Fire and Explosion Hazards in Dryers Guide

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-DRY-004

Fire and Explosion Hazards in Dryers

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Process Engineering Guide:

Fire and Explosion Hazards in
Dryers

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

FLAMMABILITY OF DUST CLOUDS

3

5

THERMAL STABILITY

4

6

IGNITION SOURCES

4

7

METHODS OF PROTECTION

4

7.1
7.2

Explosion Prevention
Explosion Protection

5
5

8

GENERAL ADVICE

6

9

BIBLIOGRAPHY

7


0

INTRODUCTION/PURPOSE

Most natural products of animal or vegetable origin are combustible, as are
approximately 70 % of synthetic organic powders and some oxidizable inorganic
compounds such as iron pyrites.
The handling of such powders poses both fire and explosion hazards, during
drying operations and also in downstream equipment.
Powders can present a number of hazards particularly when heated, for
example:
(a)

Clouds of combustible dust will burn when ignited resulting in an explosion
in an enclosed space or a flash fire.

(b)

Ignition of a dust layer may result in burning by flame or smouldering.
Smouldering may develop into a flame, depending on the nature of the
material, the local conditions, and any disturbance.

(c)

Ignition within a bulk powder can occur if it should self heat i.e., an
exothermic reaction is initiated. This may ignite the powder spontaneously
or possibly ignite some decomposition product.

The following conditions need to be satisfied simultaneously for a dust explosion
to occur:
(1)

The dust is to be combustible.

(2)

The dust needs to be in suspension in an atmosphere capable of
supporting combustion (usually air or oxygen).

(3)

The dust needs to have a particle size distribution that will propagate
flame.

(4)

The dust concentration in the suspension is to be within the explosive
range.

(5)

The dust suspension needs to be in contact with an ignition source of
sufficient energy.


Where combustible dusty materials are handled, there country specific Statutory
Requirement to take practicable precautions
(i)

Where in connection with processing a dust is likely to be generated which
is liable to explode, all practicable steps shall be taken to prevent such an
explosion by enclosure of the plant, removal of any accumulations that
may escape, and exclusion or enclosure of any possible sources of
ignition.

(ii)

Where an explosive dust is present in a plant and the plant is not
constructed to withstand the pressures likely to be produced by an
explosion, all practicable steps shall be taken to restrict the spread and
effects of such an explosion by the provision of chokes, baffles and vents,
or other equally effective appliances.

In effect the Statutory Requirement’s requires a manufacturer to identify whether
he is handling an explosive dust. If so, it needs to be contained within the
process equipment, and good housekeeping is to be employed to ensure that
deposits do not build up outside the equipment. In addition precautions need to
be taken to mitigate the consequences of any explosion.

1

SCOPE

This Process Engineering Guide covers the general principles of fire and
explosion hazards in dryers and discusses the methods of protection which may
be used. It does not deal with the hazards associated with particular products.

2

FIELD OF APPLICATION

This Guide applies to Process Engineers in GBH Enterprises worldwide.

3

DEFINITIONS

For the purposes of this Guide, no special definitions apply.
With the exception of terms used as proper nouns or titles, those terms with initial
capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.


4

FLAMMABILITY OF DUST CLOUDS

If it is possible for a dust cloud to occur, the powder should be tested to
determine whether the dust cloud will be flammable.
It should be recognized that small changes in the composition of a dust, e.g. by
the addition of dedusting agents can markedly affect both its flammability and the
violence of any resulting explosion. Materials should be re-tested if the
composition is changed.
The test sample needs to be representative of the finest dust and should always
be at least as dry as the driest material encountered in the plant. As a rule the
flammability of a combustible dust is greater if the particle size is reduced. The
minimum ignition energy is reduced and the rate of pressure rise is also
increased with a decrease in particle size. Particle sizes greater than 500 µm
diameter are not likely to cause dust explosions although the possibility of fine
dust being generated during handling will need to be carefully considered.
Dusts are classified as follows:
(a)

GROUP A - Flammable.

(b)

GROUP B - Non-flammable.

