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Process Engineering Guide:GBHE-PEG-DRY-004

Fire and Explosion Hazards in Dryers

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Process Engineering Guide: Fire and Explosion Hazards inDryers

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 57.2 Explosion Protection 5

8 GENERAL ADVICE 6

9 BIBLIOGRAPHY 7






0 INTRODUCTION/PURPOSE

Most natural products of animal or vegetable origin are combustible, as areapproximately 70 % of synthetic organic powders and some oxidizable inorganiccompounds such as iron pyrites.

The handling of such powders poses both fire and explosion hazards, duringdrying operations and also in downstream equipment.

Powders can present a number of hazards particularly when heated, forexample:

(a) Clouds of combustible dust will burn when ignited resulting in an explosionin 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 thematerial, the local conditions, and any disturbance.

(c) Ignition within a bulk powder can occur if it should self heat i.e., anexothermic reaction is initiated. This may ignite the powder spontaneouslyor possibly ignite some decomposition product.

The following conditions need to be satisfied simultaneously for a dust explosionto occur:

(1) The dust is to be combustible.

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

(3) The dust needs to have a particle size distribution that will propagateflame.

(4) The dust concentration in the suspension is to be within the explosiverange.

(5) The dust suspension needs to be in contact with an ignition source of

sufficient energy.






4 FLAMMABILITY OF DUST CLOUDS

If it is possible for a dust cloud to occur, the powder should be tested todetermine whether the dust cloud will be flammable.

It should be recognized that small changes in the composition of a dust, e.g. bythe addition of dedusting agents can markedly affect both its flammability and theviolence of any resulting explosion. Materials should be re-tested if thecomposition is changed.

The test sample needs to be representative of the finest dust and should alwaysbe at least as dry as the driest material encountered in the plant. As a rule theflammability of a combustible dust is greater if the particle size is reduced. Theminimum ignition energy is reduced and the rate of pressure rise is alsoincreased with a decrease in particle size. Particle sizes greater than 500 µmdiameter are not likely to cause dust explosions although the possibility of finedust 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 dustcloud is to be subjected to higher temperatures then it is advisable to testwhether the dust propagates a flame at temperatures more closely simulatingdryer conditions [Ref.1].

Dusts designated Group B are classified as non-explosive at ambienttemperatures. They may be explosive at elevated temperatures and pose apotential fire risk.

In the USA and Europe, a classification system based on the concepts of ignitionsensitivity and explosion severity is used.

Dusts are further classified using the St system based on the rate of pressurerise, for example:

(1) St 0 - Non-flammable.

(2) St 1 - 3 - Flammable. Depending on rate of pressure rise, i.e explosionseverity.






It is important to note that a dust cloud in air may become much more susceptibleto ignition and the violence of the resulting explosion may increase considerablyif the air contains a small quantity of flammable vapor, even though the vaporconcentration may be well below the Lower Flammability Limit (LFL). Such dust -vapor - air mixtures are termed hybrid mixtures and can occur even with waterwetted material.

If a dust is flammable, or a hybrid mixture is suspected, further tests may berequired to determine its sensitivity to ignition and explosion. These includedetermining the minimum ignition temperature (MIT) and minimum ignitionenergy (MIE) in air. These parameters are important when assessing safeoperating 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 particularproblems during drying operations. The temperature at which self heating startsis markedly test dependent and can vary with air availability, composition, andvolume. Particular care should be taken when relating test data to full scaleconditions, [Ref.2,7], where powder is bulked or accumulates in layers.Information on thermal stability of powders is required to specify safe operatingconditions 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 toreduce the likelihood of ignition.

The main sources of ignition in dryers capable of igniting flammable vapors anddusts, 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 oraccepting 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 desirablethat 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 processplant, 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 notprevent exothermic decomposition however, which could produce effects similarto 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 toreduce the concentration of oxygen below the minimum oxygen concentration(MOC). The inert gas is usually Nitrogen or Carbon Dioxide although Steam canbe used.






The working Oxygen concentration should be a safe margin below the MOCrequired to support combustion. In the case of explosive dusts, published datashould be treated with circumspection since some data have been obtained athigh temperatures (e.g. 850°C) which is a more severe condition than normallyencountered. If inerting is used as a method of prevention, the reliability of thesystem 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 handlingwater/wetted solids. The critical requirement is that the air velocity across orthrough the material should be low enough for particle entrainment to benegligible.

7.1.3 Elimination of Ignition Sources

The elimination of ignition sources can only be used as a basis of safety in dryerswhere it is certain that all potential sources of ignition have been identified andprecautions taken to prevent their occurrence. Particular care is needed inevaluating the thermal stability of the product being dried which may lead tospontaneous 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 anexplosion in one vessel damaging ancillary equipment.






7.2.1 Venting

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

7.2.2 Suppression

An explosion is not an instantaneous event but takes a finite time to attaindestructive pressures in a vessel. Explosion suppression requires that the

incipient explosion is detected very soon after ignition, usually by means of apressure transducer, and that sufficient chemical suppressant is discharged intothe developing explosion. Explosion suppression is often used where safeventing is not possible, particularly where an emission of toxic materials couldoccur. Automatic suppression techniques are described in detail in an Institutionof 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 resultingfrom 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 themaximum dust or vapor explosion without rupture. The maximum pressurereached in a closed vessel as a result of a dust explosion (7-10 bar) ischaracteristic of the dust concerned and may be determined from testexplosions.

Containment is a common method of protection for vacuum dryers since thepeak pressure generated by an explosion is dependent on the initial pressure.

However. careful consideration needs to be given to the charging anddischarging of material which occurs at atmospheric pressures.

Two different approaches for designing plant capable of withstanding this type ofexplosion are used:






(a) Pressure resistance. The plant is designed to prevent permanentdeformation or rupture.

(b) Pressure shock resistance. The plant is designed to withstand theexplosion pressure without rupture but is subject to permanentdeformation [Ref.6].

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

8 GENERAL ADVICE

Most powders are flammable and capable of forming combustible dust clouds. Inorder to ensure the safety of proposed operations with a product, it is necessaryto carry out suitable tests to ascertain the characteristics of the material and itssuitability in the type of equipment concerned.

Most dryers and ancillary equipment contain dust clouds, dust deposits andflammable vapors at some time during processing.

Many types of ignition source are to be found in dryers. It is vital therefore, thatadequate consideration is given to explosion prevention and protection.

Safety is critically dependent on the reliability of the prevention or protectionsystem used. Reliability needs to be assured by regular preventive maintenance.When considering a system of explosion prevention or protection for a processplant, it is essential to consider the plant as a whole rather than each vessel inisolation.






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 dustexplosions, Staub - Reinhalt.

[4] LUNN G A, (1992) Guide to dust explosion prevention and protection, Part1 -Venting. The Institution of Chemical Engineers. (2nd Edition)

[5] LUNN G A, (1988) Guide to dust explosion prevention and protection, Part3 - Venting of weak explosions and the effect of vent ducts. The Institutionof Chemical Engineers.

[6] ABBOTT J A, (1990) Prevention of Fires and Explosions in Dryers. TheInstitution of Chemical Engineers.

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

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