DUST EXPLOSIONS TOPICS

Transcription

1 DUST EXPLOSIONS OPERA meeting, 29 November 2005 Pieter Zeeuwen Technical Manager Process Safety Consultancy Chilworth Technology Ltd Beta House, Chilworth Science Park Southampton SO16 7NS , Chilworth Technology TOPICS Some incidents What is a dust explosion ATEX and DSEAR/EPS Control of dust explosions EUDIR mod Pieter Zeeuwen, Chilworth Technology Ltd 1

2 EXPLOSIONS DO HAPPEN 01-DSEAR DUST EXPLOSION - GRAIN STORAGE In Blaye in France a dust explosion occurred in August 1997 a grain storage plant The plant handled wheat, barley & maize The plant consisted of 44 silos with a storage volume of 47,000 m 3 The explosion occurred with no warning 04a-EXP Pieter Zeeuwen, Chilworth Technology Ltd 2

3 THE DUST EXPLOSION 11 people were killed, including one member of the public who was fishing in the river behind There was no explosion protection, consequently concrete rained down on the control room 28 silos were wrecked 04a-EXP IGNITION SOURCE Report suggests ignition started in a fan of the central dust collection plant Other ignition sources? Once the initial explosion occurred then the flame front provided ignition for other disturbed dust clouds 04a-EXP Pieter Zeeuwen, Chilworth Technology Ltd 3

4 DUST EXPLOSIONS IN DUST COLLECTORS Dust explosion occurred in nearest dust collector Explosion also occurred in the workplace One operator instantly killed and another seriously injured Explosion then propagated into second dust collector 04b-EXP DUST COLLECTORS Dust collectors had no form of explosion protection In-line fans (i.e. on the dirty side) Underground ducts from the production plant 04b-EXP Pieter Zeeuwen, Chilworth Technology Ltd 4

5 OVERALL PLANT DAMAGE Bulging of 1st dust collector Overpressure resulted in cracked and bulging walls of the workplace Fire caused soot, melted electrical fittings etc. 04b-EXP DAMAGE TO 2 ND DUST COLLECTOR Damage to 2 nd dust collector much worse than 1 st Base completely blown off Inlet elbow on 2 nd dust collector was found 50 m away in a garden 04b-EXP Pieter Zeeuwen, Chilworth Technology Ltd 5

6 DAMAGE TO 2 ND DUST COLLECTOR 04b-EXP EXPLOSION before. after. Maize dust explosion in Metz (France) Explosions do not have to be devastating to harm people 04-DSEAR Pieter Zeeuwen, Chilworth Technology Ltd 6

7 WHAT IS A DUST EXPLOSION? 2005, Chilworth Technology EXPLOSION CONDITIONS Dust explosions are comparable to gas explosions as long as dust is fine Gas, vapour or dust needs to be in right concentration range between LEL (or MEC) and UEL Ignition source or auto-ignition conditions need to be present 02-EXP Pieter Zeeuwen, Chilworth Technology Ltd 7

8 FIRE OR EXPLOSION? In a fire the fuel and oxidant are separate In an explosion fuel and oxidant are mixed 02-EXP EXPLOSION IN CLOSED VESSEL Combustion creates hot gases P max Pressure In closed vessel the pressure will increase Ideal gas law: PV/nT = constant As n changes little, expansion ratio is almost equal to temperature ratio dp [ dt ] max Time idealised pressure - time history of an explosion in a closed vessel 02-EXP Pieter Zeeuwen, Chilworth Technology Ltd 8

9 SECONDARY DUST EXPLOSIONS 1mm Layer of Dust Bulk Density of 500 kg/m 3 One of the main hazards associated with handling dusts is the potential for devastating secondary dust explosions 5m 100 g/m 3 02-EXP TYPES OF EXPLOSIONS Deflagration or explosive burning slow reaction starting from an ignition point Detonation fast (supersonic) reaction starting from an ignition point Thermal explosion run away caused by global heating no defined reaction zone Physical explosions bursting vessel, BLEVE, etc. 02-EXP Pieter Zeeuwen, Chilworth Technology Ltd 9

10 CONDITIONS FOR EXPLOSION Oxidant Mixing Confinement Fuel Ignition source 02-EXP FLAMMABILITY OF DUSTS Can only be measured Test usually based on vertical tube In UK: Group A = flammable Group B = non-flammable for T > 110 C furnace test needed before a Group B can be confirmed Group B powders may burn or be thermally unstable 03-EXP Pieter Zeeuwen, Chilworth Technology Ltd 10

