Dust explosions are a well-known hazard in many branches of indus-
try. Although, probably devastating, grain dust explosions in silo’s
and elevators attract most attention. Numerous dust explosions have
been recorded where sugar was involved.
Because, in most countries, there is no legal obligation to report dust
explosions, the available data are rather limited. Probably the best
statistics are available in Germany where BGIA (BG-Institute for
Occupatonal Safety and health), former BIA, keeps records of dust
explosions (Beck & Jeske 1982, 1987, 1996, 1997).
Since 1932, 30 sugar dust explosions have been reported. Twenty
of these incidents are well documented, enabling an inventory of the
equipment involved and the ignition sources. The results are sum-
marised in Tables 1 and 2.
Static electricity is not included in this table, but finding traces of
electrostatic discharges after a dust explosion is very difficult.
Because of the large number of sugar dust explosions, a research
program was started in Germany (Scholl 1973), which included a
number of dust explosion tests in a real life situation: a closed sugar
plant. After the first few tests, where great care was taken to prevent
an uncontrolled dust explosion, a test resulted in a very strong dust
explosion, which damaged the equipment and was the main reason
that further testing was stopped.
Apart from these German data, IRMACO has been involved in
Sugar dust explosions
By Ake Harmanny
ISMA nv, Heiveldekens 8, B-2550 Kontich, Belgium
Tel: +32 3 451 01 30 Fax: +32 3 451 01 31 E-mail: email@example.com
Following a short introduction on sugar dust explosions statistics, the paper discusses relevant explosion characteristics of sugar dust.
Pre-emptive measures to prevent sugar dust explosions or, at least, to reduce the probability of a sugar dust explosion are analysed.
In some situations the residual risk of a sugar dust explosion is too high, even if appropriate preventive measures are taken. Therefore
an overview of the available techniques to reduce the consequences of a dust explosion is supplied with particular references to a num-
ber of practical examples.
Sieves and milling 2
Table 1. Equipment involved in sugar dust
Ignition source Number
Mechanical sparks 12
Mechanical heat 4
Table 2. Ignition sources involved in sugar dust
evaluating five sugar dust explo-
sions. Of these, four explosions
did occur in a bucket elevator
(mechanical sparks or mechani-
cal heat) and one was in a sugar
hopper, due to electrical spark-
There are many characteristics of
adust, which influence the prob-
ability of a dust explosion. Since
most dust explosion characteris-
tics depend on the properties of
the dust involved (particle size,
moisture content, …) a general
overview is given, followed by a
table noting the “worst case
properties” for sugar dust (based
on Beck, Glienke & Möhlmann,
The minimum concentration of a
dust in air to enable a dust explosion is the Lower Explosion Limit
(LEL). This value is, for sugar, typically 60 – 100 g/m3.
The Upper Explosion Limit (UEL) is hardly documented for most
dusts. It is typically in the order of magnitude of several kg/m3.
In case inerting (working in a nitrogen atmosphere where there is
hardly any oxygen) is chosen as a preventive measure, the Limiting
Oxygen Concentration (LOC) is also important, in order to determine
the maximum concentration of oxygen that can be allowed in an
inerted installation. This is not very well documented for sugar, but
for most dusts this is about 10 % (rem.: the “natural” oxygen con-
centration in air is about 20 %).
The Minimum Ignition Energy (MIE) is the minimum amount of
spark energy required to ignite an optimum dust-air mixture. For
sugar it is typically in the range of 10-30 mJ, which is rather low
compared to many other dusts.
The Minimum Ignition Temperature (MIT) is the minimum sur-
face temperature of a hot object required to ignite an optimum dust-
air mixture. It is around 400°C for sugar dust.
Although a burning or smouldering dust layer is not an explosion, it
might act as an ignition source. Therefore, for a dust explosion risk
analysis, it is important to take into account the probability that dust
layers are ignited. Two relevant burning properties are:
The Glowing Temperature (GT), the temperature of a hot surface
covered with a 5 mm dust layer where the dust starts to smoulder.
(Remark.: the smouldering behaviour depends very much on the
thickness of the dust layer: a thick layer will start smouldering far
below GT). For sugar it is found that there is no GT:pure sugar does
not smoulder but melts when heated.
The Burning Group (BG): which indicates the behaviour of a dust
sample after ignition. Sugar is classified as BG 2, meaning it will
extinguish shortly after ignition.
The conclusion therefore has to be that sugar does not smoulder
nor burn. Smouldering or burning particles, which are transported
and cause a dust explosion further in the process, (very typical for
grain) do not occur for sugar. However, this is only valid for pure
sugar! It is well known that the addition of some cigarette ash to
sugar, which acts as a catalyst, will enable burning of sugar.
