Journal of Electrostatics 51}52 (2001) 387}394
Electrostatics requirement for intermediate
bulk containers intended for use in explosive
atmospheres
Ulrich von Pidoll*
Physikalisch-Technische Bundesanstalt, Section 3.33, Bundesallee 100, 38116 Braunschweig, Germany
Abstract
Intermediate bulk containers (IBC) of 1 m in volume are often needed for storing #ammable
liquids. These containers should also be suitable for use in potentially explosive atmospheres of
zone 1 without safety hazards being caused. In the past, such containers were exclusively made
of metal. Some years ago special designs made of insulating plastic material instead of metal
made their appearance on the market. However, inappropriate construction or use of these
plastic containers may create hazards as a result of static electricity discharges. For this reason,
the build-up and decay of electrostatic charges produced during "lling, emptying, transportation as well as by rubbing have been studied on several container types. On the basis of the
experimental results safety measures are proposed, which will ensure safe operation of the
container with regard to static electricity discharges. 2001 Published by Elsevier Science B.V.
Keywords: IBC; Container; Plastics; Electrostatic hazards
1. Introduction
In the chemical industry and related branches there is a great demand for intermediate bulk containers (IBC) of 1 m in volume for #ammable organic solvents,
paints etc. to be handled even in explosive atmospheres of zones 1 and 2. In the past
only metal containers were used for this purpose. The costs for the production,
transportation and cleaning of these metal containers has been a challenge to the
manufacturers of these containers to develop so-called electrostatically protected
plastic containers which had to be as safe as the metal containers. These containers
* Corresponding author. Tel.: #49-531-592-3431; fax: #49-531-592-3305.
E-mail address: ulrich.von.pidoll@ptb.de (U. von Pidoll).
0304-3886/01/$ - see front matter 2001 Published by Elsevier Science B.V.
PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 0 8 0 - 8
388
U. von Pidoll / Journal of Electrostatics 51}52 (2001) 387}394
usually consist of a plastic bubble completely wrapped in tin, conductive pasteboard, conductive foil, conductive coating, or surrounded by earthed grids of metal,
conductive plastics, conductive thread in connection with other electrostatic safety
precautions. It is important that the surrounding is conductive in order to suppress
brush discharges from the inner wall of the container, so, e.g., normal dissipative
pasteboard is not su$cient safe.
Many questions arise in connection with these types such as the question of the
maximum safe mesh size, the distance between grid and plastic bubble, etc. with regards
to electrostatic hazards. The Physikalisch-Technische Bundesanstalt as a test house for
electrical and non-electrical equipment has, therefore, made experiments with the
di!erent types of so-called electrostatically protected plastic containers for their
electrostatic behavior. The results of this investigation are presented in the following.
2. Results
2.1. Charge generation during container operations
The charge generated during operation of IBC's made of metal, plastics or tinwrapped plastics with a volume of 1 m has been determined using "eld mills, picoamperemeters and electrostatic voltmeters. The aim of this experimental series is to
determine the most hazardous operation for IBC's. During the experiments, the IBC's
stood on an insulating rubber mat. The liquids used were either petroleum (conductivity at rest 2 pS/m) or ethanol (conductivity at rest '10000 pS/m).
The IBC was top-"lled from a second IBC arranged 2 m above via a plastic tube
(about 8 cm in diameter). For emptying, the two IBC's were exchanged. The average
velocity of the liquid #owing from IBC 1 to IBC 2 was about 1 m/s. To get an estimate
of the charges generated during transportation, a "lled IBC was displaced for 3 min
along a bumpy road and then shaken for 1 min with the aid of a fork-lift truck. As the
experiments were carried out at 213C and at a relative humidity of about 50%
somewhat greater charges may be expected for drier climates.
The voltage was measured with a metal plate on bottom of the IBC which was
connected with a short wire to a contact-free voltage probe of a "eldmill. The voltage
course during "lling the IBC was recorded. The current was measured with a picoamperemeter connected between conductive bottom valve and earth. The current course
was recorded too.
Table 1 shows that "lling an IBC can result in 20 mJ electric energy being generated
mainly by the liquid streaming through insulated plastic "ll pipes. This value may even
be exceeded under more critical circumstances. Table 2 demonstrates that transportation and emptying of an IBC creates less charge than the "lling process.
2.2. Mesh size of the grid
The inside of a container for #ammable liquids may permanently contain an
explosive atmosphere and as in this case is to be classi"ed as zone 0. According to the
15
Not measured
2
2
3000
1800
1700
1200
Max. voltage
(V)
Not measured
5
20
2
Energy U I dt
(mJ)
Range obtained in three di!erent experiments.
