It can be seen that a conductivity
value is more
useful than a description that varies with the Code of Practice
referred to.
3. Suggested warning statements for “static accumulating
liquids” include:
• “This liquid may accumulate static electricity when filling
properly

grounded containers.”
• “Bonding and grounding may be insufficient to remove static
electricity.”
• “Static electricity accumulation may be significantly increased
by the
presence of small quantities of water or other contaminants.”
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April 7-8, 2009
13
• “Restrict flow velocity according to (CITE APPLICABLE
CODE)”
• “Refer to Codes of Practice” (insert applicable code, such as
CLC/TR
50404 in the EU, for guidance). We are awaiting the issuance of
an IEC
“Static Electricity” code in late 2009 or early 2010 that is
internationally
recognized and should greatly help the MSDS preparer.
To address the ignitability issue, MSDS preparers must consider
the criterion of a vapor

pressure that provides “a persistent, easily ignitable vapor-air
mixture close to the liquid
surface” in closed vessels or containers. This is simple in the
case of pure liquids because
the vapor pressure at different temperatures can be simply
related to the known
flammable limits. For mixtures, it is more complex. However,
for a first pass the criterion
may be applied to NFPA Class IB and IC liquids as discussed by
the CSB, with
exceptions made where applicable. Some Class IB liquids such
as gasoline and light
naphthas might be excluded while under cold weather
assumptions some borderline Class
IA liquids might be included. Figure 1 below shows the vapor
pressure curve of a typical
hydrocarbon Static Accumulating Liquid (Toluene in this case),
indicating the
temperatures where the Lower Flammable Limit will occur and
where the Upper
Flammable Limit will occur.
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April 7-8, 2009
14
0
10
20
30
40
50
60
-10 0 10 20 30 40 50
Figure 1: Toluene Flammability Limits at 1 atm
Temperature Limits of Flammability (TLF)
and Most Easily Ignitable Temperature
V
ap
or
P
re
ss
ur

e
(m
m
H
g)
Equilibrium Temperature (C)
UFL = 7.1 vol% (54 mmHg)
Upper TLF
= 38 C
LFL = 1.1 vol%
(8.36 mmHg)
Lower TLF
= 3.2 C
Most Easily Ignitable
~26C (Britton 1999)
A suggested warning statement is:
• “This liquid may form an ignitable vapor-air mixture in closed
tanks or
containers”
• “Additional advice on handling and processing low

conductivity liquids
can be found in the following documents –
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15
o NFPA 77 – Recommended Practice on Static Electricity,
National
Fire Protection Association
o RP-2003 – Protection Against Ignitions Arising Out of Static,
Lightning, and Stray Currents, American Petroleum Institute
o TR- 50404 – Code of Practice for the Avoidance of Hazards
Due
to Static Electricity, CENELEC, European Committee for
Electrotechnical Standardization”
o Generation and Control of Static Electricity in Coatings
Operations, National Paint and Coatings Association
o Britton, L.G., “Avoiding Static Ignition Hazards in Chemical
Operations”, AIChE-CCPS (1999)
In some cases tank inerting might be considered. This is
described in:

o NFPA 69 – Standard on Explosion Protection Systems,
National
Fire Protection Association.
Plate 1 shows a roughly two-inch long “positive brush”
discharge from a negatively
charged diesel oil surface to a grounded electrode (Britton,
L.G., and T. Williams, “Some
Characteristics of Liquid-to-Metal Discharges involving a
Charged Oil”, J. Electrostatics,
13 (1982) pp. 185-207). The picture was taken using a high gain
image intensifier so does
not show the liquid surface or the electrode. The upper
electrode was a ½-inch steel
sphere, intended to represent a probe such as the end of a
thermowell above electrically
charged liquid in a tank. Discharges of this type were able to
ignite mixtures of propane
or butane in air at liquid voltages above -25 kV.
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16
Plate 1
Two-inch Long, Incendiary Brush Discharge from Negatively
Charged
Oil to ½-inch Grounded Spherical Electrode
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1
Hazards of “Static Accumulating” Flammable Liquids
James Reppermund (Presenter)
Consulting Engineer, Howell, NJ
and
Laurence G. Britton PhD CEng CPhys

Consulting Scientist, Charleston WV
Abstract
In June 2008, The U.S Chemical Safety Board issued its report
2007-06-I-KS on its
investigation of an explosion and fire at Barton Solvents in
Valley Center, Kansas. The
recommendations issued with this report included the need for
improved MSDS
communication about the hazards associated with a particular
class of flammable
liquids. This paper attempts to explain what happened, why it
happened and offers
suggestions as to what statements can be added to future MSDSs
for these products to
alert the MSDS readers of the unusual hazards of these products
Introduction
In June 2008 the U.S. Chemical Safety Board (CSB) issued Case
Study No. 2007-06-I-
KS describing a tank explosion at Barton Solvents. The
recommendations included

improved communications for MSDS preparers. In brief:
1. Warn of liquids that are both “static accumulators” and can
form ignitable vapor-
air mixtures inside storage tanks
2. Warn that bonding and grounding may not be enough
3. Give specific examples of additional precautions needed.
4. Include conductivity testing data so that companies can apply
published guidance
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April 7-8, 2009
2
What Happened at Barton Solvents
In July of 2007, Barton Solvents experienced a catastrophic fire
at its Valley Center,
Kansas facility. The fire destroyed the tank farm and caused the
evacuation of
approximately 6000 local residents. This incident occurred

during multi-stage unloading
of a multi-compartmented tank truck of VM&P (Varnish
Makers’ and Painters’) naphtha,
a NFPA Class IB flammable liquid.
The most likely cause of ignition was considered to be a spark
caused by a loose
connection between the metal float and the grounded metal tape
in the storage tank’s
level gauge system. An analysis showed that the float might
briefly attain a high voltage
during multi-stage loading of the tank and spark to the grounded
tape. However, it could
not be ruled out that ignition might have been caused by a non-
spark static discharge
from the liquid itself. A possible location for such a static
discharge (brush discharge)
was from the liquid to the side of the float.
The particular grade of VM&P naphtha involved (flash point
58ºF) is one of a
comparatively small number of commercial hydrocarbon
products that has both a low
conductivity and a vapor pressure that provides a persistent,

easily ignitable vapor-air
mixture close to the liquid surface in closed vessels or
containers. This is where ignition
must occur in cases where static discharges are produced by the
charged liquid itself.
In the Barton Solvents case, the ungrounded component of the
float gauge was also
located close to the liquid surface and the liquid was loaded at
77ºF (about 20ºF above its
flash point). The most easily ignitable vapor-air mixture
typically occurs about half way
between the Lower Flammable Limit (LFL) and the Upper
Flammable Limit (UFL). This
condition can be exhibited by many NFPA Class IB liquids and
(at higher ambient
temperatures) by many Class IC liquids. Some Class IB liquids,
such as most gasolines,
Houston, Texas
April 7-8, 2009
3
quickly exceed the UFL close to the liquid surface as a tank is

filled, owing to the
presence of volatile “light end” components. However, many
pure liquids such as
toluene, and hydrocarbon mixtures such as some VM&P
naphthas, lack volatile “light
ends” and do not exceed the UFL during tank filling. They are
therefore more prone to
ignite by static discharges. Toluene, for example, maintains its
most easily ignitable
vapor-air mixture near the liquid surface throughout tank filling
in the high-70s
Fahrenheit and many static ignitions of toluene have been
reported.
What is Static Accumulation?
The liquids that are the object of the CSB recommendations are
low conductivity liquids,
also known as Static Accumulating liquids.
The defining characteristic of electrical conductivity is how
quickly electrical charge
moves over the surface of a material or through the body of a
material. When electrical

charges can move easily, the material is defined as a conductor.
When electrical charges
move very slowly, the material is defined as a non-conductor or
an insulator. Solid
materials can be classified both by volume resistivity and
surface resistivity, since the
movement of electrical charges across a solid surface is distinct
from movement through
the bulk material. Since liquids have electrical charges
distributed throughout the body of
the liquid, they are classified only by volume resistivity. It is
customary to use the inverse
of resistivity, conductivity, to electrostatically classify liquids.
The units of volume
conductivity are Siemens per meter. One Siemens is the
conductance of a material in
which an electric current of one ampere is produced by an
electrical potential of one volt.
The Siemens is the SI equivalent of the “Mho” (which, in turn,
is an inverse Ohm).
Low conductivity liquids (also called non-conductive liquids or
insulating liquids) have a
high resistance to the flow of electrons and will retain
significant electrical charge for

seconds or even minutes. Virtually all refined, petroleum-based
hydrocarbon products
Houston, Texas
April 7-8, 2009
4
have low conductivity and are considered to be “static
accumulating”. Many of these also
create a persistent, easily ignitable vapor-air mixture close to
the liquid surface in closed
vessels or containers filled at ordinary ambient temperatures.
Examples include the
following Class IB flammable liquids:
Benzene Cyclohexane
Heptane VM&P Naphtha
Hexane Toluene
Various non-petroleum based liquids such as simple ethers,
carbon disulfide and
hexamethyldisilazane fall into this category as well.