The explosion properties that characterise the intensity of an explo-
sion (Figure 1) are determined in a standardised test in a volume of 1
m3or in a 20 litre vessel where the results, by means of calculations,
are upscaled to 1 m3.
The maximum explosion pressure (Pmax)and the maximum rate
of pressure rise (Kst)are the maximum values recorded, where the
concentration of the dust involved is varied.
Table 3 gives the “worst case” explosion properties for fine sugar
(< 63 µm). These values can be used as a safety guide for most appli-
cations, also for more coarse sugar. As an example: when sugar is
loaded into a silo, the coarse particles will fall down immediately, but
the fines (which are always present due to friction) remain airborne
LEL (g/m3)MIE (mJ) MIT (°C) Pmax (bar) Kst (bar.m/s)
30 8-10 360 9 140
Table 3. Explosion properties of sugar
Figure. 1. Explosion intensity
for quite some time. The properties of these fines are decisive for the
From the statistics and the explosion characteristics of sugar dust fol-
lowing can be concluded with respect to the explosion hazards with-
in a sugar plant:
•Mechanical equipment (mills, bucket elevators), which run at a
relatively high speed are dangerous as ignition sources can be creat-
•“Static” equipment, such as filters, hoppers or silo’s usually
does not generate ignition sources. Although in general this kind of
equipment is very often involved in dust explosions, this is less fre-
quent for sugar dust, because it is rather difficult to transport ignition
sources into such equipment. The probability of an explosion within
such equipment mainly depends on the presence of inherent ignition
sources, such as electrostatic discharges and/or electrical equipment.
•The decision if protective measures are necessary does not only
depend on the probability of an explosion, but also on the conse-
quences. Therefore large silos are often protected by venting, because
of the devastating effect of an explosion in a large silo. Also filters
are often protected, because (de-dusting) filters are usually linked to
all kinds of equipment and therefore may provoke explosion propa-
Avery important measure to prevent dust explosions in moving
equipment is to prevent the entrance of foreign objects (metal, stone),
by installing sieves, metal detectors, gravity separators or other
equipment intended to remove foreign objects. However,although
such systems are very important in reducing the probability of a dust
explosion, it is impossible to entirely prevent an ingress of foreign
objects: a sieve may fail, metal detectors or gravity separators do not
have a 100 % efficiency.
Especially for bucket elevators a number of measures are avail-
able to reduce the probability of mechanical sparks:
• Slippage detection (by measuring the speed of the
• Out-of-line detection
• Control of the power consumption
In silos or bunkers the probability of a sugar dust explosion can
be reduced substantially by the choice of the filling method:
•With gravity filling there is only minor dust generation: occur-
rence of explosible mixtures is unlikely. As sugar does not burn it is
also unlikely that ignition sources will be transported into the silo
•With pneumatic filling the occurrence of explosible mixtures is
very likely. Even with conductive ducting, the sugar may be charged
to such a level that cone discharges can arise. Especially in large silos
the energy in these discharges may reach such a level that a sugar
dust cloud can be ignited.
Depending on the situation it might be necessary to use anti-stat-
ic materials to prevent electrostatic discharges, such as an anti-static
FIBC (“big bag”) or anti-static filter elements. If such anti-static
materials (meaning conductive materials) are chosen, it is very
important that they are grounded properly: non-earthed anti-static
materials usually are much more dangerous as non-anti-static materi-
als. Recently we were involved in a dust explosion investigation,
where it was found that the ignition most likely had been caused by
an anti-static filter bag, which was not earthed. Whenever flexibles
are used in pneumatic conveying or de-dusting lines, it is very impor-
tant that these are anti-static. A standard flexible, with metal rein-
forcement, even if the metal is properly grounded, is not anti-static,
but might enable the very dangerous propagating brush discharges
(energy content > 1000 mJ!).
Apart from these specific measures, there are also several gener-
alguidelines, which should always be followed to reduce the proba-
bility of dust explosions:
•Cleaning dust spills, in order to prevent secondary dust
•Apply proper (“Dust-Ex”) electrical equipment whenever
• Proper system of “hot work” permits.
• Awareness training for operators. As just an example: investing
in IP 65 enclosures, to prevent dangerous dust ingress into cabinets
which contain switching and sparking equipment, is absolutely use-
less, if the operators keep the doors permanently open.
