ISMA
StuvEx
PAGG
IExT
Product Guide
Main Office Belgium
Heiveldekens 8
B-2550 Kontich
Tel. +32-3-458 25 52
Fax +32-3-458 25 27
E-mail: [email protected]
www.irmaco.be
Introduction
For over 30 years the IRMACO Group has developed new products and services in the field of explosion protection and fire
protection in the processing industry. With 5 companies, established in 3 different countries, IRMACO has evolved into an
international Group, active in many countries, from Europe to Australia.
To do even better justice to this product guide, we gladly offer you our “Explanation On Explosion Safety”: a unique overview
which can be used combined with the index to look up and fathom certain terms.
We do hope that you will find in this guide what you are looking for and that you will get convinced of the quality of our
products and services. For further questions, our customers service team will be gladly at your service (tel. +32-3-458 25 52).
Yours sincerely,
The IRMACO-team
INTRODUCTION
In this product guide you will find a detailed description and a conveniently arranged technical data sheet for the different
products and services offered by these companies. We did classify them per field: from advice (ISMA), via products and systems
(the StuvEx companies and PAGG), up to distribution (IExT).
EXPLANATION ON EXPLOSION SAFETY
xxxxxxxxxx
The most efficient searching procedure? Consult the index on page 19!
There you will find the page numbers for a fast indication on terminology and abbreviations.
1. INTRODUCTION
An explosion can be best described as a process in which a large quantity of energy is released in a very short time. As a
touchstone for the difference between a fast fire and an explosion it has been suggested that it has to go that fast that a “bang”
is obtained.
There are different kinds of explosions. Usually they are distinguished into physical and chemical explosions. With chemical
explosions the energy has its origin in a chemical reaction, this is not the case with a physical explosion.
Some examples of physical explosions:
• A pressure vessel bursting;
• The bang and the flame jet developing at a sudden short circuit;
• An explosion as a consequence of melted metal falling in a small puddle of water:
the water instantaneously forms an enormous steam cloud;
• A nuclear explosion.
Chemical explosions are subdivided into:
• Deflagrations: a reaction front propagates through a gas/air mixture (or a dust/air mixture) with a subsonic speed, typically
around 10 m/sec. Most gas and dust explosions are typical deflagrations.
• Detonations: a deflagration in a long pipe can accelerate and eventually transit into a detonation; the speed is now (much)
higher than the speed of sound. Sensitive mixtures (explosives, but for example also a mixture of hydrogen and oxygen) can
detonate immediately, certainly if the ignition source is sufficiently strong.
In this text we will first discuss the phenomenon of gas and dust explosion. Then we will examine the various protection
techniques and last but not least this information is placed in the ATEX framework.
Result of a dust explosion in the neighbourhood of Zwolle (The Netherlands)
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EXPLANATION ON EXPLOSION SAFETY
• A thermal explosion, also called “run-away reaction”, is an example of a chemical reaction “running out of control”: the
reaction accelerates and eventually results in an explosion.
2. GAS AND DUST EXPLOSIONS
2.1. WHAT IS AN EXPLOSION?
A gas or dust explosion is in fact nothing else but a fast combustion of fuel (combustible gas or dust) in the air. The high
combustion speed is caused by the fact that a more or less ideal mixture has been formed. This can be clarified with the
following example:
When a log of wood is put into the fire, it will burn but slowly. But if it is tried to burn a heap of wood chips, one will
discover that these will burn very fast. The reason is that the fuel (wood) and the oxygen from the air are in this case much
better mixed as in the case of the log. If a quantity of saw dust is thrown in the fire, even a flame jet might develop. If fine
dust is well mixed with air, the mixture is so ideal that it will burn very fast: a huge flame jet. If this is done in a confined
space, a huge pressure (caused by the hot gases) will be built up. If the size of the dust particles is further reduced,
eventually loose molecules will remain: a gas explosion.
Demonstration
The consequences of a heavy explosion (France)
2.2. EXPLOSION CHARACTERISTICS
For the assessment of the explosion risks a large number of characteristics are important. These will successively be clarified
in brief hereafter. It is important to keep in mind that all these characteristics (one more, one less) are strongly dependent on
the kind of gas or dust, but also on for example, on particle size, humidity, temperature and pressure.
The explosion limits
An explosive mixture means that the fuel concentration has to situate itself between two well defined limits: the lower and the
upper explosion limit. At concentrations below the lower explosion limit (in short LEL), the fuel quantity is too low to support an
independent flame propagation. If the concentration is higher than the upper explosion level (short UEL), the quantity of oxygen
is insufficient for an independent flame propagation. For powders the LEL is situated in most cases above 30 g/m3. The UEL is
situated in most cases in the order of kg/m3 and for this reason hardly significant.
Some rules of thumb:
• A dust layer of less than 0.5 mm is sufficient to cause, when whirled up, an explosive mixture.
• A dust cloud is explosive if you cannot see your hand with your arm fully stretched.
