explosion protection in the grain industry

Explosion protection in the grain industry

S.Radandt

Dust ExplosionStatistics

0

20

40

60

80

100

120

140

Events (ten year total)

TotalGrain ElevatorFeed MillFlour MillCorn MillingRice Mill

Dust-ExplosionStatistics

0

10

20

30

40

50

60

70

Commodity(Total Explosions)

CornWheatSoybeansCorn StarchRiceBarley(malted)Wheat FlourMixed Feed


02468

101214161820

Amount of events

Silo/BinDust ExhaustingMilling/CrushingConveyingDryingHeatingMixingGrinding/PolishingSieving/Screening


05

1015202530354045

Amount of most affected plants in dustgroups

Wood(Silos)Paper(Mills)Coal(Silos)Food/Feed(Conveyors)Plastics(Mixers)Metal(Exhausters)

Grain elevator after dust explosion

Destroyed silo bins after dust explosion in a maltery

Schadensbild nach Malzstaubexplosion

Maintaining in a maltery

Zerborstene Silozellen

Beginn der Zündung durch Trennschleifen an Rohrleitung

Explosionsablauf

Explosion in Grain-Elevator

Silos after Dustexplosion

Generic Fault Tree of Accident Causation

Damage/Injuries

and

Explosion Failed mitigation

and

Ignition Failed suppression

and

Explosive athmosphereEther pre-existing or produced Ignition source

or

Normal operation Malfunction/rare malfunction

or

Normal operation Abnormal operation

Function/task/intended use

•Phases of equipment life•Limits of equipment•Operating process

•Operating state•Energy

•Chemical, physical conditions

Hazard identification/Estimation of risk

Risk evaluationList of techniques

Technical aspects andInfluences

•Selection of material•Combustion properties

•Presence of ignition sources•Undesired events,

malfunctions•Releases

•Investigation processparameters

Human factors and organisational aspects

•Physiological/psychologicalcapacity

Interaction between persons,operation units, products

and processesTraining, communication

Risk reduction options•Redundancy•Efficiency•Availability•Reliability

Explosion Event Influencing FactorsProperties of explosive mixture

Dust, Gas/vapour, Oxygen { concentrationInitial temperature and pressure

TurbulenceFlame-extinguishing constituents

EXPLOSION EVENT

DimensionGeometry

Surface quality Obstacles

Pressure relief openings

Characteristics of explosion room

Ignition parameters

LocationPressure, shock wave

TemperaturePower density

Energy

Kind of hazardous situation

Deflagration

Detonation

E l e v a t o r

Drag chain conveyor

Destoner

Scale

Pre-bin

Bunker

= Zone 20

= Zone 21

= Zone 22

Roller mill

Mashing in

Drag chain conveyorScrew

Aspiration

E l e v a t o r

The materials processed in the plant are malt and special malt as well as a certain portion of fine dust which is generated by abrasion. The dust amounts to approx. 0.5%.

Lower explos. limit [g/m3]

Max. explosion overpressure [bar]

kst-value [bar • m • s-1]

Ignition temperature (BAM oven) [°C]

Minimum ignition energy [mJ]

Smoldering temperature [°C]

Combustibility (< 250 µm particles) [combustion number]

Malt 60 8.8 110 380 > 10 310 2Crushed malt

60 9.0 110 380 > 10 310 2

Special crushed malt, barley (dark)

60 8.1 120 370 > 5 300 3

Applicable safety-relevant characteristics in a maltery

Occurrence of explosive dust/air mixtures inside the plantProject: Malt receiving, storage and grinding plant

No. Part of plant Zone Notes

1 Dump 21 Under the dump grate; uncleaned malt with high dust portion. Dump encased.

2 Receiving conveyor 21 Following visual inspection, dust deposits are present in this area as a rule. Swirling of dust is expected occasionally during start and stop procedures.

3 Receiving screw conveyor 22 Following visual inspection, dust deposits are present in this area as a rule. But swirling of dust is not likely to occour.

4 Elevator 21 Regular visual inspections show considerable dust deposits which may create dangerous explosive atmospheres both during idle operation and during start and stop procedures. This elevator in the plant undergoes occasional start and stop procedures.

5 Chain conveyor silo floor 21 Following visual inspection, dust deposits are present in this area as a rule. Swirling of dust is expected occasionally during start and stop procedures.

6 Malt silos 21 Filling via downpipes without much dust swirling. Formation of dust clouds (dangerous explosive atmosphere) - does not occur most of the time.