Post-operative explosion protection
If, with the application of all preventive measures the dust explosion
hazards are still unacceptably high (meaning high probability and/or
serious consequences) measures are required, which will reduce the
effect of an explosion.
There are several "protective" or "post-operative" measures, each
with its own advantages and disadvantages and consequently its own
range of application. These measures are:
-Containment by explosion resistant construction: making
equipment strong enough to withstand the explosion loading.
-Explosion venting, providing openings that will relieve the
explosion pressure to a safe place, thus reducing the pres-sure load-
ing in the vessel to be protected.
-Explosion suppression: detecting an explosion in an early stage
and injecting a suitable suppressant (extinguishing agent) to extin-
guish the explosion flame, thus ensuring that the pressure never
exceeds a certain (low) value.
In all cases the propagation of an explosion from one vessel to
another should be preven-ted by so-called isolating measures. This
must be done because escalation might occur to a level that cannot be
controlled by the protection methods of the individual vessels.
Containment of an explosion relies on the "explosion resistant con-
struction" of the equip-ment, i.e. the ability of equipment to with-
stand the loading by the explosion. As a separate mode of protection,
"containment" usually refers to the protection against the pressure
from an uncon-trolled explosion. However, containment is also nec-
essary when using other protection measures because with ven-ting
and suppression a "reduced explosion pressure" is still generated.
The basis for the explosion resistant construction must be the
"maximum pressu-re to be expec-ted", that is not necessarily equal to
If one chooses for explosion containment, all equipment includ-
ing all connected piping, flanges, manhole covers, instrumentation
mountings etc., must be able to withstand the design explosion pres-
sure, and must be "explosion resistant" for that pressure, usually
somew-here around 10 bar. One major cause for concern in such
installations is maintaining the level of resistance over the lifetime of
Two types of explosion resistant design are usually distinguished,
that can be simplified to:
-Explosion Pressure Resistant Design (EPRD): the vessel under
consideration has to be designed according to the rules for a pressure
-Explosion Pressure Shock Resistant Design (EPSRD): the ves-
sel will be designed in accordance with the pressure vessel codes, but
the safety margin to yielding is set equal to 1. As it cannot be avoid-
ed that, locally, stress concentrations will arise, the EPSRD implica-
tes that local yielding is accepted. Failure, however, is certainly not
For equipment of complicated shape, that is therefore not very
accessi-ble to strength calcula-tions, sometimes one prototype is sub-
mitted to explosion testing. If the item does withstand some specific
explosion pressure, this appa-ratus is accepted now to be EPSRD for
that pressure (less a safety margin).
With explosion venting the effects of the explosion are trans-ferred to
asafe place outside the equipment and the building. This is obtained
by installing a weak part of sufficient dimen-sions with a known
opening pressure. Instead of the maximum explo-sion pressure,
which would arise in a closed vessel, a reduced explosion pressure
has to be taken into account, often just a few tenths of a bar.
For the design of dust explosion vents clear guidelines (such as
the German VDI 3673 or the American NFPA 68) are available for
most practical situations. With these guidelines the required vent area
can be found if the violence of the explosion (Kst-value), the volume
and strength of the vessel and the opening pressure of the vents are
It is very important that there are no obstructions, e.g. filter bags,
in front of the vent opening and that the opening is created very fast:
explosion doors usually have a high mass and, because of mass iner-
tia, will open slowly. The reduced efficiency of these doors must be
established and taken into account in the design of the vents.
Because of the blast wave and the fire ball which escape from the
vent opening, a hazard is created in the area in front of the vent.
Therefore it is very important that the vent directs into a safe area.
Alternatively, special venting equipment must be used that is proven
to quench (extinguish) the flame in the vent opening.
Explosion suppression relies on an early detection of an incipient
explosion and the rapid injection of a suitable suppressant (sup-press-
ing agent) into the volume to be protected. The suppressant extin-
guishes the flame within a short period, typically 50-100 millisecond
and therefore the explosion pressure rises only slightly above the
pressure at the moment of detection.
Typically the explosion pressure at the moment of detection is 35
to 100 mbar g, and the suppressed (reduced) explosion pressure is 0.2
-0.5 bar g.
Toachieve explosion suppression any vessel to be protected is
equipped with explosion detectors and suppressors as well as a con-
trol unit to control and monitor the system. The suppressors are locat-
edon the vessel wall in such a way that the suppressant can reach any
location in the vessel within a very short time.
For many years explosion suppression was only used as the ulti-
mate solution, because of the disadvantages involved:
•High explosives to open the suppressors
•Pressure detectors which need frequent calibration
(high maintenance costs)
With the development of modern suppression techniques, such as
the Stuvex FLASH system (Figure 2), which does not use pressure
vessels, or explosives and applies detectors with a fixed setting, the
use of explosion suppression is widely applied.