Why is Static Accumulation a Hazard?
Low conductivity liquids, also called non-conductive liquids,
have a high resistance to
the flow of electrons and will retain an electrical charge for
significant lengths of time.
As with insulating solids such as plastics, once these materials
are charged, they will
remain charged even when in contact with grounded metal
surfaces. Since the electrical
charges are unable to move quickly to ground, they can build up
or accumulate in the
liquid receiver (tank, container, etc) provided there is a
continuous source of charging.
This is why low conductivity liquids are also called Static
Accumulating Liquids.
When a static accumulating liquid becomes charged, it can
cause ungrounded conductors
that are in contact with the liquid or near to the liquid to
become charged. If the charged,
ungrounded conductor becomes grounded, there can be a spark.
If the spark has sufficient
energy and if the spark occurs in an ignitable vapor-air mixture,

the result will be a fire.
Other hazards are more insidious and less obvious. When a
charged, static accumulating
liquid is pumped into a tank the surface voltage on the liquid in
the tank increases as
Houston, Texas
April 7-8, 2009
5
liquid level builds. The voltage can in some cases be
sufficiently high for static
discharges to occur from the liquid itself, even if the tank is
properly bonded and
grounded. Discharges known as “brush discharges” often occur
to grounded projections
above the liquid surface such as the ends of filling pipes (which
is one reason a slow start
is often used until dip pipes are submerged). Static discharges
may also in some cases
form “streamers” at the tank wall that travel across the liquid
surface. Both types of static
discharges may ignite flammable vapors in air under the right

conditions, which might
only occur once during years of operation.
To avoid ignition various precautions are required to limit the
accumulation of charge.
These are given in codes of practice and include such measures
as restriction of flow
velocity, depending on the size of tank, filling pipe diameter
and other conditions.
How do You Determine the Potential for Static Accumulation?
The propensity of a liquid to accumulate static electricity can
quickly be determined by
measuring the liquid’s electrical conductivity. Instruments are
commercially available
that can quickly and (relatively) inexpensively make this
measurement. The instrument
selected to make these measurements must be capable of
measurements in the pico
Siemens (pS) range. One pico Siemens is equal to 1 x 10-12
Siemens. Typical laboratory
conductivity meters only measure in the micro Siemens range,
roughly 6 orders of

magnitude larger than what is needed. While there is no longer
a single ASTM Standard
Test Method that is applicable to the testing of all liquids,
including high conductivity
liquids such as alcohols, ASTM D2624 Standard Test Methods
for Electrical
Conductivity of Aviation and Distillate Fuels addresses the
conductivity range up to 2000
pS/m, which includes the low conductivity liquids discussed in
this paper. However,
instruments are commercially available that measure
conductivities over very wide
ranges, based on other standardized test procedures. Hence it is
possible to determine the
conductivity of almost any liquid for MSDS reporting purposes.
Houston, Texas
April 7-8, 2009
6
Liquids having a measured conductivity of 100 pS/m or less are
considered to be “Static
Accumulating” liquids.

Liquids having a measured conductivity greater than 100 pS/m
are considered to be either
“Semi-Conductive” liquids or “Conductive Liquids”. The
hazards of such liquids are
specific to the handling conditions and will be mentioned only
briefly in this paper.
The demarcation of 100 pS/m as given above is arguably
conservative for hydrocarbons
(at least in the case of tank filling), and the Petroleum Industry
in particular uses a lower
demarcation of 50 pS/m. However, the conductivity of “Static
Accumulating” liquids is
highly dependent on temperature and purity. The conductivity
of a liquid handled in a
chemical plant on a cold day might be only one-third of that
measured in the laboratory.
Also, at a given temperature, it is common for samples of the
“same” liquid to have quite
different conductivities depending on the source of the liquid. A
“pure” liquid such as n-
heptane has virtually no intrinsic conductivity and what is
measured is the effect of trace

contaminants. Different samples of n-heptane could have
conductivities varying by at
least two orders of magnitude. Another complication is that the
rate at which a charged
liquid loses its charge depends on its dielectric constant. This
typically ranges from about
2 for hydrocarbons to about 4 for other “Static Accumulating”
liquids such as simple
ethers. Hence, for general reporting purposes such as MSDS, the
higher demarcation of
100 pS/m should be used.
Hazards of Suspended Water Droplets & New Hypothesis for
“Water Slug”
Hazards
According to CSB, the Barton tank likely contained sediment
plus water. It was an air-
breathing tank so water (condensed from humid, ambient air)
would gradually
accumulate in the tank bottom over time. The Barton tank
volume (~15000 gallons)

Houston, Texas
April 7-8, 2009
7
exceeded 50 cubic meters and places it in the “large” tank size
category defined in
CENELEC 50404.
It is well known that static charging can be greatly increased
when mixtures of oil and
water are pumped together, especially when subjected to high
shear such as during
passage through a component such as a partly closed valve that
produces small water
droplets having a large interfacial area relative to the
continuous oil phase. CENELEC
50404 (2003) warns of this hazard in Chapter 5.4.4.2.1 with
respect to “medium” tanks
(1-50 cubic meters) where it states: “For two-phase flow or if
water bottoms could be
stirred up in the tank, the filling velocity should be restricted to
1 m/s”. Here the usual
“two-phase flow” warning has been extended to address
suspension of water droplets
derived from water bottoms already in the receiving tank.

Most air-breathing tanks are likely to contain water bottoms that
could be stirred up
(provided water is insoluble in the lading) and the authors are
unaware of what (if any)
procedures are used to limit its accumulation. It is not
uncommon for a “side-bottom-
entry” fill pipe to double as the outlet pipe, in which case the
limiting factor would be
entrainment of settled water bottoms during tank emptying. The
inlet/outlet pipe is often
located close to the tank floor.
CENELEC’s “1 m/s” flow velocity restriction with respect to
water bottoms in “medium”
tanks (1-50 cubic meters) is not currently provided in other
codes such as NFPA 77.
Also, CENELEC does not apply the 1 m/s flow rate restriction
to tanks larger than 50
cubic meters, which are designated as “large” tanks. The reason
behind the selected
volume cut-off is likely based on the maximum capacity of
single compartment tank
trucks (<50 cubic meters and typically about 26 cubic meters).

The 50 cubic meter cut-
off allows ready differentiation between tank trucks and rail
cars, which have a larger
capacity (typically about 89 cubic meters for single
compartment cars). Greater flow
velocities are allowed for rail cars than for tank trucks. The
reader should refer to
Houston, Texas
April 7-8, 2009
8
CENELEC 50404 and NFPA 77 for specific recommendations.
It is important to
recognize the practicalities involved in “tank size” definitions.
Although the Barton tank
strictly exceeded 50 cubic meters capacity, the geometry of
vertical storage tanks results
in faster accumulation of liquid level and larger surface
voltages than would apply to a
rail car of equal capacity, all other factors being equal. It is
prudent to apply significant
latitude when considering what flow rate restrictions should be
applied to vertical storage

tanks.
The discussion has so far focused on hazards caused by
suspension of small water
droplets. We now propose a new hazard scenario in which large
“slugs” of water derived
from tank water bottoms pose a spark ignition hazard, even
where all tank components
are properly bonded and grounded. This hazard is different than
that of “increased static”
caused by suspension of small water droplets; it is potentially
far more severe and has not
previously been recognized.
If a tank has significant water bottoms and is side filled at the
bottom, as was the Barton
tank, it is possible that large water “slugs” will be launched into
the liquid and convected
to the surface. Electrostatic charging of water slugs may occur
via a variety of
mechanisms once they are adrift in the charged oil. Collision
with grounded tank
components and break-up of slugs, particularly in regions of
high electric fields, is in

