INTRODUCTION TO DUSTS AND THEIR EXPLOSIVE PROPERTIES

Transcription

1 INTRODUCTION TO DUSTS AND THEIR EXPLOSIVE PROPERTIES Olof Bekker Pr. Eng. BSc. Eng. BML S.MSAIEE, M.NACE Engineering Manager, Electrical and Facilities

2 DUST EXPLOSION! 1

3 DEFINITION Dust consists of small solid particles in the atmosphere which settle due to their own weight, but which remain suspended in air for a time (this includes dust and grit, as defined in ISO 4225)European Standard EN Combustible dusts are hazardous because when they are dispersed in air by any means they form potentially explosive atmospheres. Furthermore, layers of combustible dust may ignite and act as ignition sources for an explosive atmosphere. SANS /IEC :2004-Electrical apparatus for use in the presence of combustible dust Part 10: Classification of areas where combustible dusts are or may be present 2

4 DUSTS Dust explosions - one of the least recognised of industrial fire hazards. Can occur within any process where a combustible dust is produced, can be triggered by any energy source, including static sparks, friction and incandescent material. Over 70% of all organic materials are combustible and have the potential to result in a dust explosion. The manufacture of food products involves the storage and handling of materials such as grains, flours, sugars, starches, etc. all of which can give rise to fire and dust explosion hazards. Processing operations such as milling, spray drying, blending, agglomeration, etc. can also produce potentially hazardous dust clouds. In addition, high temperature process operations such as heating, milling, baking and frying can also give rise to further fire and explosion hazards. 3

5 DUST EXPLOSION IN A FLOUR MILL 4

6 BASIC RULES There are a few basic rules to observe to see whether a dust is capable of causing an explosion: Must be combustible. Must be capable of becoming airborne. Must have a size distribution capable of flame propagation. concentration must be within the explosible range. An ignition source must be present. The atmosphere must contain sufficient oxygen to support and sustain combustion. If all of these criteria are fulfilled then an explosion can occur. 5

7 FACTS Gas & vapours cannot be seen; dusts can usually be seen. Gas and vapours will normally disperse in time; dust can build up overtime to form layers The best way to avoid dust explosions is to invalidate as many of the aforementioned properties. 6

8 Factors Affecting Ignition Sensitivity and Explosion Violence Other factors that have an influence: Chemical Composition (and its moisture content) Chemical Composition, pressure and temperature of the gas Particle shape and size distribution Degree of dispersion of the dust cloud Concentration distribution in the dust cloud Turbulence in the dust cloud Amount of turbulence caused by the explosion in un-burnt parts of the cloud Flame front disturbance by mechanisms other than turbulence Radiation heat transfer from the flame (dependant on chemistry) 7

9 Mount Mulligan mine disaster in Australia These cable drums were blown 50 feet (15 m) from their foundations following a coal dust explosion. 8

10 Dust Chemistry and Moisture Content If the substance is not combustible there cannot be an explosion. Dust chemistry is affected by the elements that make up the molecule and their specific configuration in the molecule. Dust chemistry is one of the fundamental considerations when investigating dust explosions. This influences the thermodynamics directly regarding, how much heat is released, and the kinetics, how fast heat is liberated, of the reaction, which in turn directly affects the severity of the explosion. The heat of combustion per mole of Oxygen used is a commonly available and useful number for calculating the total heat release in a dust explosion. Moisture content will affect the ability of a dust cloud to be ignited and its ability to sustain an explosion. Increasing moisture content pushes the ignition energy up exponentially with some dusts. The moisture works in several main ways. The heating and evaporating the moisture provides an inert heat sink. Once evaporated the water vapour mixes with pyrolysis gases and makes them less reactive, and can also increase intermolecular cohesion of the dust meaning a larger effective particle size. 9

11 Aerial view of Imperial Sugar following 2008 dust explosion and fire 10

12 Oxygen Content of Oxidising Gas Less oxygen in the air causes the explosion to be much less severe as it limits the rate of combustion of the dust, thus limiting the oxygen in process vessels can minimise the possibility of a dust explosion (fire can only be sustained if oxygen concentration is greater than 10% in air). 11

13 IGNITION A cloud of dust, within its flammable concentration limits, will not burn unless sufficient energy is provided to ignite it. Possible ignition sources include: Open flames (welding, cutting, matches, smoking etc) Hot surfaces (process equipment, dryers, bearings, heaters, etc) Heat from mechanical impacts (sparking or frictional sparking or heating associated with the apparatus) Arcing or sparking of electrical apparatus Electrical discharges Electrostatic discharges Smouldering or burning dust 12

14 MEASURES TO AVOID IGNITION HAZARDS Temperature of surfaces be kept below the temperature that would ignite the dust; Electrical sparking parts, are contained in an enclosure, which prevents the ingress of dust; Energy of electrical circuits is limited to avoid arcs, sparks or temperatures capable of igniting dust; Other forms of ignition must be avoided or prevented; these include RF energy (e.g. radio, sunlight), adiabatic compression and exothermic reactions, static, to name but a few. Housekeeping by proper continuous cleaning to avoid dust build up. 13