It should be emphasised that experts must design explosion sup-
pression systems, since the safety of the plant depends on the per-
formance of all components.
It has been noted before that isolation is very important. The simplest
way of achie-ving isolation is avoiding unnecessary con-nections.
If this is impossible, special measures must be taken to create bar-
riers to avoid propagation of an explosion. Two types of barri-ers can
be distinguished: mechanical bar-riers and chemical barri-ers.
Three types of mechanical barriers can be distinguished:
Figure 2. Stuvex FLASH explosion suppression,
where the suppressant is expelled with the help
of a gas generator
•rotary valves and feeder screws: equipment that is already
present in the installation;
•explosion relief stacks (diverters): a local venting arrangement;
•shut-off valves: quick-closing slide valves (Figure 3), Ventex
valves and Quench valves.
It is always a first requirement that mechanical barriers must be
tested to establish the ability to stop the flame and withstand the
explo-sion load. For shut-offvalves the delay between actua-tion and
comple-te closing is important, as this determines the required dis-
tance between detection and barrier location.
In the explosion diverter,the fact is used that a propagating explo-
sion cannot suddenly change direction by 180∞. The greatest for-ce
of the explosion is discharged to a safe place outside by means of a
co-ver plate or explosion panel.
With a chemical or suppressant barrier, the flame front or pressure
wave is detected by a detector, which activates a suppres-sor located
at some distance downstream of the detector. The explosion thus runs
into a cloud of suppressant that extinguishes the flame, but does not
stop the pressure wave.
Examples of protective measures in sugar factories
In this section a number of typical examples of protective measures
will be discussed. It has to be stressed that every practical situation is
different, thus it is best to note the underlying principles from these
Sometimes (usually small) mills are used in between two rotary
valves. These can be protected easily: the mill has to be explosion
resistant. By installing spark detectors the rotary valves can be
stopped, in order to prevent propagation of sparks and/or dust explo-
Many mills are placed on a receiving bin, often with an integrat-
ed filter. Here, usually, mill, receiver bin (and filter) can be treated as
one volume. Because of the dimensions it is impractical to use explo-
sion resistant design.
An appropriate method is venting, where the bursting disc is usu-
ally installed on the receiving bin. In order to prevent condensation
onthe bursting disc, it can be provided with an insulating layer
(Figure 4). In many situations “standard” venting is impossible,
because the equipment is far away from an exterior wall. In such sit-
uations bursting discs with quenching devices can be used instead of
explosion suppression. Also all inlets and outlets are to be protected
by barriers. In case venting is applied to protect the main volume,
rotary valves or feeder screws usually protect the product inlet and
outlet, whereas for the air inlet and outlet Ventex valves are used. If
it was opted for suppression it is possible to use chemical barriers
instead of mechanical barriers and a barrier on the clean air outlet of
the filter is not required.
From explosion incidents it is known that dust explosions in bucket
elevators usually start in head or boot, although we have once been
involved in an incident where a sugar dust explosion started some-
where halfway down the bucket elevator!
If an explosion starts in head or boot, this does not mean that the
explosion is restricted to head or boot. Without appropriate isolation
measures the explosion will propagate (and accelerate) in the legs.
Installing mechanical barriers on elevator legs is impossible.
Therefore, for the explosion protection of bucket elevators, there are
two “standard” solutions:
1. Explosion venting protects the elevator. Apart from vents at
head and boot, vents are also required at regular distances all along
the legs. In practice this means, especially for long elevators, a large
number of explosion vents.
2. Explosion suppression is applied. Now advantage is taken of
the fact that an explosion usually starts at head or boot. Explosion
detectors and suppressors are installed at head and boot, together
with chemical barriers on the legs near head and boot. Now, with a
limited number of detectors and suppressors the elevator is protected
against most explosion hazards (Figure 5).
Sometimes a combination of suppression with venting is used
where, to reduce costs, a vent replaces the suppressor on the head.
Figure. 3. Quick-closing slide valve
Figure 4. Bursting disc equipped with insulation,
to prevent condensation
This is rarely done for the boot, as the boot of a bucket elevator fre-
quently is in an area where large vented flame jets are unacceptable.
Although rather exceptional, it is possible, especially for rather short
bucket elevators, to use containment: build the bucket elevator for the
maximum explosion pressure.