With gas explosions the LEL and UEL are, in most cases, situated relatively near to each other, for methane for example they lie
by respectively 5 and 15% in air. Exceptions are highly explosive gases such as hydrogen and acetylene. For hydrogen the
limits lie by about 4 and 75%.
Beside this, with gas explosions also the flash point is important. If a flammable liquid is stored below its flash point, this means
there is so little evaporation that, above the liquid surface, no explosive mixture is formed: no risk for gas explosion.
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The ignition sensitivity
The ignition sensitivity of a mixture is in most cases defined by two variables: the minimal ignition energy (Emin) and the
minimal ignition temperature (Tmin).
The minimal ignition energy (also indicated as MIE) is a measure for the easiness with which an explosive mixture can be
ignited by electrical sparks. This is defined as the lowest spark energy with which a fuel/air mixture under ideal conditions can
still be ignited. The value of Emin is especially important in order to be able to define if certain types of electrostatic discharge
are dangerous. The value of Emin can vary enormously, from less then 0,1 mJ up till more than 1000 mJ.
The minimal ignition temperature of a cloud (also indicated as MIT) is the lowest temperature that, in accordance with certain
test methods, can make a fuel/air mixture to explode. The Tmin can be used very directly to define the maximum admissible
temperature of objects to be placed in an explosive atmosphere. The variation in temperature is much less. This often lies around
400-500°C.
Emin and Tmin together define the sensitivity of the mixture for impact or frictional sparks. There are graphics in which, for a
certain type of spark, can be found if this, as a function of Emin and Tmin, is dangerous.
With gases the expression “gas group” is often used. This is a practical value, indicating the maximum experimental safe gap
(for a pressure resistant casing) to stop the flame. This value is coherent with Emin.
The maximum explosion pressure and the maximum rate of pressure rise
The maximum explosion pressure (Pmax) is the highest pressure occurring with an explosion in a closed vessel at an optimal
concentration. The maximum rate of pressure rise (dp/dtmax) is the highest value of the rate of pressure rise occurring during an
explosion in a confined vessel at optimal concentration. The determination of these values has to be done using standardised
test methods.
Important:
Dust class
Dust explosion constant
Examples
Kst in bar.m/sec
St 0
Kst = 0
Cement, chalk, sand, ashes, aerosol, salts
St 1
0 < Kst ≤ 200
Flour, malt dust, many corn starches, sugar dust, many wood dusts,
sulphur dust, coal dust, PVC dust, carbon black
St 2
201 < Kst ≤ 300
Some corn starch, beech wood dust, epoxy resin dust,
methyl cellulose dust, some pigments, some powdered flavourings
and vitamins and paraformaldehyde dust
St 3
Kst > 301
Aluminum dust, magnesium dust, red phosphorus and antrachinon
Table I: Classification of dust explosible substances
By analogy with the Kst value, for gas there is the Kg value. This is, however, far less used.
The speed of a gas explosion is often measured as the flame speed, which obviously is directly connected with Kg.
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EXPLANATION ON EXPLOSION SAFETY
If the maximum rate of pressure rise (dp/dtmax) of a dust is determined in a standard 1m3 vessel according to the standardised
test procedures, this value equals the so called dust explosion constant 'Kst'. This maximum rate of pressure rise, or Kst
value, is an indication for the vehemence of the explosion and also defines the speed required to be able to release the
explosion pressure at an early stage (explosion venting), or to extinguish it (explosion suppression). With the help of this
dust explosion constant the substances can be divided into 4 dust explosion classes as indicated in table I.
Dust layers
Dust layers cannot explode: explosions are only possible with a dust cloud. But a dust layer can easily be dispersed and form a
dust cloud. It is a “potential” dust cloud. Therefore it is important to know the behaviour of a dust layer. In this context two
figures are relevant.
The smouldering temperature or minimal ignition temperature of a dust layer is the lowest temperature of a heated free
surface, on which a dust layer with a certain thickness (standard thickness 5 mm) ignites. A smouldering fire may expand, but
also develop into a full fire with flames. In both cases a smouldering fire can act as a direct ignition source for an explosion.
The burning group (BG) of a powder is defined according to the observed behaviour of a dust layer after ignition and gives an
indication of the ease and the speed with which an ignited dust layer expands as a dust fire or a smouldering fire.
Observed behaviour
Burning Group
dust does not ignite
BG 1
dust ignites but extinguishes fast
BG 2
dust ignites and glows locally but does not spread
BG 3
spreading of the glowing (smouldering fire)
BG 4
spreading as an open fire with flames
BG 5
explosive burning and expansion
BG 6
3. PREVENTION
3.1. MIXTURES
The best way to prevent explosions is preventing explosive mixtures. Things that matter in this context:
• Choice of products (is it necessary to use an explosive gas or dust, is there no safer alternative?)