15 IGNITION TEMPERATURE The ignition temperature of a dust layer is a function of the type of dust and its physical and chemical properties and is often less than the cloud ignition temperature. The ignition temperature of a layer of organic dust on heat-producing equipment can decrease over time if the dust dehydrates or carbonizes. For this reason, the heat-producing equipment is not permitted to exceed the lower of either the ignition temperature or 165 C To understand the concerns one must understand the basics of Cathodic Protection. To function, the system requires 3 inputs, namely an Anode, a Cathode and the Electrolyte. In most instances the use of Impressed current is the preferred method in our industry. The Anode is either a precious metal Oxide (Titanium coated) Anode or a Some Centrifugally Cast dusts Silicon Iron in tube, layers the Cathode could is the buried melt steel pipe, before structure or reaching tank bottom and the their Electrolyte layer is the soil. ignition The power source is in the form of a Transformer Rectifier Unit that typically has the ability to put out 50 volts DC and 100 amps or more temperatures. This melted material could then act more like a combustible liquid than a dust. An ignited dust layer introduces an open-flame ignition source that can ignite a dust cloud in the vicinity and also can stir up the dust layer, creating a dust cloud. No matter how combustible the dust, if it's in big lumps it isn't going to cause a dust explosion. There is a clear dependence on size and surface area of dust particles. 14

16 GRADES OF RELEASE Continuous grade: examples are inside of silos, blenders and mills in which dust is introduced or formed; Primary grade: examples are the close vicinity around an open bag filling or emptying point; Secondary grade: example is a dust handling plant where deposits of dust are formed. ZONE Determination Guidelines Based on Dust Layer Thickness Thickness of Dust Layer Classification Greater than 3.0 mm Zone 21 Less than 3.0 mm, but surface colour not discernible Surface colour discernible under the dust layer Zone 22 Unclassified 15

17 EXTENT OF THE ZONE The Extent of a zone is defined as the distance in any direction from the edge of a source of release to the point where the hazard associated with the release is considered to exist no longer. SANS /IEC :2004- Zone 20 is found inside of dusts, vessels etc; these conditions generally occur only inside containers, pipe conduits, devices etc. Zone 21 is: Inside some dust handling equipment; usually within 1-1.5m around the source of release also depends on dust amounts, flow rates, particle size and moisture content of the product. Outside areas, the extent of the zone can be altered due to weather conditions e.g. wind, rain etc; Walls and other structures can be taken as the boundary of the zone; Zone 22 as zone 21 plus an additional 1-1.5m around zone 21 (depends on ventilation and housekeeping). Practical considerations could make it desirable for the whole area to be classified into zone

18 NFPA 654 NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, contains comprehensive guidance on the control of dusts to prevent explosions: Minimize the escape of dust from process equipment or ventilation systems; Use dust collection systems and filters; Utilize surfaces that minimize dust accumulation and facilitate cleaning; Provide access to all hidden areas to permit inspection; Inspect for dust residues in open and hidden areas, at regular intervals; Clean dust residues at regular intervals; Use cleaning methods that do not generate dust clouds, if ignition sources are present; Only use vacuum cleaners approved for dust collection; Locate relief valves away from dust hazard areas; and Develop and implement a hazardous dust inspection, testing, housekeeping, and control program (preferably in writing with established frequency and methods). Use appropriate electrical equipment and wiring methods; Control static electricity, including bonding of equipment to ground; Control smoking, open flames, and sparks; Control mechanical sparks and friction; Use separator devices to remove foreign materials capable of igniting combustibles from process materials; Separate heated surfaces from dusts; Separate heating systems from dusts; Proper use and type of industrial trucks; Proper use of cartridge activated tools; and 17

19 INSPECTION AND MAINTENANCE Only trained personnel familiar with the concept of protection should carry out inspection and maintenance. Before any apparatus is opened in a hazardous area it shall be isolated from all sources of supply. Electrical apparatus shall be inspected and maintained on a regular scheduled basis. All apparatus should be accompanied by instructions, together with any additional information to help maintenance. Where practical, electrical apparatus should be taken to a dust free area for maintenance. If this is not practical then you must take suitable measures that prevent dust entering the enclosure. Care shall be taken during dismantling that parts e.g. seals, etc. are not damaged. 18

20 HOUSEKEEPING Housekeeping refers to the removal of dust accumulations within the plant. Careful removal of lying dust can eliminate the possibility of secondary dust explosions occurring, and also help to prevent some ignition sources. Installing good dust extraction systems wherever there is a particularly dusty area of the plant should do this. Spilt dust should be removed immediately using either a high power explosion proof vacuum cleaner or an internal vacuuming system, which removes dust and sends it to a central filtration system. Minor but steady leaks must be looked out for, as these can cause large amounts of dusts to accumulate over a period of time. The best way to avoid dust accumulation is to be constantly aware of the surroundings and ever vigilant. 19

21 DUST CONTROL CHECKLIST 20

22 REFERENCES National Fire Protection Association, NFPA - Fire Protection Handbook 11-56, Storage Practices and Hazards, Sixteenth Edition 1986 The National Electrical Code (NEC) EN : gives guidance on design, construction and testing of electrical apparatus. EN gives guidance on the Selection, installation and maintenance of electrical apparatus. EN : relates to test methods. EN : relates to classification of areas where explosive dust/air mixtures and dust layers are present. EN Selection of equipment according to the max. allowable surface temperature Tmax of electrical equipments NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids NFPA 499 Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas 2004 Edition SANS Electrical apparatus for use in the presence of combustible dust Part 10: Classification of areas where combustible dusts are or may be present (IEC :2004, IDT, Ed. 1) SANS Electrical apparatus for use in the presence of combustible dust Part 11: Protection by intrinsic safety "id" (IEC :2005, IDT, Ed. 1) SANS Electrical apparatus for use in the presence of combustible dust Part 14: Selection and installation (IEC :2004, IDT, Ed. 1) SANS Electrical apparatus for use in the presence of combustible dust Part 17: Inspection and maintenance of electrical installations in hazardous areas (other than mines) (IEC :2005, IDT, Ed. 1) SANS Electrical apparatus for use in the presence of combustible dust Part 18: Protection by encapsulation "md" (IEC :2004, IDT, Ed. 1) 21

23 QUESTIONS 22