many ways a more plausible spark ignition scenario than the
much-studied “supertanker
water washing” explosion scenario advanced in the early 1970s
(Britton 1999 pages 217-
218).
The hypothesized water slugs will at this point be “charged
ungrounded conductors” that
may spark to the tank wall. The minimum voltage for vapor
ignition via sparking is less
than about 10 kV, depending on the size of the slug. Such
voltages are commonly
exceeded when filling medium sized storage tanks. Hence, all
that is needed is for a slug
of sufficient size (capacitance) and voltage to attain the correct
trajectory through the
Houston, Texas
April 7-8, 2009
9
liquid. Conversely, the minimum voltage for ignition via a
brush discharge is at least 25

kV (see later). This 25 kV threshold applies only to negative
charging with an optimized
electrode and vapor-air mixture above the liquid surface; in all
other cases the threshold
voltage is greater. It follows that increased static due to
suspended water droplets should
be far less of a hazard than the formation of large water slugs.
Note that the specific
gravities of some common Class IB liquids are not very
different from that of water;
while heptane has a specific gravity of about 0.7, that of typical
VM&P is about 0.8 and
toluene is about 0.9.
This “water slug” hypothesis might explain some atmospheric
tank explosions that did
not involve high flow velocities or other adverse conditions,
such as pumping oil-water
mixtures or the location of a microfilter close to the tank. If
large water slugs can float
around in a receiving tank, even slow filling velocities might
not exclude the possibility
of spark ignition; indeed, slower velocities might favor the
creation of larger slugs.

However, the use of decreased flow rates will reduce the liquid
voltage in the tank and
hence the ignition frequency.
As a practical matter, it would be helpful to gather information
on the accumulation of
water bottoms in air breathing storage tanks. If there is general
consensus that the
problem needs to be addressed, we hope that the matter will be
taken up by an
appropriate safety organization.
The “water slug” hypothesis is not currently recognized in
codes of practice. A suitable
warning statement would need to address water bottoms
directly, such as “Do not load
liquid into tank containing water bottoms that could be stirred
up”. As noted above, there
is no “safe” flow velocity associated with the hypothetical
scenario.

Houston, Texas
April 7-8, 2009
10
What Can Be Done About Static Accumulation?
Static accumulating liquids can become charged by numerous
different operations
including but not limited to:
• Spraying
• Air entrainment
• High velocity flow or agitation
• Two phase flow or mixing
• Settling of an entrained solids or immiscible second phase
• Passing through a micro-filter
Static accumulation in low conductivity liquids cannot be
prevented although it can be
reduced by reduced velocity and the addition of conductive
liquids or of conductivity
enhancing materials.

Part-per-million levels of antistatic additive can be used to raise
the conductivity of a
nonconductive liquid to above 100 pS/m depending on the needs
of the customer. This
eliminates the need to use the phrase “Static Accumulating
Liquid”. However, static
accumulation cannot be prevented under all conditions.
The phrase “Static Accumulating Liquid” (or “Static
Accumulator”) must be confined to
those liquids that may accumulate hazardous levels of static
charge when pumped into
properly grounded metal tanks or containers. The purpose of the
warning is to identify
those liquids that may accumulate sufficient surface voltage for
a so-called “brush
discharge” to occur. This is generally associated with liquid
conductivities less than 100
pS/m and usually much less than this value. However, as
discussed above, conductivity is
not constant and for communication purposes, a safety factor is
needed to account for
batch-to-batch variation plus the effect of low ambient
temperatures.

Houston, Texas
April 7-8, 2009
11
Codes of practice on “Static Electricity” warn of precautions
needed to fill tanks safely,
such as limiting the flow velocity and using dip pipes. The
codes also warn of special
hazards such as nonconductive tanks, plastic lined tanks,
entrainment of air or water,
passage through static generators such as micro-filters and
partly blocked strainers, and
suspension of water bottoms in a tank. The user should follow
these recommendations.
However, the recommendations are not consistent in different
codes and also vary with
the type and size of the tank. It is impossible to summarize all
of these on a MSDS and
reference should be made to codes such as NFPA 77 and
CENELEC CLC/TR 50404
(which has far more explicit information on tank filling
precautions). In the next year it is

expected that a new IEC document will be issued that will
update and replace the
CENELEC document.
It must be emphasized that various common process operations
such as two phase mixing
and spraying can accumulate static electricity at much higher
conductivities than 100
pS/m. For various mixing operations, it is common practice to
increase the liquid
conductivity to several thousand pS/m to avoid static problems,
such as by adding a
suitable conductive liquid to a nonconductive hydrocarbon.
Even alcohols and ketones,
which typically have conductivities of 1 million pS/m or more,
can accumulate hazardous
static on ungrounded spray nozzles such as in painting
applications.
Even where all other precautions are taken, an ungrounded
person may be the cause of a
static spark, independent of any electrical properties of the
liquid. Hence general warning
statements about static ignition should be given separately

along with boilerplate
warnings about open flames and the like.
What Should be on the MSDS to warn of Static Accumulation
Houston, Texas
April 7-8, 2009
12
1. Include a representative “standard” conductivity
measurement (25ºC) on the
MSDS for all static accumulating liquids and identify them as
both “static
accumulating liquid” and “low conductivity liquid (non-
conductive liquid)”.
Warn that the value may change with temperature and purity,
including how the
liquid is stored and handled.
2. Include where possible a representative conductivity value
for all liquids, so that
various code practices can be applied. The conductivity should
at a minimum be

given for liquids with conductivity up to 2000 pS/m, which is
within the
capabilities of various commercial instruments and includes the
demarcation of
1000 pS/m used by the Petroleum Industry for “high
conductivity liquid”. Since a
higher demarcation of 10,000 pS/m is widely used in the
Chemical Industry
(especially for operations such as liquid-solid mixing), it would
be prudent to
have a 20,000 pS/m capability, as available in some commercial
instruments.
Ideally, the instrument should be capable of wide range
determination from less
than 1 pS/m to about 10,000,000 pS/m, so conductivities can be
given not only for
“static accumulating liquids” but also for commonly used
conductive solvents
such as many esters, alcohols and ketones. Note that some
liquids have
intermediate conductivity (between 50 and 1000 pS/m, or
between 100 and
10,000 pS/m, depending on the code of practice referred to) and
are described as

“medium conductivity” or “semi conductive”. These require
special consideration
in various Codes of Practice. It can be seen that a conductivity
value is more
useful than a description that varies with the Code of Practice
referred to.
3. Suggested warning statements for “static accumulating
liquids” include:
• “This liquid may accumulate static electricity when filling
properly
grounded containers.”
• “Bonding and grounding may be insufficient to remove static
electricity.”
• “Static electricity accumulation may be significantly increased
by the
presence of small quantities of water or other contaminants.”
Houston, Texas
April 7-8, 2009
13
• “Restrict flow velocity according to (CITE APPLICABLE

CODE)”
• “Refer to Codes of Practice” (insert applicable code, such as
CLC/TR
50404 in the EU, for guidance). We are awaiting the issuance of
an IEC
“Static Electricity” code in late 2009 or early 2010 that is
internationally
recognized and should greatly help the MSDS preparer.
To address the ignitability issue, MSDS preparers must consider
the criterion of a vapor
pressure that provides “a persistent, easily ignitable vapor-air
mixture close to the liquid
surface” in closed vessels or containers. This is simple in the
case of pure liquids because
the vapor pressure at different temperatures can be simply
related to the known
flammable limits. For mixtures, it is more complex. However,
for a first pass the criterion
may be applied to NFPA Class IB and IC liquids as discussed by
the CSB, with
exceptions made where applicable. Some Class IB liquids such
as gasoline and light
naphthas might be excluded while under cold weather

assumptions some borderline Class
IA liquids might be included. Figure 1 below shows the vapor
pressure curve of a typical
hydrocarbon Static Accumulating Liquid (Toluene in this case),
indicating the
temperatures where the Lower Flammable Limit will occur and
where the Upper
Flammable Limit will occur.
Houston, Texas
April 7-8, 2009
14
0
10
20
30
40
50
60

-10 0 10 20 30 40 50
Figure 1: Toluene Flammability Limits at 1 atm
Temperature Limits of Flammability (TLF)
and Most Easily Ignitable Temperature
V
ap
or
P
re
ss
ur
e
(m
m
H
g)
Equilibrium Temperature (C)
UFL = 7.1 vol% (54 mmHg)
Upper TLF
= 38 C
LFL = 1.1 vol%
(8.36 mmHg)
Lower TLF
= 3.2 C