• Proper sealing of the equipment (to prevent leaks), and coherent with this:
• Regular inspections and maintenance;
• Also maintaining a slight underpressure in the equipment may, certainly for dust, prevent leaking to outside;
• In case of regular small gas leakages, a good ventilation is important;
• Ventilation does not work with dust leakages. In those cases regular cleaning (vacuuming) is necessary.
In spite of this explosive mixtures cannot always be avoided. Think of equipment in which, typical of the process, dangerous
concentrations have to be present and the nearly unavoidable risk of leaking gaskets.
For this reason it is customary to divide the process space into so called hazardous areas. For more details on area classification
or zoning we refer to the Dutch code of practice NPR 7910 (also often used outside The Netherlands). Part 1 deals with gas
zoning, part 2 with dust zoning. Here only the principles are given in short.
Zone 0
In a “zone 0” an explosive gas mixture will often be present. Normally a zone 0 will only be found inside certain process equipment.
Remark: in a gas pipe there is no zone 0. Inside the pipe only gas is present, no air, consequently no dangerous mixture.
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Zone 1
In a “zone 1” an explosive gas mixture will regularly be present. Think, for example, of the space around inspection traps that
open regularly.
Zone 2
In a “zone 2” an explosive gas mixture will seldom be present, but it cannot totally be excluded (otherwise it would be a
so-called unzoned area). An example could be the area around a gasket that could start leaking.
It is clear that the nature and the magnitude of the zone will depend on various variables:
• The strength of the source (leaks much or little gas)
• Inside or outside (inside leaked gas will accumulate)
• The possible presence of ventilation
By analogy with gas, there are the zones 20, 21 and 22 for dust. Their definitions are more or less analogous, on the understanding that also dust layers, that might be dispersed, have to be taken into account.
• If there are almost continuously dust layers it is considered a zone 21
• If there are regularly dust layers it is considered a zone 22
With dust zoning, ventilation is not important, but the extent of cleaning is.
Gas and dust zoning are executed in quite a different way. With gas various calculations have to be made (what is the source
strength, what is the ventilation ratio) whilst dust zoning is mainly done visually: “here it is rather dusty, so it will be a zone 22”.
The partition in gas and dust zones defines if, and which, measures have to be taken to prevent ignition sources.
3.2. IGNITION SOURCES
It is impossible to consider all ignition sources within this framework. Some important ones will be singled out.
Mainly in the petrochemical industry it has already been common use for a long time, to use so called Ex equipment in danger
zones. Well known examples are: Eexd, Eexe, Eexi, Eexp, etc. equipments.
Non-explosion safe electrical equipment
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EXPLANATION ON EXPLOSION SAFETY
3.2.1. Electrical equipment
According to the ATEX legislation equipment has to be divided into categories. Then, dependent on the zones certain categories
of equipment can be used.
Remark: this division in categories is not only valid for electrical, but also for mechanical equipment.
Product:
gas/vapour
Product:
dust
Risk degree
Required category of the equipment
Zone 0
Zone 20
In which continuously, for a prolonged
time or often an explosive environment
is found
Category 1
Zone 1
Zone 21
In which an explosive environment
will probably be found
Category 2
Zone 2
Zone 22
In which probably no explosive
environment will be found, and if it is
found, only rarely and for a short time
Category 3
Table ll: division into dangerous zones
For electrical equipment in a gas explosive environment this approach has existed for many years. Within the ATEX framework
mainly the existing standards have been converted into new European standards, without many changes concerning content.
Standard
Way of protection
Code
EN 50 014
General requirements
EN 50 015
Immersion in oil
o
EN 50 016
Overpressure
p
EN 50 017
Powdery filling
q
EN 50 018
Explosion proof housing
d
EN 50 019
Increased security
e
EN 50 020
Intrinsic safe equipment
i
EN 50 039
Intrinsic safe systems
i sys
EN 50 021
Non-sparking material Cat 3G
n
EN 50 028
Encapsulated material
m
EN 50 284
Category 1G
Table lll: 'Gas-Ex' electrical equipment: Construction and test
For dust this is still fairly new. Analogous to the “Gas Ex” situation they are trying now to draw up standards for “Dust Ex”
equipment. For the greater part these are not yet EN standards, but IEC standards that will be converted to EN standards in the
years to come.
IEC old
IEC new
Denomination
Code
61241-1-1
61241-0
General requirements
61241-1-1
61241-1
Protection by enclosures
tD
61241-2
Overpressure
pD
61241-11
Protection by intrinsic safety
iD
61241-18
Protection by encapsulation
mD
EN 50281-1-1
61241-4
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Table IV: 'Dust Ex' electrical equipment: construction and test
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Also in the past, when there was not yet a “Dust Ex”, of course equipment was applied in a dusty environment. At that time the
protection consisted mainly of, what is now called “Protection by encapsulation”. With this, particularly the correct IP degree
and the correct temperature class of the equipment or system concerned are important:
• The correct protection degree (IP degree), in practice predominantly IP 54 or IP 65, means the equipment is almost, or even
fully, dust tight. As no dust can enter the equipment, possible sparks inside the equipment cannot cause a dust explosion.