7 Chain conveyor 1 silo basement 21 Following visual inspection, dust deposits are present in this area as a rule. Swirling of dust is expected occasionally during start and stop procedures.

8 Grinding plant elevator 21 Regular visual inspections show considerable dust deposits which may create dangerous explosive atmospheres both during idle operation and during start and stop procedures. Swirling of dust is expected occasionally during start and stop procedures.

9 Destoner 21 Classification into zones according to "Guideline for the preparation of an explosion protection document for grain processing, storage and trade companies".

10 Scale 22 Product is cleaned by upstream sieve. Very small portion of fine dust. Nevertheless, formation of dust clouds cannot be ruled out with certainty.

11 Pre-bin 22 Product is cleaned by upstream sieve. Very small portion of fine dust. Nevertheless, formation of dust clouds cannot be ruled out with certainty.

12 Roller mill 20 Explosive atmosphere has to be expected in the long run.

13 Grist case 20 Due to direct connection between roller mill and grist case, explosive atmosphere is present in the long term.

14 Grist screw conveyor 21 See "receiving screw conveyor"

15 Aspiration pipes 21 Dropping below LEL is not certain under flow-technical aspects; it may be exceeded occasionally.

16 Filter (raw gas) 21 Very long cleaning cycles

17 Filter (pure gas) none No dust deposits present on pure gas side (daily visual inspection).

Identification of the ignition sources

The measures for prevention of explosive atmospheres described above are accompanied by the avoidance of effective ignition sources.

The following ignition sources listed in EN 1127-1 can become effective in the present plant:

Hot surfaces;Flames and hot gases;Mechanically produced sparks;Electrical installations;Static electricity;Lightning strike, andAuto-ignition.

Explosion hazard assessment

Analysis of ignition sourcesProject: Malt receiving area and grinding plant

No

L Risk/failure Cause Effect Countermeasures

1 F Ignition of the malt dust cloud in the dump

Hot surfaces on the truck (exhaust pipe, catalytic converter, motor parts)

Explosion hazard in the area of the dump, danger to persons, transmission to subsequent plant parts

Truck should cool down for at least 15 min prior to driving into the dump hall, place respective signs.

2 E Ignition of the malt dust cloud in the dump

Lumps of smoldering material in the supplied malt

Explosion hazard in the area of the dump, danger to persons, transmission to subsequent plant parts

1. Corresponding provisions in the supplier contracts2. Inspection of incoming goods

3 D Occurrence of hot surfaces with the hazard of fire development/smoldering product

General bearing damage, resulting in overheating or axle displacement

Possible ignition of the product. Transmission of fire into subsequent plant parts, then explosion hazard with unforeseeable consequences

1. Maintenance and repair in accordance with respective schedules2. Only outside bearings(3. Monitoring of bearing temperature)

4 E Product is ignited by pressure and friction

Blockage in a drag-chain conveyor

Development of fire with explosion hazard in the subsequent plant part

1. Blockage sensor with source-side locking2. Low setting of motor protection trigger

L: Probability of occurrenceA frequently B sometimes C occasionallyD seldom E very seldom F almost impossible

No L Risk/failure Cause Effect Countermeasures

5 D Product is ignited by pressure and friction

Blockage in a screw conveyor

Development of fire with explosion hazard in the subsequent plant parts

1. Blockage sensor with source-side locking2. Low setting of motor protection trigger

6 D Product is ignited by hot surface

Screw helix drags Development of fire with explosion hazard in the subsequent plant parts

1. Use slow running screws (r< 1m/s)2. Low setting of motor protection trigger3. Provide sufficient space between wall and helix4. No inner bearings in case of long screws

7 D Product is ignited in screw due to hot surface

Tramp matter wedged in conveyor

Development of fire with explosion hazard in the subsequent plant parts

1. Low setting of motor protection trigger2. Upstream tramp matter extraction3. Use slow running screws(r< 1m/s)

8 E Explosion in elevator due to hot surface

Slipping belt drags on disk

Destruction of elevator possible (tearing at different levels), explosion transmission in buildings and subsequent plant parts possible (see no. 13, 14)