Most Easily Ignitable
~26C (Britton 1999)
A suggested warning statement is:
• “This liquid may form an ignitable vapor-air mixture in closed
tanks or
containers”
• “Additional advice on handling and processing low
conductivity liquids
can be found in the following documents –
Houston, Texas
April 7-8, 2009
15
o NFPA 77 – Recommended Practice on Static Electricity,
National
Fire Protection Association
o RP-2003 – Protection Against Ignitions Arising Out of Static,
Lightning, and Stray Currents, American Petroleum Institute

o TR- 50404 – Code of Practice for the Avoidance of Hazards
Due
to Static Electricity, CENELEC, European Committee for
Electrotechnical Standardization”
o Generation and Control of Static Electricity in Coatings
Operations, National Paint and Coatings Association
o Britton, L.G., “Avoiding Static Ignition Hazards in Chemical
Operations”, AIChE-CCPS (1999)
In some cases tank inerting might be considered. This is
described in:
o NFPA 69 – Standard on Explosion Protection Systems,
National
Fire Protection Association.
Plate 1 shows a roughly two-inch long “positive brush”
discharge from a negatively
charged diesel oil surface to a grounded electrode (Britton,
L.G., and T. Williams, “Some
Characteristics of Liquid-to-Metal Discharges involving a
Charged Oil”, J. Electrostatics,
13 (1982) pp. 185-207). The picture was taken using a high gain
image intensifier so does
not show the liquid surface or the electrode. The upper
electrode was a ½-inch steel

sphere, intended to represent a probe such as the end of a
thermowell above electrically
charged liquid in a tank. Discharges of this type were able to
ignite mixtures of propane
or butane in air at liquid voltages above -25 kV.
Houston, Texas
April 7-8, 2009
16
Plate 1
Two-inch Long, Incendiary Brush Discharge from Negatively
Charged
Oil to ½-inch Grounded Spherical Electrode

CSB • Barton Solvents Case Study 1
Case Study
U.S. Chemical Safety and Hazard Investigation Board
Barton Solvents
Static Spark Ignites Explosion Inside Flammable
Liquid Storage Tank
No. 2007-06-I-KS
ISSUeS
• Nonconductive flammable liquids can accumulate static
electricity during transfer and storage.
• Static sparks can readily ignite flammable vapor-air mixtures
inside storage tanks.
• Material Safety Data Sheets (MSDSs) often do not adequately
communicate hazard data and precautions.
1. IntroductIon
On July 17, 2007, at about 9 a.m., an explosion and fire
occurred at the Barton Solvents
Wichita facility in Valley Center, Kansas. Eleven residents and
one firefighter received
medical treatment. The incident triggered an evacuation of
Valley Center (approximately
6,000 residents); destroyed the tank farm; and significantly
interrupted Barton’s business.
An investigation by the U.S. Chemical Safety and Hazard

Investigation Board (CSB)
has concluded that the initial explosion occurred inside a
vertical above-ground storage
tank that was being filled with Varnish Makers’ and Painters’
(VM&P) naphtha. VM&P
naphtha is a National Fire Protection Association (NFPA) Class
IB flammable liquid1 that
can produce ignitable vapor-air mixtures inside tanks and,
because of its low electrical
conductivity, can accumulate dangerous levels of static
electricity.2
The CSB is publishing this Case Study to help companies
understand the hazards
associated with static-accumulating flammable liquids that can
form ignitable vapor-air
mixtures inside storage tanks. In addition, the CSB wants to
urge companies to take extra
precautions to prevent explosions and fires like the one at
Barton. This Case Study also
examines industry Material Safety Data Sheet (MSDS) hazard
communication practices
and makes recommendations to ensure that MSDSs identify
these hazards and outline
appropriate precautions.
1 Liquids most likely to form ignitable vapor-air mixtures
during tank filling at ambient operating temperatures are
normally those designated as class
IB or class Ic in nFPA 30 (flammability hazard rating of “3” in
nFPA 704). In the American Petroleum Institute (API)
classification system these liquids
usually fall into the “Intermediate Vapor Pressure Products”
category. A notable exception is motor gasoline, an nFPA class
IB liquid that is designated
as a “High Vapor Pressure Product” in the API system, implying

that (except at very low operating temperatures) the vapor-air
mixture formed during tank
filling rapidly becomes too rich to be ignitable. (See nFPA 30,
Section 4.3 “classification of Liquids” and nFPA 704 chapter 6
for a detailed discussion
of nFPA’s classification and flammability hazard rating
systems. See API 2003 (2008 edition), Section 3 “definitions”
for an explanation of “High,”
“Intermediate,” and “Low” vapor pressure product classes.
2 on october 29, 2007, fire destroyed a large portion of a Barton
facility in des Moines, Iowa. Flammable liquids and static
electricity were also involved in that
incident. Because of the incident-specific findings associated
with the Wichita incident investigation, this case Study focuses
solely on the Wichita incident.
Who’s at Risk…
Companies that transfer (pump) bulk flammable liquids into or
from storage tanks.
2. IncIdent deScrIPtIon
The initial explosion occurred soon after the tank farm
supervisor started the transfer of
the final compartment of a tanker-trailer containing VM&P
naphtha into a 15,000 gallon
above-ground storage tank (Figure 1) .
Pressure/Vacuum ValveEmergency Pressure
Relief Device

Liquid Level
Tape Gauging
System
Gauging
View Glass
Transfer Pump
Tanker-Trailer
Grounding
Float
FiGuRE 1
VM&P naphtha tank
and photo of
an example float
FiGuRE 2
Tank top projectile struck
a mobile home
FiGuRE 3
Pressure vacuum valve
projectile struck
neighboring business

The explosion sent the VM&P tank rocketing into the air,
trailing a cloud of smoke and
fire from the burning liquid; it landed approximately 130 feet
away. Witnesses heard the
explosion and saw the fireball from several miles away. Within
moments, two more tanks
ruptured and released their contents into the rapidly escalating
fire that was concentrated
inside the earthen spill containment area surrounding the tank
farm.3 As the fire burned,
the contents of other tanks over-pressurized or ignited,
launching steel tank tops (10-12 feet
in diameter); vent valves; pipes; and steel parts off-site and into
the adjoining community.
A tank top struck a mobile home in the community
(approximately 300 feet away) and a
pressure/vacuum valve hit a neighboring business nearly 400
feet away (Figures 2 and 3).
3 Approximately 20,000 gallons of flammable liquid were
released into the spill containment. the tank farm included 43
above-ground storage tanks with
capacities ranging from 3,000 to 20,000 gallons. tank heights
ranged from approximately 15 to 40 feet.
3. FLAMMABLe LIquIdS And StAtIc eLectrIcIty
Fire occurs when there is an ignitable vapor-air mixture and a
source of ignition, such as a
static electric spark. At normal handling temperatures,
flammable storage tanks, like those
containing gasoline, may contain vapor-air mixtures that

typically cannot be ignited by a
static electric spark because the vapor-air mixture is too rich
(i.e., contains too much fuel
and not enough oxygen) to burn. VM&P naphthas, however, and
other flammable liquids
(e.g., many NFPA Class IB Flammables), may form ignitable
vapor-air mixtures inside tanks
at normal handling temperatures.
Static electricity is generated as liquid flows
through pipes, valves, and filters while being
transferred. 4 It can also be produced by en-
trained water or air, splashing or agitation,
and when sediment in the bottom of the
tank becomes suspended (Britton, 1999).
Because nonconductive liquids, such as
VM&P naphtha and other flammable
liquids, dissipate (or “relax”) static electricity
slowly, they pose a risk of dangerous static
electric accumulation that can produce sparks
inside tanks.5
4 the rate of static charge generation during flow through pipe
increases roughly with the square of the flow velocity. A liquid
whose conductivity is less
than 100 pico siemens per meter (pS/m) is generally considered
nonconductive (Britton, 1999). the VM&P naphtha involved in
the Barton incident had a
conductivity of 3 pS/m. Some common nonconductive liquids
are listed in nFPA 77 (Annex B – table B.2). See the resources
Section at the end of this
case Study for web access instructions.
5 the length of the transfer piping from the pump to the storage
tank was approximately 215 feet (66 meters); the piping was 2.5

inch nPS Schedule 40,
(6.3 cm inside diameter); and the pump flow velocity was 4.6
meters per second (15 feet per second). A 425 micron (0.017
inch) mesh strainer was
located at the pump outlet.
Normal Bonding and Grounding May Not Be enough!
Companies that handle, transfer, and store flammable liquids
should contact manufacturers
to determine if these liquids can accumulate dangerous levels of
static electricity, and if they
can form explosive vapor-air mixtures inside storage tanks. if
so, extra precautions—beyond
normal bonding and grounding—may be necessary.
4. Key FIndIngS
The CSB determined that several factors likely combined to
produce the initial explosion:
• The tank contained an ignitable vapor-air mixture in its head
space.
• Stop-start filling, air in the transfer piping, and sediment and
water (likely present in the
tank) caused a rapid static charge accumulation inside the
VM&P naphtha tank.
• The tank had a liquid level gauging system float with a loose
linkage that likely separated
and created a spark during filling.
• The MSDS for the VM&P naphtha involved in this incident
did not adequately
communicate the explosive hazard.
Common Static-Accumu lating