• The temperature class means that the temperature of those parts of the concerned equipment reachable for an explosive
atmosphere can never rise above a certain value. This way it is prevented that the hot surface can function as an ignition
source. Important: the temperature limitation, dependent on the category, not only applies at normal operation, but also in
faulty situations. In practice this often means that a temperature control is present, that in case the temperature rises too
high, the equipment concerned is shut off.
Table V shows the division in temperature classes, table VI gives an overview of the protection degrees (IPxx code).
T class
Temperature
First digit: objects and dust
Second digit: water
T1
450 °C
0 no protection
0 no protection
T2
300 °C
1 object > 50 mm
1 dripping water, vertical
T3
200 °C
2 objects > 12 mm
2 dripping water, 75-90°
T4
135 °C
3 objects > 2,5 mm
3 spray water
T5
100 °C
4 objects > 1 mm
4 water hose (rain)
T6
85 °C
5 harmful penetration of dust
5 waterspout
6 completely dust tight
6 as on deck of a ship
Table V: Temperature classes
7 effects of immersion
8 specific immersion
Tabel VI: Protection Degree (IP code) 3.2.2. Mechanical equipment
EN 13463
Type of protection
Codes
Even an IP 65-housing is not dust tight when
the door is left open!
1
General rules and requirements
2
Limited breathing enclosure
fr
3
Explosion proof enclosure
d
4
Intrinsic safety
i
5
Safe by construction
c
6
Control over ignition sources
b
7
Protection by overpressure
p
8
Protection by immersion
k
Tabel VII: `Ex'-mechanical devices: construction and test
Mechanical spark formation
Furthermore it will also be often necessary, especially for mechanical equipment, to assess to what extent moving parts in a
process (a stirring rod, but for instance also a milling installation) might be potential ignition sources. It is recommended, if
necessary, to contact an expert. Mechanical ignition sources can, of course, also arise when executing mechanical works such
as drilling and grinding. Here the point is to reduce the possible risks, applying the correct procedures.
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EXPLANATION ON EXPLOSION SAFETY
Completely new in ATEX, is the demand that the requirements on mechanical
equipment should be comparable with those on electrical equipment.
Dependent on the zone it is applied in, this should also be certified for a
certain category (see also table II). Think of equipment such as for instance
power drives and pumps. Table VII shows an overview of the various
standards (some of them are still in draft).
3.2.3. Static electricity
Static electricity is often created by friction: think for instance of a pneumatic transport of (non-conductive) powders through a
metal tube. But also the fast unrolling of plastic foil or the fast emptying of a plastic bag, may cause heavy charging.
Once there is charging, discharging may follow. The energy involved can cause an explosion. There are different kinds of
electrostatic discharges, each with a largely different energy content:
1. Lightning discharge: from a charged cloud to earth. This has a huge energy content. In case of an explosive atmosphere,
this can certainly be ignited. Prevention: a good lightning rod.
2. Spark discharge. This is generated when two conductive objects, one of them charged, come near each other. The energy
is strongly dependent on the object's size: a charged truck discharging can cause a huge spark and is dangerous for most
gases and dusts. A charged paper clip discharging can hardly be called dangerous. Prevention: see to it that all conductors
are well interconnected (for instance: earth connections over all flanges).
3. Brush discharge. This is generated when for example a charged plastic foil (or clothing) discharges. This is accompanied by
some crackling. It is dangerous for most gases, not for most dusts. Prevention: avoid using non-conductive foils.
4. Propagating brush discharges. These are extremely dangerous discharges with an energy content of over 1000 mJ. They
occur mainly in pneumatic transport through non-conductive hoses. The only good solution to avoid this is the use of antistatic
hoses. Remark: a hose with a metal spiral inside, even if this spiral is earthed, is in most cases not antistatic.
5. Corona-discharges. These are found around very high charged metal points. They have a very low energy content and for
this reason are in most cases not dangerous.
6. Cone discharges. These occur when a very bad conductive powder is blown pneumatically to a silo. It is a kind of discharge
from the charged powder to the silo wall. Sometimes this can be dangerous. Prevention: no pneumatic transport, or a smaller
silo diameter.
Recording of a propagating brush discharge (courtesy of Ciba Geigy AG, Basel)
3.2.4. Other ignition sources
Apart from the earlier mentioned ignition sources, there still are numerous others. Think for instance of:
• Fire or hot surfaces caused by welding, grinding and similar actions. This can be avoided by proper training and a hot work
permit policy.
• Product deposits that start smouldering or heat. This is often the result of product impurity or too long storage.
• Sparks or hot surfaces caused by mechanical friction in the equipment. For instance the rubbing of elevator buckets against
the casing.
• Stray currents due to welding in the vicinity.
• Sudden compression of an explosive mixture (diesel engine effect).