Like for all other equipment isolation is required for inlet and out-
let and the de-dusting line (if present). In case of explosion suppres-
sion mainly chemical barriers are used, with venting usually rotary
valves (with Ventex valves for de-dusting lines) are applied.
Rem.: It is very hard to state, in general, if the presence of de-
dusting reduces the explosion hazard in a bucket elevator, by reduc-
ing the presence of dust. If the dust concentration can always be kept
below the LEL, no dust explosion is possible. However, some years
ago the dust concentration was measured inside a bucket elevator,
during operation, with and without de-dusting. It was found that
without de-dusting the dust concentration was close to the UEL
whereas, with de-dusting, almost an optimum dust concentration was
created. In this specific situation therefore de-dusting increases the
explosion hazard considerably. For other situations, depending on the
type and speed of the bucket elevator and the dust properties, it might
just be the other way around.
Figure 5. Bucket elevator protected by explosion
Figure 6. Principle of explosion barrier in a sugar gallery
Sugar is often stored in large horizontal silos, which have hardly any
resistance against explosion pressures and therefore can not be pro-
tected by venting. As the entrance of smouldering material into such
asilo is unlikely (sugar does not burn), sometimes it is accepted that
such a silo is not vented. It is, however, especially in such situations
very important that all measures are taken to prevent explosions.
Apart from preventing inherent ignition sources in the silo, such as
improper electrical equipment, the presence of bucket elevators has
to be avoided or, if they are present, they should be protected prop-
erly (venting or suppression).
Because of a number of devastating dust explosions in silos, in
France it was required for several years for all silos to be protected
(by venting, suppression or containment).
A major problem for many large sugar silos is that they are con-
nected to the rest of the plant by transport belts in galleries. If, even
with all preventive measures taken, an explosion should occur inside
the silo, it may not only have a large effect on the surroundings but it
is very likely that the explosion will propagate through the galleries
and destroy the rest of the plant. Especially for such galleries Stuvex
has developed a barrier system, which mainly consists of an encase-
ment of the belt over a certain length, which is filled with suppressant
in case of an explosion (Figure 6).
Explosion venting protects the vast majority of de-dusting filters,
although also sometimes explosion suppression or containment (for
small filters) is used. A major concern is the proper positioning of the
vents. Very often the vents are placed such that the filter elements are
in front of the vent, although the VDI 3673 states clearly that “filter
cages must not cover the vent area”. During explosion venting, the air
velocity near the vent may easily become 100-200 m/s, meaning that
(flexible) filter cages will be blown into the vent area and reduce the
If the clean air from the filter is returned into the building (to save
on heating costs) it is essential that this duct be provided with a bar-
rier, usually a Ventex valve. If the air outlet is into a safe direction,
no barrier is required.
It is very important that barriers protect the air inlets, in order to
prevent explosion propagation towards all connected equipment.
With low dust concentrations Ventex valves can be used. Otherwise
quick acting slide valves or chemical barriers are the best solutions.
Of course, also the product outlet has to be protected, usually a
rotary valve or a feeder screw.
Beck, H. & Jeske, A. 1982, Dokumentation Staubexplosionen – Analyse
und Einzelfalldarstellung. BIA-Report Nr. 4/82, Hauptverband der
gewerblichen Berufsgenossenschaften (HVBG), Sankt Augustin
Jeske, A. & Beck, H. 1987, Dokumentation Staubexplosionen – Analyse
und Einzelfalldarstellung. BIA-Report Nr. 2/87, Hauptverband der
gewerblichen Berufsgenossenschaften (HVBG), Sankt Augustin
Beck, H. & Jeske, A. 1996, ‚Berichte über Staubexplosionen –
Einzelereignisse und Dokumentation VDI Berichte 1272, Tagung Sichere
Handhabung Brennbarer Stäube, Nürnberg, pp. 365-387
Jeske, A. & Beck, H. 1997, Dokumentation Staubexplosionen – Analyse
und Einzelfalldarstellung. BIA-Report Nr. 11/97, Hauptverband der
gewerblichen Berufsgenossenschaften (HVBG), Sankt Augustin
Scholl, E.W. 1973, Explosionsversuche mit Zuckerstaub in
Entstaubungsanlagen einer stillgelegten Zuckerfabrik. Forschungsbericht Nr.
109, Bundesanstalt für Arbeitsschutz und Unfallforschung, Dortmund
Beck, H., Glienke N. & Möhlmann, C. 1997, Brenn und
Explosionskenngröβen von Stäuben. BIA-Report Nr. 12/97, Hauptverband
der gewerblichen Berufsgenossenschaften (HVBG), Sankt Augustin