Flammable Liquids That May
Form Ignitable Vapor-Air Mixtures
• VM&P naphtha
• Cyclohexane
• n-Heptane
• Benzene
• Toluene
• n-Hexane
• Xylene
• Ethyl benzene
• Styrene
4.1. FlammaBIlIty oF Vm&P NaPhtha
The CSB tested the VM&P naphtha involved in the Barton
explosion to determine if an
ignitable vapor-air mixture could have been present inside the
tank at the time of the
explosion.6 The results revealed that, at approximately 77˚F
(25˚C) (the handling temperature
of the VM&P naphtha at the time of the incident), the tank head
space likely contained a
readily ignitable vapor-air mixture. The energy from a static
spark would have been adequate
to ignite this vapor-air mixture.7
4.2. taNK leVel Float DeSIgN
The design of the tank liquid level gauging system float used by
Barton incorporates a loose
linkage at the float/tape junction that can separate slightly,
interrupting grounding (see Section
4.3) and creating the potential for a spark (Figure 4).8 The CSB

concluded that turbulence and
bubbling during the stop-start transfer pumping, in addition to
creating rapid static charge
accumulation, also likely created slack in the gauge tape
connected to the float, causing the
linkage to separate and spark.9
6 Its flashpoint was 58 ̊F (14 ̊c); its vapor pressure was
approximately 0.7 kPa (5 mmHg) at 68 ̊F (20 ̊c) using an
Isoteniscope; and its flammable range
was approximately 0.9-6.7% in air. the reid VP of the VM&P
naphtha was 3.1 psia (21.4 kPa) at 100 ̊F (38 ̊c).
7 the cSB estimates that the minimum ignition energy required
for a spark to ignite the Barton VM&P naphtha was 0.22 mJ
(plus/minus 0.02 mJ).
8 electrical testing of an exemplar tank level float indicated
that a loose linkage could produce a spark with sufficient
energy to ignite a flammable vapor-air
mixture inside a tank.
9 While the cSB has concluded that the loose linkage level
float was the most likely spark location, a spark from a “brush
discharge” cannot be ruled out.
Brush discharges encompass a variety of “non-spark” static
discharges that occur between a charged liquid surface and a
grounded conductive object,
such as a dip pipe or other metal component acting as an
electrode, or even the tank wall itself. Brush discharges can
occur even when all equipment is
properly bonded and grounded (Britton, 1999). See the
resources Section at the end of this case Study for more
information on brush discharge.
Float Body

Tape
Linkage
Assembly
(Side View)
Spark
Area
FiGuRE 4
Float linkage and
area where the spark
likely occurred
4.3. BoNDINg aND grouNDINg
Bonding is the process of electrically connecting conductive
objects, like tanker-trailers, to
transfer pumps to equalize their individual electrical potentials
and prevent sparking (Figure 5).
10 the transfer hose was severely damaged during the fire,
however, which prevented investigators from determining if
bonding/grounding was effective.
11 Barton indicated that it had no records of the VM&P tank
ever being cleaned, and the tank had no manway or access
opening to facilitate cleaning.
employees stated that they scooped sediment from the bottoms
of similar tanks to prepare them for inspection.

FiGuRE 5
Bonding and grounding Storage
Tank
Storage
Tank
Storage
Tank
Bonding
Grounding
Bonding & Grounding
Grounding (earthing) means connecting a conductive object to
the earth to dissipate electricity,
like accumulated static, lightning strikes, and equipment faults,
into the ground, away from
employees/equipment and ignitable mixtures.
According to witnesses at Barton, the tanker-trailer, pump,
piping, and storage tank were
bonded and grounded at the time of the incident.10 However,
published safety guidance indicates
that bonding and grounding measures applied to typical transfer
and storage operations may
not be enough if nonconductive flammable liquids are involved.
Nonconductive liquids
accumulate static electricity and dissipate (relax) it more slowly
than conductive liquids,
and therefore require additional precautions (see Section 5).

4.4. StatIc accumulatIoN IN the PumPeD lIquID
Barton pumped the VM&P naphtha from three separate
compartments in the tanker-trailer
to the VM&P tank. Air pockets were introduced into the fill
piping, and then transferred into
the tank when the transfer hose was reconnected to the tanker-
trailer after compartments
were changed. Studies have found that static electricity
accumulates rapidly during pump
startup when nonconductive liquids are transferred to storage
tanks (Walmsley, 1996). In
this case, the static electricity accumulation was likely
exacerbated by the air pockets (bub-
bling) and the likely presence of suspended sediment and water
in the tank.11 In addition,
the VM&P tank was approximately 30 percent filled at the time
of the explosion, which
would have produced a liquid surface potential (voltage) close
to the maximum expected
during filling.
4.5. materIal SaFety Data SheetS
According to the Occupational Safety and Health
Administration (OSHA) Hazard
Communication Standard (HCS),12 employees both need and
have the right to know the
identities and hazards of the chemicals they are exposed to
when working. The purpose of
the HCS is to ensure that chemical manufacturers and importers
evaluate the hazards
and communicate them, along with appropriate precautionary
measures, to employers

and employees through a hazard communication program.13 The
primary method of
communicating this information is via detailed technical
bulletins called Material Safety
Data Sheets (MSDSs).
The MSDSs supplied by the manufacturer of the Barton VM&P
naphtha indicated that
the material may accumulate a static electrical charge that could
discharge and ignite
accumulated vapors. It did not, however, provide critical
physical and chemical property
data and warnings that the material may form an ignitable
vapor-air mixture inside storage
tanks. Nor did it list any precautionary measures, beyond
normal bonding and grounding
practices, or reference relevant consensus guidance that Barton
could have used to help
prevent this explosion.
To prevent explosions with flammable liquids like VM&P
naphtha, MSDSs
should communicate
• warnings that the material is a static accumulator and can form
an ignitable vapor-air
mixture inside storage tanks;
• that bonding and grounding may not be enough;
• specific examples of additional precautions (see Section 5)
and references to the published
guidance targeted at preventing static electric discharge; and
• conductivity testing data,14 so that companies know the
degree to which the material will
accumulate static and can compare it to the published guidance.