Especially for dust explosion risks it is often very difficult to prevent all ignition sources with enough certainty. Think on the
milling of grain. There fine powder is made. If, together with the grain, a small piece of stone or metal enters the mill, a shower
of sparks is generated and possibly also hot surfaces. This risk can be reduced (by installing stone and metal separators), but not
totally prevented.
If it appears that the risk is still unacceptably high, measures must be taken to limit the effects of the explosion.
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4. RESTRICTIVE MEASURES
Important preliminary remark: these “measures to limit explosion effects” can never replace the preventive measures to be
taken. One must always try to prevent an explosion, by avoiding explosive mixtures or ignition sources. Only in those cases
where this (due to process circumstances) cannot be realised with enough certainty, additional measures must be taken.
The restrictive measures can be divided into 2 groups:
1. Measures to prevent an explosion, through for instance pipes, from propagating to adjacent equipment. These are the so-called
isolating measures.
2. Measures to prevent the equipment from collapsing in case of an explosion in a certain volume. These measures can, depending
on the protection technique chosen, be subdivided into:
• Explosion resistant design
• Explosion venting
• Explosion suppression
These different techniques will be discussed hereafter. With each technique, and the elements possibly to be used, the diverse
application possibilities and their limitations will be mentioned.
4.1. EXPLOSION ISOLATION
4.1.1. Necessity
The misunderstanding that, if only the diverse equipment is protected against explosions, for instance with explosion venting,
no isolating measures would be necessary, is still alive.
A second misunderstanding is that an explosion in most cases will not propagate against the air flow. In dust exhaust pipes for
instance speeds of 20 m/sec can easily be reached. The flame speed of a gas or dust explosion reaches at most a few meters
per second. This might lead to the (erroneous) conclusion that an explosion in a filter will not propagate backwards in an exhaust
pipe. One has to consider, however, that an explosion, even vented, leads to a considerable overpressure in the filter. The
consequence is that the air is no longer sucked towards the filter, but blown back. The explosions flame jet does not have to
“row against the flow” but “drifts” with the air flow.
Sometimes it is said that there would be no, or nearly no, chance for explosion propagation to happen if the fuel concentration
in the connected pipes is sufficiently low. This also is incorrect. As a consequence of an explosion in a vessel a fuel/air mixture
is blown into the connected pipes, even before the flame jet has arrived. Possible dust deposits in the pipes can easily be
dispersed by the explosion.
Finally the discussion goes sometimes as follows:
Isolation is often an expensive measure. The chances of an explosion occurring is rather small. If all equipment is protected
against the consequences of an explosion, isolation is not necessary. After all, the propagation of an explosion to adjacent equipment is now acceptable, because that equipment is also protected. This also is a totally wrong conclusion. The problem is that
an explosion, when propagating from one vessel to an other tends to become more and more violent. This is illustrated in the
following drawing.
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EXPLANATION ON EXPLOSION SAFETY
An explosion is a dynamic phenomenon accompanied by a very fast pressure increase. This means that, even if an opening is
created (= explosion venting), still a considerable overpressure with a tendency to propagate in all directions, will remain. In
case of a vented explosion there is not only a flame jet emanating from the vent, but in most cases there will be also flame jets
passing through all connected pipes.
pipe A-B
vessel
vessel
A
B
Explanation:
• An explosion in vessel A will make the pressure rise in vessel A.
• As the speed with which the overpressure expands (= sound speed = 340 m/sec) is much higher than the flame speed, the
pressure will also rise in vessel B, long before the flame jet arrives.
• After a certain time the flame jet, coming from A, reaches the connecting pipe.
• A flame, travelling through a pipe tends to accelerate.
• At the moment the flame reaches vessel B its speed has increased vastly. In addition there is already a considerable overpressure in vessel B.
• Result: a very heavy explosion in vessel B, at which both the explosion speed and the maximum explosion pressure are
(much) higher than in vessel A.
In general explosion isolation is for this reason always required. Exceptions on this rule are:
• As the connecting pipe leads to a safe environment, for instance the air inlet of a milling installation, where the air is sucked
in from outside at a safe place.
• If the connecting pipe is so small that, as a result of cooling, no flame acceleration is to be expected. Example: measuring
capillaries with a diameter of a few millimetres will, in many cases, halt the dust explosion, and certainly not cause great
flame accelerations.
• If, at regular distances the pipe is equipped with explosion vents, as a result of which no huge overpressures and flame
accelerations may occur.
• If the design of the protection on the second vessel takes into account the possible acceleration of the explosion.
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4.1.2. Passive systems
Passive systems are autonomous working systems: no external detection and control are required.
Flame arrestor
Flame arrestors are much used to prevent propagation of gas explosions. A flame arrestor is in fact a finely meshed grid. The
openings in the grid are so small that the flame, as a result of its cooling at the wall, will extinguish.
Flame arrestors can, obviously, only be used in 'clean air'. If dust or droplets are present, the flame arrestor will soon become
filthy and get clogged.