11 PARTICLE SIZE Larger surface area leads to greater explosion violence and ignition sensitivity 1 cubic cm = 6 cm square 5 mm squares = 12 cm square Relationship is more complex, e.g. particle shape surface structure Predictions are impossible: testing is only option 03-EXP FLAMMABLE DUSTS Agricultural corn, milk powder, sugar Carbonaceous coal, peat, activated charcoal Chemical adipic acid, sulfur, anthraquinone Pharmaceutical aspirin, paracetamol, ibuprofen Metal aluminium, iron, zinc 30% Flammable Non-flammable 70% 03-EXP Pieter Zeeuwen, Chilworth Technology Ltd 11

12 MINIMUM EXPLOSIBLE CONCENTRATION Lower explosion limit (LEL) for dusts usually gm -3 visually dense cloud measured in 20 litre sphere 25W 2 m UEL not measurable or useful several kg m g/m³ Coal Dust 03-EXP HYBRID MIXTURE Gas, vapour and dust in any combination Can be flammable even if both components below LEL Usually a flammable dust cloud with gas/vapour below the LEL MIE of dust strongly reduced severity becomes that of turbulent gas air mixture 03-EXP Pieter Zeeuwen, Chilworth Technology Ltd 12

13 LIMITING OXYGEN CONCENTRATION Reducing the oxygen concentration narrows the explosion range The LOC is the O 2 concentration below which no explosion can occur The LOC is found near the LEL The LOC depends on the inert gas used 03-EXP IGNITION SOURCES IN DUST EXPLOSION INCIDENTS Legend: A = mechanical friction/heating B = smouldering C = static discharges D = fire, flames E = self heating F = hot surfaces G = hot work H = electrical equipment I = unknown, not determined J = other % ABCDEFGH I J Ref: BIA Report 11/97: 600 incidents 01-EXP Pieter Zeeuwen, Chilworth Technology Ltd 13

14 MINIMUM IGNITION ENERGY MIE is smallest electrical spark that can ignite the optimum mixture of fuel and air Spark is generated using a capacitor discharge Spark details are important for dusts two methods for electrostatics pure capacitive discharge is relevant 03-EXP MINIMUM IGNITION TEMPERATURE MIT is AIT for dusts Sets maximum surface temperature Needs to be considered alongside LIT where appropriate Godbert-Greenwald furnace is internationally standardised 03-EXP Pieter Zeeuwen, Chilworth Technology Ltd 14

15 LAYER IGNITION TEMPERATURE Normally determined for 5 mm thick layer of dust Really a thermal stability parameter, but used with MIT Usually lower than MIT Depends on the layer thickness 03-EXP EXPLOSION IN CLOSED VESSEL Combustion creates hot gases that expand, or in a closed vessel the pressure will increase P max Pressure P max is volume independent, but the rate of pressure rise is volume dependent dp [ dt ] max dp/dt is normalised to1 m 3 volume using the cubic law : K= (dp/dt) max. V 1/3 (K st for dust, K g for gas/vapour) Time idealised pressure - time history of an explosion in a closed vessel 07-DSEAR Pieter Zeeuwen, Chilworth Technology Ltd 15

16 EXPLOSION SEVERITY 20 Litre sphere is standard test vessel for dusts, compatible with the 1 m 3 vessel Turbulence level simulates severe plant conditions For gases tests can be carried out in smaller vessels (> 5 litre), using quiescent mixtures 03-EXP THE LAW ATEX CAD DSEAR EPS 2005, Chilworth Technology Pieter Zeeuwen, Chilworth Technology Ltd 16

17 ATEX 100a (Directive 94/9/EC) On equipment and protective systems intended for use in potentially explosive atmospheres Official Journal of the European Communities, L100, Volume 37, 19 April 1994 Corrigendum published in OJEC L21, 26 January 2000 In Great Britain the ATEX 100a Directive has been adopted as The Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations, 1996 EUDIR ATEX 137 (Directive 1999/92/EC) formerly referred to as ATEX 118a On minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres 15th individual Directive within the meaning of Art. 16(1) of Directive 89/391/EEC Official Journal, L23, (corrigendum L134, ) EUDIR Pieter Zeeuwen, Chilworth Technology Ltd 17

18 CHEMICAL AGENTS DIRECTIVE CAD (Directive 98/24/EC) On the protection of the safety and health of workers from the risks related to chemical agents at work 14th individual Directive within the meaning of Art. 16(1) of Directive 89/391/EEC Official Journal, L131, , p EUDIR DANGEROUS SUBSTANCES & EXPLOSIVE ATMOSPHERES REGULATIONS 2002 (DSEAR) Health CAD Safety CAD & ATEX COSHH Amendments New Safety Regs DSEAR Asbestos Lead Repeal old legislation Dangerous substance is used in lieu of hazardous chemical agent because the latter is not defined in British legislation EUDIR Pieter Zeeuwen, Chilworth Technology Ltd 18