Information about the pub-
lished guidance is included in the Information Resources
section at the end of this report.
12 29 cFr 1910.1200.
13 29 cFr 1910.1200(a)(1) and (2).
14 the units routinely used to report conductivity are pico
Siemens per meter (pS/m).
Material Safety Data Sheets (MSDSs)
MSDSs do not typically communicate critical physical and
chemical properties, and specific
precautions or reference guidance for flammable liquids that
may pose a static ignition hazard.
Companies should contact the manufacturer (or an expert
familiar with the relevant consensus
guidance) for this information. Manufacturers should in turn
update their MSDSs to provide this
critical safety information.
4.5.1. INDuStry mSDSs reVIew
The CSB reviewed 62 MSDSs of some of the most widely used
nonconductive flammable
liquids to determine if they provided the warnings,
precautionary measures and references,
and conductivity testing data discussed above.
• Static Accumulator and Storage Tank Ignitable Vapor-Air
Mixture Potential: Of the
MSDSs reviewed, 39 (67 percent) contained a warning about the
potential for the material

to accumulate static electricity. Nearly all (97 percent) included
a warning about ignitable
flammable vapors. However, only one specifically warned of the
potential for the material to
form an ignitable vapor-air mixture inside a storage tank.
• Specific Precautions and References to Prevent Explosions: Of
the MSDSs reviewed, 52
(84 percent) advised companies to properly bond and ground
equipment, but only seven
(all prepared by the same manufacturer) indicated that bonding
and grounding alone
may not be enough to prevent a static discharge. Each of the
seven also referenced NFPA
77 and API 2003,15 and 11 others referenced NFPA 77 and/or
API 2003, but did not
specifically warn that bonding and grounding may not be
enough. Only eight of the 62
provided one or more specific precautionary measures such as
adding nonflammable
(inert) gases to tank head spaces, adding an anti-static agent, or
reducing the pump flow
velocity during transfer.
• Conductivity Testing Data: Only three MSDSs (all prepared by
the same manufacturer)
included conductivity testing data.
4.5.2. regulatory aND coNSeNSuS guIDaNce For PreParINg
mSDSs
The three chemical hazard classification systems discussed in
this section contain guidance
to assist manufacturers who prepare MSDSs. OSHA establishes
the regulatory requirements
governing the content of an MSDS.

• Occupational Safety and Health Administration: OSHA
describes the HCS as largely a
performance-oriented standard that gives employers the
flexibility to adapt the rule to the
needs of the workplace, instead of having to follow specific,
rigid requirements. Consequently,
the HCS generally identifies categories of information to be
included in the MSDS, including
physical and chemical characteristics, physical hazards, and
applicable precautions and/
or control measures for handling materials safely. However,
neither the standard nor its
compliance directive16 identifies the specific physical and
chemical data, hazard warnings or
precautions necessary to address some chemical hazards. The
HCS places the responsibility
on the preparer to identify the specific hazards within these
broad categories.
The OSHA advisory document, “Guidance for Hazard
Determination For Compliance with
the OSHA Hazard Communication Standard (29 CFR
1910.1200),” is intended to help
MSDS preparers identify and communicate chemical hazards.
While the document lists cer-
tain data and physical hazards recommended for inclusion in
labels and MSDSs, it does not
address relevant data and hazards associated with static-
accumulating flammable liquids.
15 nFPA 77 and API 2003 are consensus standards that provide
static electric safety guidance.
16 cPL 02-02-038 – cPL 2-2.38d, “Inspection Procedures for the
Hazard communication Standard.”

• Globally Harmonized System of Classification and Labeling of
Chemicals (GHS): The GHS,
first adopted by the Sub-Committee on the Globally Harmonized
System of Classification
and Labeling of Chemicals (SCEGHS) in December 2002, is an
initiative to establish inter-
national consensus on criteria for classifying chemical hazards
for international distribution,
and to create consistent requirements for MSDSs. The GHS has
been revised twice: once in
2005, and again in 2007. According to the GHS Sub-Committee
of Experts, the GHS is
now ready for worldwide implementation.
The GHS provides specific criteria for identifying and
classifying flammable liquids, but it
does not provide identification criteria or warning guidance for
liquids that, in addition to
being ignitable inside tanks at ambient temperatures, also
accumulate static electricity that
can ignite them. In addition, the GHS does not require a
preparer to include conductivity
testing data in an MSDS, data that are essential to identify a
material as nonconductive.
OSHA participates in the GHS criteria development process,
and on September 12, 2006,
published an Advance Notice of Proposed Rulemaking (71 FR
53617), indicating its intent
to adopt the GHS guidance into the requirements of the HCS.
• American National Standards Institute (ANSI) Z400.1-2004
“American National

Standard for Hazardous Industrial Chemicals - Material Safety
Data Sheets - Preparation”:
ANSI Z400.1-2004 is a voluntary consensus standard, and is
recognized by OSHA’s
HCS compliance directive as a consensus standard that provides
valuable guidance to
MSDS preparers.
Because the OSHA HCS is performance-based, it provides
minimal substantive guidance
for MSDS preparers. ANSI Z400.1 was developed to provide
such guidance; it identifies
information that must be included in an MSDS to comply with
OSHA’s HCS, and includes
additional guidance to help MSDS preparers comply with state
and federal environmental
and safety rules.
ANSI Z400.1 gives the following example of a general warning
about what practices to avoid
or restrict: “To reduce the potential for static discharge, bond
and ground containers when
transferring material.” However, the example does not warn that
bonding and grounding may
be insufficient to eliminate the potential for static discharge,
particularly if the material is a
nonconductive flammable liquid. The standard includes no
additional precautions or relevant
consensus guidance references, and no requirements for a
preparer to include conductivity
testing data in an MSDS.

5. AddItIonAL PrecAutIonS
Companies that handle, transfer, and store nonconductive
flammable liquids, such as
naphthas, toluene, benzene, and heptane, should take additional
precautions to avoid an
incident like the one at Barton.
5.1. requeSt aDDItIoNal maNuFacturer guIDaNce
As discussed, MSDSs do not typically provide conductivity
testing data or specific examples
of additional precautions that should be observed, and do not
typically reference the relevant
consensus guidance pertaining to static electricity and storage
tank vapor-air mixture hazards.
Therefore, to determine if additional precautions to eliminate
the potential for an explosion
are necessary, companies that transfer flammable liquids should
contact the manufacturers,
or a qualified expert, to determine if the flammable liquid is
• nonconductive (a static accumulator); and
• capable of producing an ignitable vapor-air mixture inside a
storage tank.
5.2. aDD a NoNFlammaBle, NoNreactIVe (INert) gaS to taNK
heaD SPaceS17
Using an inert gas such as nitrogen, if done correctly, is
effective in reducing the potential
for an ignitable incident (explosion) as it renders tank head
spaces incapable of supporting
ignition from a static spark.18 However, because this practice
can produce oxygen-deficient
environments inside tanks, extreme caution should be exercised
when opening tanks for
routine inspections and maintenance.19

Additional Precautions
• Request additional manufacturer guidance
• Add an inert gas to the tank head space
• Modify or replace loose linkage tank level floats
• Add an anti-static agent
• Reduce flow (pumping) velocity
17 See nFPA 69 “Standard on explosion Prevention Systems”
(2008) for guidance pertaining to proper inerting practices.
18 Before using inert gases in tanks, companies should contact
the liquid manufacturer to determine if the proposed gas is
appropriate for the particular liquid.
19 employers who require employees to enter confined
spaces—particularly those with oxygen-deficient or other
hazardous atmospheres—must comply with
the requirements of the oSHA “Permit required confined Space
Program” (29 cFr 1910.146).
5.3. moDIFy or rePlace looSe lINKage taNK leVel FloatS
Companies with tanks that may contain ignitable vapor-air
mixtures and that are equipped
with conductive loose linkage level floats should take one or
more of the following measures:
• Use an appropriate gas to inert tank head spaces.
• Inspect and replace, as appropriate, floats with level
measuring devices that will not

promote sparks inside the tank.
• Modify floats so that they are properly bonded and grounded
(see Figure 6).20
• Reduce the liquid flow (pumping) velocity.21
• Remove any slack in the tape connected to the float
mechanism that could allow a spark
gap to form.
5.4. aNtI-StatIc aDDItIVeS
Anti-static (conductivity-enhancing) additives increase the
conductivity of liquids, helping
reduce static accumulation. Before relying solely on these
additives, however, companies should
contact the flammable liquid manufacturer to determine if such
an additive is appropriate
and effective for the particular liquid.
5.5. reDuceD Flow (PumPINg) VelocIty
Various guidance suggests that nonconductive flammable
liquids capable of forming ignitable
vapor-air mixtures inside tanks should be transferred at reduced
flow (pumping) velocities to
minimize the potential for a static ignition.22
FiGuRE 6
Tank level float
bonding wire
Float Body
Tape
Linkage

Assembly
Bonding Wire
20 this figure illustrates the modification recommended by the
manufacturer of the floats used at Barton’s Wichita facility.
companies with floats equipped
with loose linkages should contact the manufacturer for
modification recommendations.
21 nFPA 77 (2007); API 2003 (2008); and Britton (1999)
recommend a flow (pumping) velocity of 1 meter per second
when the risk of static ignition is
high. until the spark potential inside the tank is eliminated,
companies should use a pump flow velocity at (or near) 1 meter
per second to transfer
nonconductive flammable liquids.
22 the guidance pertaining to reduced flow (pumping)
velocities include API 2003 (2008), Sections 4.2.5.6 and 4.5.1;
nFPA 77 (2007), table 8.6
(footnote f); and Laurence Britton, “Avoiding Static Ignition
Hazards in chemical operations”, chapters 2-1.6 and 5-4. While
toluene and heptane
are specifically identified in nFPA 77, table 8.6 (footnote f),
typical VM&P naphthas exhibit similar characteristics and
should also be transferred
at reduced flow rates. recommended maximum flow (pumping)
velocities provided in the various guidance differ. However, the
most protective
recommended flow (pumping) velocity is 1 meter per second.