Rotary valve
Rotary valves are often used in powder transport systems. If a rotary valve is explosion proof it can also be used as an isolation
device for an explosion.
Lock
In situations where the use of a rotary valve is impossible (for instance sticky product or too little space), sometimes two gate
valves are placed one above the other. The valves open in turn to be able to transport the product. It is evidently important that
the second valve should open only when the first valve is completely closed.
Screws
When a screw, during product transportation, is fully filled with product, the explosion cannot pass. However: if the screw is
emptied, or if there is still room above the screw, the flame can shoot through. Screws can be specially adapted to prevent flame
passage.
Explosion relief stack
Explosion relief stacks or explosion diverters are often used on the inlet duct of (dust) filters. In an explosion relief stack the
direction of the airflow is suddenly turned 180°. The upper side of the stack is closed by an element that easily opens. In case
of an explosion the flame will shoot through this opening rather than take the 180° U-turn and follow the pipe.
The functioning of a VENTEX valve can be compared more or less with a check valve. In the pipe there is an oblate metal spheric
body, mounted springily on a spindle. In case of an explosion this body, through the combined influence of the pressure wave
and the increased air speed, is pushed in the direction of the explosion and the pipe is closed off. Ventex valves are mostly used
in clean air pipes, for instance on the clean air outlet of a filter or on the clean air suction line of a mill.
Ventex check valve
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EXPLANATION ON EXPLOSION SAFETY
Ventex-valve
4.1.3. Active systems
With active systems the explosion is detected, based on pressure or optical detection. Subsequently a control unit will trip off
the barrier.
Detection
There is still quite a discussion going on about the detection techniques to be used.
• Pressure detection is relatively insensitive to pollution. The disadvantage of pressure detection is however that, in case of a
very slow explosion, detection will be very late. It may occur that in such case the flame jet has already passed the barrier.
• Optical detection does not have this disadvantage. Optical detection in most cases is placed on the pipe itself. This way one
can be sure that, also in case of a slow explosion, detection takes place at the very moment the flame passes the detector.
A disadvantage of optical detection is its vulnerability to pollution. Some detectors can see through several millimeters of
product, under condition that the product is not too light absorbing (no black product). If necessary the detector can be kept
clean using an air purging device.
Mechanical barriers
To stop an explosion a fast shutting valve can be used. Traditionally fast shutting gate valves with a closing time of, in most
cases, below 50 ms are used. Besides them, nowadays other fast shutting valves exist, such as fast shutting ball valves and
butterfly valves.
The most important application is in pipes for pneumatic transport or on exhaust systems with relatively high dust load.
Fast shutting gate valve
Chemical barrier
An alternative to the mechanical barrier is the so-called chemical barrier. A huge quantity of extinguishing agent (in most cases
extinguishing powder) is blown locally into the pipe. Such a chemical barrier stops the explosion's flame jet, but not the pressure.
Chemical barriers are often used in combination with explosion suppression. But they are also a (cheaper) alternative for fast
shutting gate valves in piping with large diameters.
Chemical barriers may in general not be used in combination with explosion resistant equipment.
4.1.4. Dimensioning
The correct positioning is of the utmost importance, especially for active systems: what is the necessary distance between the
detector and the barrier to be sure the barrier is triggered in time, without running the risk of such vast accelerations that
detonations could occur.
In most cases this distance is chosen based on test experiences. The problem here might be that in testing circumstances there
is mostly a vehement explosion, whilst in practice a lot of explosions are weak. Especially when pressure detection is used such
explosions are often detected very late.
On the other hand it is also possible to define the exact position for the barrier by calculating the possible flame accelerations
in the pipe and combine these with the system's delay time. For this the necessary knowledge and experience are of course
needed.
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4.2. EXPLOSION RESISTANT CONSTRUCTION
The principle of explosion resistant construction is simple: the installation to be protected is designed to resist the maximum
explosion pressure to be expected (in practice usually 10 bar).
Lower design pressures are nevertheless also used: if explosion venting or suppression is applied, the equipment must still be
able to resist the reduced explosion pressure to be expected. In practice distinction is made between explosion resistant design
and explosion pressure shock resistant design.
This has to do with whether or not permanent deformation is allowed.
4.3. EXPLOSION VENTING
This is by far the technique most used, especially for dust explosions, to limit the consequences of an explosion.
Here the equipment to protect is equipped with one or more weak elements (rupture discs or hinged explosion doors) that open
at a low pressure. This way the pressure inside the equipment to be protected stays restricted. Some short comments:
• In case of venting the explosion is not extinguished, the effects are only moved outside. In front of the vent area one must count
on huge flame jets and pressure waves. Explosion venting can only be done in a safe direction, and never inside a building.
• There exist all kinds of venting elements, such as rupture discs and explosion doors, and also systems with integrated flame
extinguisher which can, under certain circumstances be used inside a building. All have their pros and cons.