1. Slipping monitoring2. Running off-track monitoring3. Cambered disks4. Pressure venting through roof


9 D Explosion in elevator due to hot surface

Belt running off track, buckets dragging along (metal) wall


1. Slipping monitoring2. Running off-track monitoring3. Cambered disks4. Pressure venting through roof


Belt become loose, buckets dragging along (metal) wall


1. Running off-track monitoring2. Cambered disks(3. Pressure venting through roof)4. Suitable material selection5. Maintenance/repair


Wedged tramp matter


1. Running off-track monitoring2. Upstream tramp matter extraction(magnet and safety grid)3. Pressure venting through roof


Elevator bearing defect, axle displacement and buckets dragging along the wall


1. Slipping monitoring2. Running off-track monitoring3. Cambered disks(4. Pressure venting through roof)

13 E Explosion in elevator Different causes Tearing of the elevator and explosion propagation to extremely dusty process areas with subsequent explosion in the room, static of building in danger

1. Measures to avoid ignition sources, see above2. Cleaning of room (external firm)3. Constructional measures (reinforcement of elevator housing, pressure venting through the roof)4. Constructional measures (reinforcement of the elevator base)

14 E Explosion in elevator Different causes Explosion propagation to the adjoining silos, static of building in danger

1. Measures to avoid ignition sources, see above2. Explosion decoupling (rotary valve with product availability)


No

L Risk/failure Cause Effect Countermeasures

15 E Fire/explosion in one or several silosIntroduction of lumps of smoldering material

In case of explosion affecting the silo, the static of the building is always endangered; in case of fire in the silo, always explosion hazard due to decomposition gases or the breaking of product bridges

1. Measures to avoid ignition sources, see above2. Corresponding provisions in the supplier contracts3. Inspection of incoming goods4. Prohibition of open flames, work involving fire only with clearance, no smoking

16 F Fire and subsequent explosion in one or several silos

Auto-ignition In case of explosion affecting the silo, the static of the building is always endangered; in case of fire in the silo, always explosion hazard due to decomposition gases or the brea-king of product bridges

Avoid introduction of water into silos (rearrangement according to work instruction)

17 F Explosion in one or several silos

Electrostatic discharge

In case of explosion affecting silo, static of building always in danger

Grounding of all plant parts, when non-conductive material is used (e.g. plastic silos), consult specialist


18 E Ignition of malt dust/air mixture in the sieve shaker or in the installation location

Mechanical damage in or on sieve shaker at dust concentration > LEL

Damage or destruction of sieve shaker, possibly explosion transmission into installation locations or subsequent plant parts

1. Maintenance and repair2. Provide conditions so that LEL is not achieved3. Cleaning of room4. Secure start-up bridging


No L Risk/failure Cause Effect Countermeasures

19 D Ignition of a malt dust/air mixture in the roller mill

Machine partor introduction of tramp matter which generates sparks or hot surface

Damage to the mill, explosion transmission to upstream and downstream plant parts, explosion transmission to the installation location

1. Tramp matter extraction2. Maintenance/repair (maintenance contract with Messrs. Mill Construction)3. Explosion decoupling of upstream and downstream aggregates (product availability)4. Pressure venting5. Cleaning of room

20 E Ignition of a malt dust/air mixture in the roller mill

Hot surface due to bearing damage

Damage to the mill, explosion transmission to upstream and downstream plant parts, explosion transmission to the installation location

1. Monitoring of bearing temperature2. Maintenance/repair3. Explosion decoupling of upstream and downstream aggregates4. Pressure venting5. Cleaning of room

21 D Ignition of a malt dust/air mixture in the filter

Electrostatic discharge Damage or destruction of the filter, possibly explosion transmission to the aspirated aggregates or the installation location

1. Permanent and secure grounding of the filter and all built-in parts2. Pressure venting of filter3. Upstream bursting protection4. Cleaning of room


22 E Ignition of a malt dust/air mixture in the filter

Flame jet ignition into the filter due to explosion flames from the aspiration pipes

Damage or destruction of the filter, possibly explosion transmission into unaffected aspiration pipes or the installation location

1. Pressure venting of filter2. Upstream bursting protection3. Cleaning of room

23 E Ignition of a malt dust/air mixture in the filter

Mechanical damage to the ventilator (bearing, axle displacement, broken axle, startup)

Damage or destruction of the filter, possibly explosion transmission to the aspirated aggregates or the installation location

1. Avoid explosive mixtures in the pure gas area of the filter by means of maintenance2. Maintenance of the ventilator3. Pressure venting of filter4. Cleaning of room5. Nephelometer

