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 Explosion Prevention 5 7.2 Explosion Protection 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 no-one 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.