Explosion venting test in a 250 m3 vessel (Switzerland)
Explosion suppression
4.4. SUPPRESSION
In case of explosion suppression the explosion is detected at a very early stage and subsequently an extinguishing agent is
blown in very fast. As for the choice of detectors the same remarks can be made as for the active containment systems.
Previously Halon was used as an extinguishing agent; nowadays mainly extinguishing powders are used. In some cases however water or steam is applied.
Compared to venting, suppression has the advantage that it can be applied also in the middle of a building as there are no
effects to the outside anymore. In addition to this, nowadays suppression is often chosen because in most situations there is no
fire afterwards: there is no, or nearly no damage to the installation, as a result of which the latter can be started up again very
soon.
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EXPLANATION ON EXPLOSION SAFETY
• European standards are being made to calculate the necessary surface for a concrete situation. For dust this standard runs
closely into the range of the (up till now predominantly used) German standard VDI 3673.
5. ATEX-DIRECTIVES
There are two ATEX directives, with a totally different background and a completely different target group. Both will be discussed
separately.
5.1. DIRECTIVE 94/9/EC: ATEX 95
This directive evolved out of the wish to remove as many trade barriers as possible in Europe. In the past it was in fact often so
that for instance Ex equipment that had been certified in one country, was not accepted in another. ATEX 95 serves to counter
this situation: equipment, ATEX 95 certified in one European country, must be accepted in all other countries.
The directive is in the first place meant for equipment manufacturers who want to bring these on the market.
The directive makes a distinction between equipment destined to be placed in an explosive environment and explosion
protection systems.
The equipment includes the old 'Ex' electrical equipment, but expressly also the mechanical equipment. As indicated in 3.2.1
the equipment is subdivided into categories. Dependent on the category more severe requirements are set.
• Category 1 is the heaviest class. Equipment in this category can be placed in environments where an explosive mixture is
more or less continuously present: Zone 0 or 20 (of course it can be placed also in all other zones). Here ignition sources have
to be avoided even in extremely faulty situations. Category 1 equipment must be tested by institutions especially recognised
for this (the so-called Notified Bodies).
• Category 2 is meant for zone 1 or 21. Here the requirements are a bit less severe. Electrical equipment still has to be
tested by a Notified Body, mechanical equipment however can be tested by the manufacturer himself, but the file concerned
must be deposited at a Notified Body.
• Category 3 is meant for the lightest zone: 2 or 22. Here it is enough to prove that no ignition sources are present during
normal operation. The manufacturer can test himself, without a Notified Body.
Under protection systems we understand the diverse systems described in the previous chapter (flame arrestors, fast valves,
rupture discs, explosion suppression). This equipment too must, when brought on the market, be tested; during the test the good
functioning in stopping the explosion or (for instance) in venting, and the reliability are controled. Also these basic rules, that are
momentarily worked out in diverse new standards, are given in ATEX 95.
5.2. DIRECTIVE 1999/92/EC: ATEX 137
5.2.1. Background
Directive 1999/92/EC, better known as ATEX 137, is subtitled:
Minimum requirements for improving the safety and protection of workers potentially at risk from explosive atmospheres.
It is a so-called minimal directive. This means that all European member countries, when converting this directive into national
legislation, can sharpen the rules, but not soften it.
5.2.2. Target group
Above section II (after the 'general rules' in I and the 'diverse definitions' in III), in fact the core of the directive, it reads: obligations
of the employer. The directive does not aim at manufacturers nor at suppliers of equipment, but at employers.
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5.2.3. Contents
What these obligations of the employer imply is worded in article 5:
When in a work environment explosive atmospheres can occur in such a concentration that the health and the safety of the
employees or of others is endangered, the employer has to make sure that the work environment is organised in such a way
that safely can be worked and he has to take care of appropriate supervision.
5.2.4. Explosion protection document
The preceding is not really new: the employer has always been bound to protect his employees against risks. New is that
ATEX 137 insists on putting down a few things in a so-called Explosion Protection Document of the installation.
From this document must emerge that:
• The explosion risks are identified and assessed;
• Adequate measures will be taken to reach the purpose of the directive;
• Zone classification has been executed;
• On which places the minimum requirements are to be applied (according to annex II of the directive);
• The work place and work equipment including warning devices are designed, operated and maintained correctly.
The under point 4 mentioned annex II implies in a nutshell the following:
• Organisational measures;
• Training of employees related to explosion risks;
• Written instructions, work permits;
• Explosion protection measures.
5.2.5. Drawing up in practice
For new installations the explosion safety aspect can be included from the first sketch design on. But: also for existing installations such a document must be drawn up. In most cases one proceeds as follows:
Point 1. Explosion risk analysis and evaluation.
One starts with a description of the installation, in which the most important process data and the explosion characteristics
of the products concerned are noted.
Based on this follows a complete investigation of the installation:
1) Where can possible explosive mixtures be present?
2) Where and which ignition sources can occur?
3) How is the possible development of an explosion?
4) What are the possible consequences?