2 pS/m
2 pS/m
2 pS/m
'10 nS/m
Tin-wrapped
Tin-wrapped
Tin-wrapped
Tin-wrapped
plastics
plastics
plastics
plastics
Conductivity of
the liquid
IBC type
12
(0.1
20
3
Max. earth
current (nA)
350
120
1200
(5
Max. voltage
(V)
Charge I dt
3
;1
3
;1
150 J
1 to 5 J
2 mJ
(1 J
Not measured
4
15
3
Charge I dt
(C)
Energy U I dt
Table 2
Maximum earth current, voltage, energy and charge created during di!erent operations of an IBC
60
20
2
2
Metal
Tin-wrapped
plastics
200 cm grid
wrapped plastics
Plastics
Max. earth
current (nA)
Conductivity
of the liquid
(pS/m)
IBC type
Table 1
Maximum earth current, voltage, energy and charge produced during "lling of di!erent IBC types
Emptying
Transportation
Filling
Filling
Operation
Filling
Filling
Filling
Filling
Operation
U. von Pidoll / Journal of Electrostatics 51}52 (2001) 387}394
389
390
U. von Pidoll / Journal of Electrostatics 51}52 (2001) 387}394
Cenelec Report R044-001 [1], Section 4.4.3, the maximum chargeable surface area in
this zone is to be limited to 50 cm for liquids of explosion group IIA according to EN
50014 [2]. According to Section 4.4.5 of that Report, this value can be increased to 200
cm if earthed meshes with a distance not greater than 2 mm mark the boundaries of
the area.
However, in commercially available IBC's the distance usually is somewhat greater.
For this reason, experiments were carried out to determine the maximum mesh size
acceptable for liquids of explosion group IIA. At a temperature of 23 3C and a relative
humidity of 30% the maximum possible charge transferred by forced brush discharges
was measured under di!erent conditions and compared to the maximum transferred
charge of 60 nC being just not capable of igniting gases and vapors of group IIA [3,4].
Here, an earthed sphere electrode 15 mm in diameter with a coulombmeter in the
earth line was approximated to the plastic bubble of an IBC surface until a single
brush discharge did occur.
Tables 3 and 4 con"rm the recommendations of Section 4.4.5 of the Cenelec
Report [1]. The experiments did not reveal di!erences between grids of metal
wire, thread with metal wire and conductive plastic thread concerning transferred
charges.
It is well known that if an earthed grid is in contact with an outer surface of an
insulating material with a thickness of about 2 mm, propagating brush discharges
occur after spraying charges on the other side. However, when the other side is rubbed
with only a cloth, the potential electrostatic discharges are largely reduced.
Table 4 shows that the grid is even e!ective at a distance of 20 mm.
The results obtained, especially from Table 3, allow the conclusion to be drawn that
a mesh size of 100 cm will be su$cient if the grid is very close to the plastic bubble.
This value is also proposed in the literature [5] and by the German TRbF [6]. In
practice, however, the distance between grid and plastic bubble may be somewhat
greater. As shown in Table 4, for this container type a smaller mesh size of 25 cm at
a higher grid distance of 20 mm would also be safe.
Table 3
Charge transferred by forced brush discharges outside an IBC at 233C and 30% relative humidity as
a function of the rubbed surface area outside
Rubbed surface area outside (cm)
Maximum transferred
charge (nC) outside,
surface area not metal
enclosed
Maximum transferred
charge (nC) outside,
surface area metal
enclosed
25
72
225
400
700
25
70
180
Not measured
Not measured
5
15
40
70
120
U. von Pidoll / Journal of Electrostatics 51}52 (2001) 387}394
391
Table 4
Charge transferred by a forced brush discharge inside of an IBC at 233C and 35% relative humidity as
a function of the distance of a 1 m earthed mesh outside, mesh size 25 cm, attached to the outer surface area
Distance between outer surface area and grid (mm)
0
Maximum transferred
charge (nC) inside
after spraying
charges inside
Maximum transferred
charge (nC) inside
after rubbing inside
3}4
Propagating
Overload
brush discharge
(10
60
6}7
10}11
20}21
In"nite
360
190
150
75
60
60
60
75
In another series of experiments it was veri"ed that 1 m IBC's having such mesh
sizes will produce only a negligible charge if mesh and plastic bubble are quickly
separated by pressure e!ects.