This classification only applies to process temperatures of < 110°C. If the dust
cloud is to be subjected to higher temperatures then it is advisable to test
whether the dust propagates a flame at temperatures more closely simulating
dryer conditions [Ref.1].
Dusts designated Group B are classified as non-explosive at ambient
temperatures. They may be explosive at elevated temperatures and pose a
potential fire risk.
In the USA and Europe, a classification system based on the concepts of ignition
sensitivity and explosion severity is used.
Dusts are further classified using the St system based on the rate of pressure
rise, for example:
(1)

St 0 - Non-flammable.

(2)

St 1 - 3 - Flammable. Depending on rate of pressure rise, i.e explosion
severity.


It is important to note that a dust cloud in air may become much more susceptible
to ignition and the violence of the resulting explosion may increase considerably
if the air contains a small quantity of flammable vapor, even though the vapor
concentration may be well below the Lower Flammability Limit (LFL). Such dust vapor - air mixtures are termed hybrid mixtures and can occur even with water
wetted material.
If a dust is flammable, or a hybrid mixture is suspected, further tests may be
required to determine its sensitivity to ignition and explosion. These include
determining the minimum ignition temperature (MIT) and minimum ignition
energy (MIE) in air. These parameters are important when assessing safe
operating temperatures for dispersion dryers (e.g. spray dryers, fluid bed dryers,
and pneumatic conveyer dryers).

5

THERMAL STABILITY

Heating of powders can lead to their decomposition which can cause particular
problems during drying operations. The temperature at which self heating starts
is markedly test dependent and can vary with air availability, composition, and
volume. Particular care should be taken when relating test data to full scale
conditions, [Ref.2,7], where powder is bulked or accumulates in layers.
Information on thermal stability of powders is required to specify safe operating
conditions of both test and full scale dryers e.g. inlet and outlet temperatures,
and product collection systems.

6

IGNITION SOURCES

When flammable dust clouds are handled, precautions should always be taken to
reduce the likelihood of ignition.
The main sources of ignition in dryers capable of igniting flammable vapors and
dusts, both as dispersions and in layers include:
(a)

Naked flames.

(b)

Hot surfaces.

(c)

Welding or cutting operations.

(d)

Electrical equipment.


(e)

Friction heating or impact sparks.

(f)

Thermite sparks.

(g)

Electrostatic discharges.

(h)

Spontaneous heating.

7

METHODS OF PROTECTION

In dryers, safety can be based either on preventing an explosion occurring or
accepting that an explosion can take place and providing means of ensuring noone is injured by it (explosion protection). In the latter case it is also desirable
that the plant is not damaged by the explosion.
In most cases it is not sufficient to take preventative steps to avoid explosions.
Measures will need to be taken to limit the spread of an explosion.
When specifying a system of explosion prevention or protection for a process
plant, it is important to look at the plant as a whole and not just a specific item,
e.g., a dryer, in isolation.

7.1

Explosion Prevention

The following methods are designed to prevent an explosion. They will not
prevent exothermic decomposition however, which could produce effects similar
to an explosion if the volume of gas evolved is great enough.

7.1.1 Inerting
Inerting is the process of introducing an inert gas to a combustible mixture to
reduce the concentration of oxygen below the minimum oxygen concentration
(MOC). The inert gas is usually Nitrogen or Carbon Dioxide although Steam can
be used.


The working Oxygen concentration should be a safe margin below the MOC
required to support combustion. In the case of explosive dusts, published data
should be treated with circumspection since some data have been obtained at
high temperatures (e.g. 850°C) which is a more severe condition than normally
encountered. If inerting is used as a method of prevention, the reliability of the
system should be routinely checked.

7.1.2 Avoiding Dust Cloud Formation
This is a feasible basis of safety in many tray dryers and band dryers handling
water/wetted solids. The critical requirement is that the air velocity across or
through the material should be low enough for particle entrainment to be
negligible.

7.1.3 Elimination of Ignition Sources
The elimination of ignition sources can only be used as a basis of safety in dryers
where it is certain that all potential sources of ignition have been identified and
precautions taken to prevent their occurrence. Particular care is needed in
evaluating the thermal stability of the product being dried which may lead to
spontaneous heating.

7.2 Explosion Protection
There are three methods of explosion protection:
(a)

Venting - relieves the pressure of the explosion.