6. recoMMendAtIonS
6.1. occuPatIoNal SaFety aND health aDmINIStratIoN
2007-06-I-KS-r1
Revise the “Guidance for Hazard Determination for compliance
with the OSHA Hazard
Communication Standard” to advise chemical manufacturers and
importers that prepare
MSDSs to
• Evaluate flammable liquids to determine their potential to
accumulate static electricity and
form ignitable vapor-air mixtures in storage tanks.
• Test the conductivity of the flammable liquid and include the
testing results in the MSDS.
2007-06-I-KS-r2
Prior to the next revision, communicate to the Sub-Committee
on the Globally Harmonized
System of Classification and Labeling of Chemicals (SCEGHS)
the need to amend the GHS
to advise chemical manufacturers and importers that prepare
MSDSs to
• Identify and include a warning for materials that are static
accumulators and that may
• Advise users that bonding and grounding may be insufficient
to eliminate the hazard from
static-accumulating flammable liquids, and provide examples of
additional precautions
and references to the relevant consensus guidance (e.g., NFPA
77, Recommended Practice
on Static Electricity (2007), and API Recommended Practice
2003, Protection Against

Ignitions Arising Out of Static, Lightning, and Stray Currents
(2008)).
• Provide conductivity testing data for materials that are static
6.2. amerIcaN NatIoNal StaNDarDS INStItute (aNSI) Z400.1
commIttee
2007-06-I-KS-r3
Revise ANSI Z400.1 to advise chemical manufacturers and
importers that prepare MSDSs to
• Identify and include a warning for materials that are static-
form ignitable vapor-air mixtures in storage tanks;
• Advise users that bonding and grounding may be insufficient
to eliminate the hazard from
static-accumulating flammable liquids, and provide examples of
additional precautions and
references to the relevant consensus guidance (e.g., NFPA 77,
Recommended Practice on
Static Electricity (2007), and API Recommended Practice 2003,
Protection Against Ignitions
Arising Out of Static, Lightning, and Stray Currents (2008));
and
• Provide conductivity testing data for materials that are static
6.3. INDuStry aSSocIatIoNS
AMERiCAN CHEMiSTRy CouNCiL

2007-06-i-KS-R4
AMERiCAN PETRoLEuM iNSTiTuTE
2007-06-i-KS-R5
NATioNAL ASSoCiATioN oF CHEMiCAL DiSTRiBuToRS
2007-06-i-KS-R6
NATioNAL PAiNT AND CoATiNGS ASSoCiATioN
2007-06-i-KS-R7
The u.S. Chemical Safety and Hazard investigation Board (CSB)
is an independent federal agency charged with investigating
industrial
chemical accidents. The agency’s board members are appointed
by the president and confirmed by the Senate. CSB
investigations
look into all aspects of chemical accidents, including physical
causes such as equipment failure as well as inadequacies in
regulations,
industry standards, and safety management systems.
The Board does not issue citations or fines but does make safety
recommendations to companies, industry organizations, labor
groups,
and regulatory agencies such as oSHA and EPA. Please visit our
website, www.csb.gov.
No part of the CSB’s conclusions, findings, or recommendations
may be admitted as evidence or used in any action or suit for
damages;
see 42 u.S.C. § 7412(r)(6)(G).

NATioNAL PETRoCHEMiCAL AND REFiNERS ASSoCiATioN
2007-06-i-KS-R8
SoCiETy FoR CHEMiCAL HAzARD CoMMuNiCATioN
2007-06-i-KS-R9
Recommend to your membership companies that prepare MSDSs
to update the MSDSs to
• Identify and include a warning for materials that are static
• Include a statement that bonding and grounding may be
insufficient to eliminate the
hazard from static-accumulating flammable liquids, and provide
examples of additional
precautions and references to the relevant consensus guidance
(e.g., NFPA 77, Recommended
Practice on Static Electricity (2007), and API Recommended
Practice 2003, Protection Against
Ignitions Arising Out of Static, Lightning, and Stray Currents
(2008)).
• Include conductivity testing data for the materials that are
static accumulators and that
may form ignitable vapor-air mixtures in storage tanks.
7. InForMAtIon reSourceS
The following references include additional information on the
safe use of static-accumulating
flammable liquids:
1. American Petroleum Institute (API), “API Recommended
Practice 2003: Protection

Against Ignitions Arising Out of Static, Lightning, and Stray
Currents,” 7th ed., 2008.
2. Britton, L.G., and J.A. Smith, “Static Hazards of Drum
Filling,” Plant/Operations
Progress, Vol. 7, No. 1 (1988) pg. 53-78.
3. Britton, L.G., “Avoiding Static Ignition Hazards in Chemical
Operations,” AIChE-CCPS
Concept Book, 1999.
4. National Fire Protection Association (NFPA), “NFPA 30:
Flammable and Combustible
Liquid Code,” 2008.
5. NFPA, “NFPA 69: Standard on Explosion Prevention
Systems,” 2008 ed.
6. NFPA, “NFPA 77: Recommended Practice on Static
Electricity,” 2007 ed. NFPA 77 can
be viewed, free of charge, on the NFPA website
(www.nfpa.org). Access directions: At
the NFPA Homepage, go the “Codes and Standards” pull down
tab, then click on “Code
development process” and scroll down to “Online access.”
7. Walmsley, H.L., “The Electrostatic Potentials Generated by
Loading Multiple Batches of
Product into a Road Tanker Compartment,” J. Electrostatics,
Vol. 38, 1996, pg.177-186.

Case Study
U.S. Chemical Safety and Hazard Investigation Board
Barton Solvents
Static Spark Ignites Explosion Inside Flammable
Liquid Storage Tank
No. 2007-06-I-KS
ISSUeS
• Nonconductive flammable liquids can accumulate static
electricity during transfer and storage.
• Static sparks can readily ignite flammable vapor-air mixtures
inside storage tanks.
• Material Safety Data Sheets (MSDSs) often do not adequately
communicate hazard data and precautions.
1. IntroductIon
On July 17, 2007, at about 9 a.m., an explosion and fire
occurred at the Barton Solvents
Wichita facility in Valley Center, Kansas. Eleven residents and
one firefighter received
medical treatment. The incident triggered an evacuation of
Valley Center (approximately
6,000 residents); destroyed the tank farm; and significantly
interrupted Barton’s business.
An investigation by the U.S. Chemical Safety and Hazard
Investigation Board (CSB)
has concluded that the initial explosion occurred inside a
vertical above-ground storage
tank that was being filled with Varnish Makers’ and Painters’

(VM&P) naphtha. VM&P
naphtha is a National Fire Protection Association (NFPA) Class
IB flammable liquid1 that
can produce ignitable vapor-air mixtures inside tanks and,
because of its low electrical
conductivity, can accumulate dangerous levels of static
electricity.2
The CSB is publishing this Case Study to help companies
understand the hazards
associated with static-accumulating flammable liquids that can
form ignitable vapor-air
mixtures inside storage tanks. In addition, the CSB wants to
urge companies to take extra
precautions to prevent explosions and fires like the one at
Barton. This Case Study also
examines industry Material Safety Data Sheet (MSDS) hazard
communication practices
and makes recommendations to ensure that MSDSs identify
these hazards and outline
appropriate precautions.
1 Liquids most likely to form ignitable vapor-air mixtures
during tank filling at ambient operating temperatures are
normally those designated as class
IB or class Ic in nFPA 30 (flammability hazard rating of “3” in
nFPA 704). In the American Petroleum Institute (API)
classification system these liquids
usually fall into the “Intermediate Vapor Pressure Products”
category. A notable exception is motor gasoline, an nFPA class
IB liquid that is designated
as a “High Vapor Pressure Product” in the API system, implying
that (except at very low operating temperatures) the vapor-air
mixture formed during tank
filling rapidly becomes too rich to be ignitable. (See nFPA 30,
Section 4.3 “classification of Liquids” and nFPA 704 chapter 6

for a detailed discussion
of nFPA’s classification and flammability hazard rating
systems. See API 2003 (2008 edition), Section 3 “definitions”
for an explanation of “High,”
“Intermediate,” and “Low” vapor pressure product classes.
2 on october 29, 2007, fire destroyed a large portion of a Barton
facility in des Moines, Iowa. Flammable liquids and static
electricity were also involved in that
incident. Because of the incident-specific findings associated
with the Wichita incident investigation, this case Study focuses
solely on the Wichita incident.
Who’s at Risk…
Companies that transfer (pump) bulk flammable liquids into or
from storage tanks.
2. IncIdent deScrIPtIon
The initial explosion occurred soon after the tank farm
supervisor started the transfer of
the final compartment of a tanker-trailer containing VM&P
naphtha into a 15,000 gallon
above-ground storage tank (Figure 1) .
Pressure/Vacuum ValveEmergency Pressure
Relief Device
Liquid Level
Tape Gauging
System