24 D Ignition of malt dust or a malt dust/air mixture

Failure in the electrical system

Damage or destruction of plant parts, possibly resulting in fire or secondary explosions

1. Use electrical equipment approved for the respective zone only 2. Maintenance/repair3. Check old equipment


25 D Ignition of malt dust or a malt dust/air mixture

Electrostatic discharge


1. Use of conductive or dischargeable materials only2. Grounding of all plant parts3. Maintenance/repair

26 C Ignition of malt dust or a malt dust/air mixture

"Work involving fire" such as cutting, welding, grinding


1. Organizational measures:Procedures for permission to perform work involving fire; work involving fire should be performed outside of hazardous areas, if possible

Organizational measures

Training of employeesCoordinationVerificationCleaningMaintenancePermits to work

Training of employees

The employees working in places where explosive atmospheres may occur will be provided with sufficient and appropriate training with regard to explosion protection. In the course of this training, the employees will be informed about the explosion hazards present at the workplace and the protective measures implemented.Training will be provided to employees in general:-at the time of employment (prior to commencement of work!),-in case of transfer or modification of the work task,-upon introduction or change of work equipment,-upon introduction of a new technology.

Coordination

Where workers from several undertakings are present at the same workplace, each employer shall be responsible for all matters coming under his control. In addition to this individual responsibility of each employer, the employer responsible for the workplace is in charge of the coordination among each other and has to ensure a safe operation. He has to obtain information about explosion hazards, coordinate protection measures with the persons involved, give instructions and monitor that these are complied with.The coordinator shall have the following qualification with regard to explosion protection:-Specialized knowledge in the field of explosion protection,-Knowledge of safety and health legislation,-Knowledge of the company's organizational structure,-Leadership qualities to implement the required instructions.

VerificationThe following issues must be verified:-The fault-free condition and functioning of the plant.-The fault-free condition and functioning of protective equipment (slip and off-track running control, bursting discs, shutdown of the plant).-The proper and permanent grounding of all plant parts.-The proper category of equipment used in potentially explosive areas (rotary valves, bursting discs, level indicators).-Whether the plant components are operated in accordance with their intended use (see documentation of plant).-The existence of proper documentation.

Cleaning

Intensive cleaning work ensures that the areas surrounding the equipment do not become areas in which explosions may occur. In order to maintain this condition, cleaning is of extraordinary significance.For the cleaning process, ignition source-free industrial vacuum cleaners or wet cleaning methods will be used so that as little dust as possible is swirled. Particular attention will be paid to surfaces which are difficult to see (e.g. located higher) or reach. On the silo floor, horizontal surfaces which are difficult to access and clean are sealed by boarding to prevent fine dust from collecting there.

MaintenanceMaintenance includes inspection and testing as well as servicing and repair. Maintenance work will only be performed by persons with a sufficient qualification. Prior to commencement of maintenance work, all persons involved will be informed, and the work will be cleared by means of a permit to work system. During maintenance work, explosion hazards and unintended startup are avoided by mechanical and electrical separation of equipment and plant parts. If work involving fire is carried out in a vessel, the piping system from which hazardous explosive atmosphere may escape or which is connected to other vessels which might contain such explosive atmosphere, will be separated from the vessel and/or closed.In case of maintenance work involving ignition hazards in areas with potentially explosive atmospheres, appropriate measures will be taken to rule out the occurrence of a hazardous atmosphere. If welding, grinding or cutting work is scheduled in or in the vicinity of areas with potentially explosive atmospheres, the protective measures will also take into account the adjoining rooms. Hot metal or slag particles may cause lumps of smoldering material in dust deposits which may result in an ignition a long time after the work involving fire has been completed.

Permits to workIf work which might lead to an explosion is carried out in areas with potentially explosive atmospheres or in their vicinity, this work will be authorized by the responsible person prior to its commencement. This also applies to work processes overlapping with other work and therefore causing hazards. For these cases, a permit to work system has been introduced. It is put into practice by issuing a permit to work which all persons involved must receive and sign. The following minimum information must be provided on the permit to work:-precise location in the plant where the work is to be performed,-who will carry out the work and who is responsible for this area,-start and anticipated completion of the work,-what protective measures have been taken, and-contact person.

Venting of Silo-Buildings

Vent

Fire Protection of Dust Collectors

explosion protection in the grain industry

Documents

dust deposits

dust amounts

special malt

high dust portion

special crushed malt

explosion overpressure

explosion protection

certain portion of fine