(b)

Suppression - quenches the explosion.

(c)

Containment - contains the explosion.

Where explosion protection is used, care should be taken to prevent an
explosion in one vessel damaging ancillary equipment.


7.2.1 Venting
The principle of explosion relief venting is that at a predetermined pressure rise
an aperture opens to vent the explosion products [Ref.3]. Venting is common,
often the cheapest method of protection provided that a safe discharge area for
the products can be found. Due to environmental concerns, its use in the
chemical industry is diminishing. Guides to the use and designs of venting as a
method of explosion relief have been prepared by the Institution of Chemical
Engineers [Ref.4] and [Ref.5].

7.2.2 Suppression
An explosion is not an instantaneous event but takes a finite time to attain
destructive pressures in a vessel. Explosion suppression requires that the
incipient explosion is detected very soon after ignition, usually by means of a
pressure transducer, and that sufficient chemical suppressant is discharged into
the developing explosion. Explosion suppression is often used where safe
venting is not possible, particularly where an emission of toxic materials could
occur. Automatic suppression techniques are described in detail in an Institution
of Chemical Engineers Guide to dust explosion and prevention [Ref.7].
Sometimes suppression is used in conjunction with venting to protect a vessel.
Suppression will not provide protection against developing pressures resulting
from large quantities of gas evolved due to exothermic decomposition of material.

7.2.3 Containment
It is possible to achieve protection by making plant strong enough to contain the
maximum dust or vapor explosion without rupture. The maximum pressure
reached in a closed vessel as a result of a dust explosion (7-10 bar) is
characteristic of the dust concerned and may be determined from test
explosions.
Containment is a common method of protection for vacuum dryers since the
peak pressure generated by an explosion is dependent on the initial pressure.
However. careful consideration needs to be given to the charging and
discharging of material which occurs at atmospheric pressures.
Two different approaches for designing plant capable of withstanding this type of
explosion are used:

(a)

Pressure resistance. The plant is designed to prevent permanent
deformation or rupture.

(b)

Pressure shock resistance. The plant is designed to withstand the
explosion pressure without rupture but is subject to permanent
deformation [Ref.6].

Further details of the use of explosion containment as a protective measure can
be found in the Institution of Chemical Engineers Guide [Ref.7].

8

GENERAL ADVICE

Most powders are flammable and capable of forming combustible dust clouds. In
order to ensure the safety of proposed operations with a product, it is necessary
to carry out suitable tests to ascertain the characteristics of the material and its
suitability in the type of equipment concerned.
Most dryers and ancillary equipment contain dust clouds, dust deposits and
flammable vapors at some time during processing.
Many types of ignition source are to be found in dryers. It is vital therefore, that
adequate consideration is given to explosion prevention and protection.
Safety is critically dependent on the reliability of the prevention or protection
system used. Reliability needs to be assured by regular preventive maintenance.
When considering a system of explosion prevention or protection for a process
plant, it is essential to consider the plant as a whole rather than each vessel in
isolation.


9

BIBLIOGRAPHY

[1]

FIELD P, (1982) Dust explosions. Handbook of powder technology,
Volume 4, Elsevier Scientific Publishing Company.

[2]

BOWES P C, (1984) Self Heating: Evaluating and controlling the Hazards.
Building Research Establishment. HMSO.

[3]

DONAT C, (1971) Selection and sizing of pressure relief devices for dust
explosions, Staub - Reinhalt.

[4]

LUNN G A, (1992) Guide to dust explosion prevention and protection, Part
1 -Venting. The Institution of Chemical Engineers. (2nd Edition)

[5]

LUNN G A, (1988) Guide to dust explosion prevention and protection, Part
3 - Venting of weak explosions and the effect of vent ducts. The Institution
of Chemical Engineers.

[6]

ABBOTT J A, (1990) Prevention of Fires and Explosions in Dryers. The
Institution of Chemical Engineers.

[7]

SCHOFIELD C AND ABBOTT J A, (1988) Guide to dust explosion
prevention and protection, Part 2 - Ignition prevention, containment,
inerting, suppression and isolation. The Institution of Chemical Engineers.


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