Gauging
View Glass
Transfer Pump
Tanker-Trailer
Grounding
Float
FiGuRE 1
VM&P naphtha tank
and photo of
an example float
FiGuRE 2
Tank top projectile struck
a mobile home
FiGuRE 3
Pressure vacuum valve
projectile struck
neighboring business
The explosion sent the VM&P tank rocketing into the air,
trailing a cloud of smoke and
fire from the burning liquid; it landed approximately 130 feet

away. Witnesses heard the
explosion and saw the fireball from several miles away. Within
moments, two more tanks
ruptured and released their contents into the rapidly escalating
fire that was concentrated
inside the earthen spill containment area surrounding the tank
farm.3 As the fire burned,
the contents of other tanks over-pressurized or ignited,
launching steel tank tops (10-12 feet
in diameter); vent valves; pipes; and steel parts off-site and into
the adjoining community.
A tank top struck a mobile home in the community
(approximately 300 feet away) and a
pressure/vacuum valve hit a neighboring business nearly 400
feet away (Figures 2 and 3).
3 Approximately 20,000 gallons of flammable liquid were
released into the spill containment. the tank farm included 43
above-ground storage tanks with
capacities ranging from 3,000 to 20,000 gallons. tank heights
ranged from approximately 15 to 40 feet.
3. FLAMMABLe LIquIdS And StAtIc eLectrIcIty
Fire occurs when there is an ignitable vapor-air mixture and a
source of ignition, such as a
static electric spark. At normal handling temperatures,
flammable storage tanks, like those
containing gasoline, may contain vapor-air mixtures that
typically cannot be ignited by a
static electric spark because the vapor-air mixture is too rich
(i.e., contains too much fuel
and not enough oxygen) to burn. VM&P naphthas, however, and

other flammable liquids
(e.g., many NFPA Class IB Flammables), may form ignitable
vapor-air mixtures inside tanks
at normal handling temperatures.
Static electricity is generated as liquid flows
through pipes, valves, and filters while being
transferred. 4 It can also be produced by en-
trained water or air, splashing or agitation,
and when sediment in the bottom of the
tank becomes suspended (Britton, 1999).
Because nonconductive liquids, such as
VM&P naphtha and other flammable
liquids, dissipate (or “relax”) static electricity
slowly, they pose a risk of dangerous static
electric accumulation that can produce sparks
inside tanks.5
4 the rate of static charge generation during flow through pipe
increases roughly with the square of the flow velocity. A liquid
whose conductivity is less
than 100 pico siemens per meter (pS/m) is generally considered
nonconductive (Britton, 1999). the VM&P naphtha involved in
the Barton incident had a
conductivity of 3 pS/m. Some common nonconductive liquids
are listed in nFPA 77 (Annex B – table B.2). See the resources
Section at the end of this
case Study for web access instructions.
5 the length of the transfer piping from the pump to the storage
tank was approximately 215 feet (66 meters); the piping was 2.5
inch nPS Schedule 40,
(6.3 cm inside diameter); and the pump flow velocity was 4.6
meters per second (15 feet per second). A 425 micron (0.017
inch) mesh strainer was

located at the pump outlet.
Normal Bonding and Grounding May Not Be enough!
Companies that handle, transfer, and store flammable liquids
should contact manufacturers
to determine if these liquids can accumulate dangerous levels of
static electricity, and if they
can form explosive vapor-air mixtures inside storage tanks. if
so, extra precautions—beyond
normal bonding and grounding—may be necessary.
4. Key FIndIngS
The CSB determined that several factors likely combined to
produce the initial explosion:
• The tank contained an ignitable vapor-air mixture in its head
space.
• Stop-start filling, air in the transfer piping, and sediment and
water (likely present in the
tank) caused a rapid static charge accumulation inside the
VM&P naphtha tank.
• The tank had a liquid level gauging system float with a loose
linkage that likely separated
and created a spark during filling.
• The MSDS for the VM&P naphtha involved in this incident
did not adequately
communicate the explosive hazard.
Common Static-Accumu lating
Flammable Liquids That May
Form Ignitable Vapor-Air Mixtures
• VM&P naphtha

• Cyclohexane
• n-Heptane
• Benzene
• Toluene
• n-Hexane
• Xylene
• Ethyl benzene
• Styrene
4.1. FlammaBIlIty oF Vm&P NaPhtha
The CSB tested the VM&P naphtha involved in the Barton
explosion to determine if an
ignitable vapor-air mixture could have been present inside the
tank at the time of the
explosion.6 The results revealed that, at approximately 77˚F
(25˚C) (the handling temperature
of the VM&P naphtha at the time of the incident), the tank head
space likely contained a
readily ignitable vapor-air mixture. The energy from a static
spark would have been adequate
to ignite this vapor-air mixture.7
4.2. taNK leVel Float DeSIgN
The design of the tank liquid level gauging system float used by
Barton incorporates a loose
linkage at the float/tape junction that can separate slightly,
interrupting grounding (see Section
4.3) and creating the potential for a spark (Figure 4).8 The CSB
concluded that turbulence and
bubbling during the stop-start transfer pumping, in addition to
creating rapid static charge
accumulation, also likely created slack in the gauge tape

connected to the float, causing the
linkage to separate and spark.9
6 Its flashpoint was 58 ̊F (14 ̊c); its vapor pressure was
approximately 0.7 kPa (5 mmHg) at 68 ̊F (20 ̊c) using an
Isoteniscope; and its flammable range
was approximately 0.9-6.7% in air. the reid VP of the VM&P
naphtha was 3.1 psia (21.4 kPa) at 100 ̊F (38 ̊c).
7 the cSB estimates that the minimum ignition energy required
for a spark to ignite the Barton VM&P naphtha was 0.22 mJ
(plus/minus 0.02 mJ).
8 electrical testing of an exemplar tank level float indicated
that a loose linkage could produce a spark with sufficient
energy to ignite a flammable vapor-air
mixture inside a tank.
9 While the cSB has concluded that the loose linkage level
float was the most likely spark location, a spark from a “brush
discharge” cannot be ruled out.
Brush discharges encompass a variety of “non-spark” static
discharges that occur between a charged liquid surface and a
grounded conductive object,
such as a dip pipe or other metal component acting as an
electrode, or even the tank wall itself. Brush discharges can
occur even when all equipment is
properly bonded and grounded (Britton, 1999). See the
resources Section at the end of this case Study for more
information on brush discharge.
Float Body
Tape
Linkage

Assembly
(Side View)
Spark
Area
FiGuRE 4
Float linkage and
area where the spark
likely occurred
4.3. BoNDINg aND grouNDINg
Bonding is the process of electrically connecting conductive
objects, like tanker-trailers, to
transfer pumps to equalize their individual electrical potentials
and prevent sparking (Figure 5).
10 the transfer hose was severely damaged during the fire,
however, which prevented investigators from determining if
bonding/grounding was effective.
11 Barton indicated that it had no records of the VM&P tank
ever being cleaned, and the tank had no manway or access
opening to facilitate cleaning.
employees stated that they scooped sediment from the bottoms
of similar tanks to prepare them for inspection.
FiGuRE 5
Bonding and grounding Storage

Paper presented at SCHC Spring 2009 Meeting Houston, Texas .docx

Paper presented at SCHC Spring 2009 Meeting Houston, Texas .docx

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