2.3. Unshielded areas
Almost every IBC with plastic bubble contains areas of plastics which are not
shielded by earthed parts. These are, e.g., the area around the "ll hole and the area
around the bottom valve. As these areas usually are greater than allowed by the
Cenelec Report R044-001 [1], Section 4.4.3, it has to be made sure by experiment that
the strongest possible electrostatic discharge produced by rubbing of this area under
dry conditions does not lead to gases or vapors of explosion group IIA according to
EN 50014 [2] being ignited.
For this reason, the maximum charge transferred by forced brush discharge to
a sphere electrode 15 mm in diameter has to be determined. According to present
knowledge a transferred charge of 60 nC is just not capable of igniting gases and
vapors of group IIA [3,4]. Due to the shielding e!ect of the earthed parts nearby only
unshielded surface areas much greater than 100 cm do not ful"ll this condition.
Unshielded areas around the bottom valve are considered to be safe if they are so
hidden that they will only scarcely be rubbed. This is acceptable because the charging
of the wall during emptying of the IBC was found to be low.
Unshielded areas on top of the container are dangerous because they can be
charged by rubbing clothes on these surfaces when the "ller cap is removed. The
resulting charged plastic area may produce incendive brush discharges with transferred charges of more than 140 nC in the region of escaping solvent vapors or even
spark discharges if, e.g., an insulated metal #ange is placed on it.
2.4. Conductive connection between liquid and earth
According to the Cenelec Report R044-001 [2], Section 5.4.6.3, IBC's with no
conductive wall inside shall have a conductive connection between liquid and earth.
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U. von Pidoll / Journal of Electrostatics 51}52 (2001) 387}394
Table 5 shows that such a connection will relax the charges in a charged liquid in a few
hours even in the case of insulating liquids.
Usually, the conductive path between liquid and earth is realized via the bottom
valve. This valve may be made of insulating plastics provided with an earthed
conductive tin inside or of a conductive plastic material or of metal. The earth wire is
to be securely fastened to the valve.
2.5. Filler cap
According to the Cenelec Report R044-001 [1], Section 4.2, materials with a surface
resistance of 10 W at most, measured at 30% relative humidity, or 10 W at most,
measured at 50% relative humidity, are considered not to be charged by normal
rubbing processes. Usually the "ller cap of an IBC is not made of such a dissipative
material. In this case, the manufacturer has to structure the surface of the "ller cap or
to glue e.g. paper stickers to the "ller cap to reduce the chargeable surface area. In the
former case, it has to be proven by measuring the transferred charge of forced brush
discharges after rubbing in dry climate that the discharges are not incendive. In the
second case, it has to be proven that the paper sticker permanently adheres also in
cold and wet climate. If, however, the "ller cap is made of dissipative material, it has to
be earthed. Electrostatic hazards at "ller caps mainly result when the "ller caps have
diameters greater than the common value of 16 cm.
2.6. Earthing and bonding
According to the Cenelec Report R044-001 [1, Section 10.1], all conductive parts of
an IBC are to be bonded together and earthed. According to Section 10.2.2, the
connections shall be reliable and permanent and they shall not deteriorate. The same
section demands that the product of capacitance and earth resistance shall be less than
0.01 s. A 1 m IBC with a capacitance of about 500 pF may, therefore, only have
a resistance to earth smaller than 2;10 . This resistance will, e.g., be exceeded if the
container feet are made of dry wood or standard plastic.
2.7. Charge generation during manufacture of an IBC
It is well known that the charges generated by rubbing on the plastic bubble of
an unshielded IBC can endure for weeks. Therefore, the question arises whether
charges generated by the manufacturing process of the plastic bubble also endure
and lead to hazards during the "rst "lling operation. For this reason, the charge
generation during manufacture of a 1 m IBC was monitored by a "eld mill in
a climate of 25 3C and 50% relative humidity outside and 25% relative humidity
inside the bubble.
Another series of experiments was performed to monitor the decrease of the charge
on three 1 m IBC's. After spraying charges inside at 23 3C and 35% relative humidity
and then laying an earthed 1 m grid, mesh size 25 cm, on the outer surface, distance
between grid and outer surface not constant as in real IBC's (0}12 lmm), the charge
U. von Pidoll / Journal of Electrostatics 51}52 (2001) 387}394
393
Table 5
Decrease of charge as a function of time in di!erent IBC types
Connection between
liquid and earth
Conductivity of
the liquid
(pS/m)
Time at "eld
strength of
100% (s)
Time to decrease
"eld strength to
37% (s)
Time to decrease
"eld strength to
1% (s)
Yes
Yes
No
'10000
2
2
0
0
0
2
3600
100000
10
18000
Not measured
transferred by a forced brush discharge inside an IBC was measured as a function of
time. After 0.1 h, the transferred charge on IBC 1 was 75 nC, after 18 h it was 30 nC on
IBC 2 and IBC 3 "nally gained 5 nC after 70 h. The results obtained show that the
charge generated by the manufacturing process for a plastic IBC will be negligible
if the time interval between manufacturing process and "rst "lling process is at
least 24 h (Table 6).