19 EXPLOSIVE ATMOSPHERES In the ATEX Directives, the following definition is given: Mixture with air, under atmospheric conditions, of flammable substances in the form of gases, vapours, mists or dusts in which, after an ignition has occurred, combustion spreads to the entire unburned mixture ATEX 100a, article 1.3 ATEX 137, article 2 EUDIR CONTROL 2005, Chilworth Technology Pieter Zeeuwen, Chilworth Technology Ltd 19

20 BASIS OF SAFETY Inherent safety Explosion prevention Equipment Material properties Acceptable residual risk Human factors Explosion protection Procedures Deviations 05-EXP FAMOUS LAST WORDS We only do it once We have done it like this for 20 years It worked perfectly fine at a smaller scale opt02-exp Pieter Zeeuwen, Chilworth Technology Ltd 20

21 EXPLOSION PREVENTION flammable atmosphere ignition source flash fire or explosion 05-EXP EXPLOSION PREVENTION Avoid the fuel is it the product? attrition can be problem with dust Work outside flammable range difficult with dusts (settling) Eliminate all ignition sources ignition sensitivity data needed Remove oxygen inerting >requires LOC data work under vacuum 05-EXP Pieter Zeeuwen, Chilworth Technology Ltd 21

22 AVOID THE FUEL FOR DUSTS ONLY: Use large particle size >0.5 mm diameter will not form dust clouds watch out for fines Keep material damp water or high flash point liquid will not form dust clouds Add an inert material prevents ignition large amount are needed limited use 05-EXP REMOVE THE FUEL 05-EXP Pieter Zeeuwen, Chilworth Technology Ltd 22

23 REMOVE THE OXIDANT Air is 21 % oxygen Reduce the oxygen concentration to below the LOC Safety factors must be applied Two main procedures pressure swing inerting flow through inerting 05-EXP AREA CLASSIFICATION PROCEDURE Get the flammability data Get the drawings & procedures Find sources of release Identify the duration determine grade of release Consider ventilation Assign zone numbers Estimate zone size Report! MOST OF THE TIME CONTINUOUS (ZONE 0) WHERE CAN IT GET OUT? HOW OFTEN? A LOT OF THE TIME PRIMARY (ZONE 1) RARELY SECONDARY (ZONE 2) 05-EXP Pieter Zeeuwen, Chilworth Technology Ltd 23

24 HAZARDOUS AREA DEFINITIONS Zone 0 (Zone 20) A place in which an explosive atmosphere of flammable gas (dust) in air is present continuously or for long periods or frequently Zone 1 (Zone 21) An explosive atmosphere of gas (dust) in air is likely to occur in normal operation occasionally Zone 2 (Zone 22) An explosive atmosphere of gas (dust) in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only Note : gas includes vapour and mist 05-EXP HAZARDOUS AREA SIGN 05-HAC A Pieter Zeeuwen, Chilworth Technology Ltd 24

25 EQUIPMENT SELECTION (DSEAR, Regulation 7(2) and Schedule 3) Unless, the risk assessment finds otherwise, equipment and protective systems must be selected as follows: Zone 0 and 20 >category 1 Zone 1 and 21 >category 1 or 2 Zone 2 or 22 >category 1, 2 or 3 Equipment must be suitable for gases, vapours or mists and/or dusts as appropriate 03-DSEAR ATEX EQUIPMENT MARKING CE conformity mark... II 1 G number of ExNB involved in production phase specific marking of explosion protection II equipment group II 1 equipment category of conformity 1 (1, 2 or 3) G suitable for gas (G = gas, D = dust, GD = gas and dust) EUDIR Pieter Zeeuwen, Chilworth Technology Ltd 25

26 EQUIPMENT UNDER ATEX 100a Article 1.3 Equipment means machines, apparatus, fixed or mobile devices, control components and instrumentation thereof and detection or prevention systems which, separately or jointly, are intended for the generation, transfer, storage, measurement, control and conversion of energy and/or the processing of material and which are capable of causing an explosion through their own potential sources of ignition EUDIR NOT EQUIPMENT UNDER ATEX If it has no ignition source, it is not equipment covered by ATEX 100a: drums FIBCs pipes hoses ladders hammers screwdrivers etc. MEIRA Pieter Zeeuwen, Chilworth Technology Ltd 26