If it appears that the consequences are inacceptable, explosion risk reducing measures are proposed and a new analysis
is done to investigate if, with those measures taken, the consequences are acceptable.
With risk reducing measures there is, according to ATEX, an order of priorities:
• Prevent explosive atmospheres;
• Prevent ignition sources;
• If the risk is still too high, effect limiting measures are necessary.
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EXPLANATION ON EXPLOSION SAFETY
An Explosion Protection Document is a living document which means that, dependent on modifications in the installation, the
document too has to be adapted. This means the drawing up of the document is no once-only matter and never finished.
Point 2. Are the measures taken sufficient?
Only after execution of all measures mentioned under point 1 can point 2 be dealt with.
Point 3. Execution of gas and dust zonings
Zoning implies in fact that the area concerned is divided into zones, dependent on the possibility of explosive mixtures
to be present. Gas zoning has already been common practice for a long time and can be realised based on well-defined
rules. Dust zoning however is relatively new and asks for a thoroughly different approach.
With dust zoning one has to take into account dust deposits. These might after all be dispersed into dangerous clouds.
This cannot happen with gas.
With gas zoning one mainly “calculates”: what is the possible leak flow rate? What are the ventilation folds? With dust
zoning one mainly “observes”: where are there dust deposits present that possibly could lead to the formation of a
dangerous cloud.
In practice this often means that, in order to relieve a gas zone, measures such as supplementary ventilation are
necessary. With dust zoning “good housekeeping” (frequent and thorough cleaning) are often sufficient.
The Dutch NPR 7910 is (also outside the Netherlands) often used as a very practical directive for zoning.
Point 4. The minimal rules are applied according to annex II.
Here the different points mentioned in this annex have to be checked against the real situation with regard to organizational
measures (training of personnel and work permits) and to explosion protection measures (removal of dangerous dusts,
measures based on the most dangerous dust present, prevention of static electricity, starting up equipment, minimalising
the exposure of personnel to the effects of explosion, alarms, escape routes and escape plan).
Point 5. The work places and equipment and the alarm installations are correctly designed, operated and maintained.
For new installations this means above all the inspection of certificates: equipment to be installed in a danger zone has to
be certified for the zone concerned. But also protection systems (rupture discs, fast shutting valves etc.) must be certified.
For existing installations no certificates are required, but there an assessment on the suitability has to be executed.
It is, however, of great importance that all these equipments and systems are well operated and maintained to guarantee
their functionality in the long term.
5.2.6. Time Schedule
For new installations this document has to be finished before start-up. For existing installations ATEX 137 requires that everything
has to be ready by July 1st 2006.
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INDEX
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WORDS
16
16
16
16
10
8,16
14
3
10
10
14
8
9
11
4
13
15
17
17
17
15
13
gas-Ex
gas group
IPXX
lightning discharge
Notified Bodies
NPR 7910
physical explosion
propagating brush discharge
protection degrees
rotary valve
screw
spark discharge
static electricity
suppression
temperature class
valve lock
valves
VDI 3673
ventex valve
zone 0/1/2
zone 20/21/22
8
5
9
10
16
6
3
10
9
13
13
10
10
15
9
13
14
15
13
6,7
7
ABBREVIATIONS
b:
ignition source control
BG:
burning group
c:
safe by construction
d:
explosion proof enclosure
dp/dtmax:
maximum rate of pressure rise
e:
increased safety
minimal ignition energy
Emin:
fr:
limited breathing enclosure
iD:
protection by intrinsic safety
i (EN 13463): intrinsically safe
i (EN 50 020): intrinsically safe equipment
i sys:
intrinsically safe systems
k:
protection by immersion
Kg:
gas explosion constant
Kst:
dust explosion constant
LEL:
lower explosion limit
m:
encapsulated material
mD:
encapsulate
9
6
9
8,9
5
8
5
9
8
9
8
8
9
5
5
4
8
8
MIE:
MIT:
n:
o:
p:
pD:
Pmax:
q:
St 0 t/m 3:
T1 t/m 6:
tD:
Tglim:
Tmin :
UEL:
minimum ignition energy
minimal ignition temperature
of a cloud
non sparking material (Cat 3G 8)
immersion in oil
protection by overpressure
overpressure
maximum explosion pressure
powdery filling
dust category
temperature category
protection by enclosure
minimal ignition temperature
of a dust layer
minimal ignition temperature
of a cloud
upper explosion limit
5
5
8
8
8,9
8
5
8
5
9
8
6
5
4
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EXPLANATION ON EXPLOSION SAFETY - INDEX
1999/92/EC directive
94/9/EC directive
ATEX 137
ATEX 95
brush discharge
category 1/2/3
chemical barrier
chemical explosions
cone discharge
corona discharge
detection
dust-Ex
Ex-mechanical equipment
explosion isolation
explosion limits
explosion relief stack
explosion resistant construction
explosion risk analysis
explosion risk evaluation
explosion protection document
explosion venting
flame arrestor