2.8. Precautions necessary when using IBC's
According to the Cenelec Report R044-001 [1, Section 5.4.6.3], rapid repeated
"llings are to be avoided if non-metal IBC's are used. Section 10.3.1 demands that all
movable metal parts such as drums, funnels and containers are to be earthed during
the "lling or emptying procedure, because earthing via the dissipative #oor cannot be
relied on. According to the experimental results discussed in the section before, the
time period between manufacturing of the plastic bubble and delivery of the container
should be at least 24 h. A sign with instructions fastened to the IBC is helpful and
proposed by [5], but not demanded by the Cenelec Report R044-001 [1]. Of course,
a container with damaged wrapping must not to be used.
3. Conclusions
The experimental results obtained show that plastic containers are considered to be
safe from the electrostatic point of view if at least the following safety measures are
applied:
1. The container is provided with a wrapping made of a conductive material such as
tin, conductive pasteboard, conductive foil or conductive coating. It is also possible
to use conductive textile net, wire-lattice or an iron grid box with an area of not
more than 100 cm enclosed by a single grid mesh, the grid having several points of
contact with the plastic bubble on each side and an average distance of 5 mm. If the
grid mesh size is 25 cm or less, the maximum distance between plastic bubble and
earthed wrapping may be increased to 2 cm if contact with the plastic bubble is still
maintained in several points on each side.
2. There is no unshielded area on the outside of the container which might produce
incendive discharges when rubbed in dry climate.
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U. von Pidoll / Journal of Electrostatics 51}52 (2001) 387}394
Table 6
Typical charge generation during container manufacture
Field strength
(measure distance
10 cm)
Transferred charge
by forced brush
discharges
After blowing of
plastic bubble
After last
leakage test
At the point of
shipment
Received
container
Max.$600 kV/m
Max.$300 kV/m
Max.$200 kV/m
(20 kV/m
Max. 30 nC
Max. 7 nC
Max. 1 nC
Not detectable
Higher, but uncritical values found on areas handled with chargeable gloves.
3. A conductive connection is provided between the liquid inside, the metal parts of
the container and the #oor, e.g., via a metal bottom valve and an earth conductor
screwed to it.
4. Precautions with regard to electrostatic discharges are to be taken especially if the
"ller cap is more than 16 cm in diameter. In this case, the "ller cap is to be made,
e.g., of dissipative material.
5. The time period between manufacturing of the plastic bubble and delivery of the
container shall be at least 24 h.
6. Special charge-generating operations such as rapidly repeated "llings are to be
avoided
7. During "lling and emptying operations the container is to be earthed via a separate
earth cable.
These measures are proposed to be included in the new German Rules BGR132 of
the German Hauptverband der gewerblichen Berufsgenossenschaften [7].
Acknowledgements
The author wishes to thank Dr. Heino Bothe, Dr. Peter Krass, Dr. Claus-Dieter
Walther and Eberhard Wildfeuer for helpful discussions.
References
[1] Cenelec Report R044-001:1999, Guidance and Recommendations for the Avoidance of Hazards Due to
Static Electricity, 1999.
[2] EN 50014:2000, Electrical Apparatus for Potentially Explosive Atmospheres, General Requirements, 2000.
[3] IEC 62013-1:1999, Caplights for use in mines, General requirements, Annex, 1999.
[4] N. Gibson, F.C. Lloyd, Incendivity of discharges from electrostatically charged plastics, Brit. J. Appl.
Phys. 16 (1965) 1619.
[5] B. Maurer, ZuK ndquelle IBC, Der Gefahrgut Pro" 4/99, pp. 151}153.
[6] Technische Regeln fuK r brennbare FluK ssigkeiten TRbF 143 `Ortsbewegliche BehaK lter bis 450 la,
Appendix 2.2, Section 3, Paragraph 2.
[7] Berufsgenossenschaftliche Regeln `Statische ElektrizitaK ta BGR 132, to be published.