27 CENTRIFUGAL PUMP Electrical sparks Coupling rub Guard catching shaft Surface temperature Hot bearing Hot seal / throttle bush MEIRA EFFECTIVE IGNITION SOURCES (EN ) (a) hot surfaces; (b) flames, included smoking and hot work activities; (c) mechanically generated (friction) sparks incl. thermite sparks; (d) electrical apparatus; (e) stray electric currents within installations; (f) static electricity; (g) lightning; (h) radio frequency electromagnetic radiation; (j) visible and similar high frequency electromagnetic radiation; (k) ionising radiation; (l) ultrasonic sound waves; (m) adiabatic compression and shock waves; and (n) exothermic reactions, spontaneous combustion MEIRA Pieter Zeeuwen, Chilworth Technology Ltd 27

28 POTENTIAL IGNITION SOURCES Hot work Naked flames These ignition sources are not intrinsic to the process and must be controlled by Company Permit to Work and / or Safety Certificate systems Smoking, matches, lighters etc. Safety management systems are required 05-EXP ELECTRICAL INSTALLATION 05-EXP Pieter Zeeuwen, Chilworth Technology Ltd 28

29 CHARGE GENERATION Most common charging mechanism is contact (or tribo) charging At least one of the materials must be non-conducting (insulating) Other mechanisms charge sharing induction charging (only for conductors) 03-ES ELIMINATION OF IGNITION SOURCES Can all ignition sources be identified? ignition sources that are intrinsic to the process ignition sources are not particular to a process Is the sensitivity to ignition by these sources known for your materials? Have both normal and abnormal conditions been considered? 05-EXP Pieter Zeeuwen, Chilworth Technology Ltd 29

30 EXPLOSION IN A CYCLONE tested using a gas explosion 06-EXP EXPLOSION PROTECTION Explosion containment Explosion relief venting Explosion suppression Needs to be combined with measures to prevent propagation (isolation measures) 06-EXP Pieter Zeeuwen, Chilworth Technology Ltd 30

31 CONTAINMENT All parts of the plant made strong includes pipes, ducts, flanges, covers, etc. Must withstand the maximum pressure that is expected Maintain strength over lifetime Strong plant is expensive to build and can be difficult to operate 06-EXP CONTAINMENT Two methods Explosion pressure resistant >built as pressure vessel Explosion pressure shock resistant >deformation allowed >failure not allowed European Standard (EN 14460) in preparation 06-EXP Pieter Zeeuwen, Chilworth Technology Ltd 31

32 CONTAINMENT Two weak joints in strong equipment 06-EXP EXPLOSION RELIEF VENTING Protects plant and personnel from the effects of an explosion Uses weak panels or doors that open quickly at low pressure Panel has to be large enough Vent design has many pitfalls for the inexperienced 06-EXP Pieter Zeeuwen, Chilworth Technology Ltd 32

33 EXPLOSION VENTING filter bags obscured the vent opening in this wood dust filter in Brazil (courtesy of Richard Siwek) 06-EXP EFFECTS OF VENTING Recoil forces External effects flame blast effects missiles P1 > P2 > P3 Recoil force P3 P2 P1 Provide safe area restrict access consider air intakes etc. Flame length 06-EXP Pieter Zeeuwen, Chilworth Technology Ltd 33

34 FLAME JET AND SECONDARY DUST EXPLOSION HAZARD Dust explosion venting indoors with dust deposits in area 06-EXP EXPLOSION VENTING vent not connected to duct leaking vent panel 06-EXP Pieter Zeeuwen, Chilworth Technology Ltd 34

35 EXPLOSION SUPPRESSION Injects a suppressing agent into the vessel as soon as the explosion is detected The explosion is extinguished in the early stages Pressure is still low at that time No external effects of explosion 06-EXP EXPLOSION SUPPRESSION EXAMPLES fluid bed dryer dust collector 06-EXP Pieter Zeeuwen, Chilworth Technology Ltd 35

36 EXPLOSION ISOLATION Necessary to prevent pressure piling and flame jet ignition When isolation fails, the explosion in secondary vessels can be more severe than the design explosion explosion protection on secondary vessel may be insufficient 06-EXP PROPAGATION OF EXPLOSION Explosion test at DMT in bag filter unit without enclosure Propagation of explosion through extraction duct occurs 06-EXP Pieter Zeeuwen, Chilworth Technology Ltd 36

37 FLAME ARRESTERS Deflagration arresters in-line (A) end-of-line (B1) Flame arrester for endurance burning (B2) Detonation arrester (C) 06-EXP ISOLATION DEVICES rapid action gate valve and extinguishing barrier rapid action Ventex valve 06-EXP Pieter Zeeuwen, Chilworth Technology Ltd 37

38 ISOLATION DEVICES rotary valve and diverter principle diverter industrial example 06-EXP Pieter Zeeuwen, Chilworth Technology Ltd 38