1. DUST EXPLOSION PROTECTION IN FEED MILLS
Forschungsinstitut Futtermitteltechnik
Research Institute of Feed Technology
DUST EXPLOSION PROTECTION IN FEED MILLS
ACCORDING TO THE EUROPEAN ATEX DIRECTIVES –
RISK ASSESSMENT AND NEW FINDINGS
The following topic is completely different from the ones before. Never the less it is in very important topic for all operators handling grain and feed stuffs and also for all suppliers of electrical and non electrical equipment for grain processing plants. Dust explosion protection in grain processing is nothing new. In Germany the first legislative regulation on explosion protection specifiing requirements for electrical equipment was established in This regulation was mainly related to one of the biggest dust explosion ever in the Roland wheat mill in Bremen, when 14 persons died. So, dust explosion protection is essential for the safety of grain processing plants.
With my presentation I would like to give you a short overview on risks of dust explosions when handling grains, on Europeen legistlative regulations for dust explosion protection, risk assessment and at least I would like to show you some new findings of the research work of our Institute dealing with dusting behaviour and the formation of explosive dust-air atmosphere.
Dipl.-Ing. Alexandra Kirchner IFF-Forschungsinstitut Futtermitteltechnik
Braunschweig-Thune

2. Cases of damage
For feed mills there is a latent danger of dust explosion because grain and feed dusts are combustible. When this dust is dispersed in air, an explosive dust-air-atmosphere can occur depending on the dust-concentration and the oxygen-content, too. An explosive atmosphere can be expected at storing and processing all sorts of grains and seeds and their mealy products or by-products with fine particles smaller than 500 µm.
In the last 10 years there were several cases of damage. Some examples are shown here.
The truck was loaded with pelleted feed, so nobody was expexting an explosion. The cause for the explosion was a unsufficient cleaning of the pneumatic conveying system of the truck. Rests of feed stuff were warmed up by the compressor and than started to smoulder. An ignition of these smouldering gases leads to the explosion.
Other examples are shown here. First one is a grain storage in france. An big explosion occured while filling a silo with corn. The explosion result was 11 dead persons and material damage amount of 1 Million Euro. As possible ignition sources the input of a smouldering spot was expected.
The next case of damage was a fire damage only. The fire was caused by a smouldering spot in the pellet cooler. As safety detection system a temperature controller of the exhaust air was installed, which was to slow to give an early alarm. At the end, the whole building burned down. Nobody was hurt, it was only material damage. Now they have a brand new facility!
The last example is a dust explosion in a sugar plant in America with a total loss of the plant, dead persons and lots of injureds.
Film

3. Pre-conditions of a dust explosion
high degree of dispersion
of flammable substances
dust explosion
concentration of the flammable
substances in air within their
combined explosions limits
effective ignition source
An explosion takes place in a room or a volume for example inside a silo, if three pre-conditions are fullfilled at the same time: .....
Explosion limiting concentration for grain dust is 20 g/m³ up to round about 4 kg/m³

4. Statistics Dust explosions of food and feed powder, damaged equipment:
conveyor %
mills %
silos %
dryer %
combustion plants %
sieves %
dust-extractions systems %
other %
reference: BG Nahrungsmittel und Gaststätten
Some equipment in feed and food processing has a high danger potential of dust explosion, depending on the working principle, the likelihood of ignition sources and the material properties. This is a statistics of equipment damaged by dust explosions from the German Employer's Liability Insurance Association
There is a high explosion risk for conveyor systems especiallay bucket elevators because of high dispersing effect of particles and lots of ignition sources. Ignition sources can occur as mechanical sparks when a bucket breaks of or foreign materials for example a wrenge gets inside the equipment. Foreign materials can also cause hot surfaces when they get jamed.
For mills there is of course a relatively high explosion risks because of fine and dispersed particles and mechanical sparks and hot surfaces (for example the rotor bearing of hammer mills).
Filling grain and feed stuffs in bins or silos a explosive atmosphere occurs over the whole filling period. Typical ignition sources are smouldering spots caused by hot surfaces or material friction and electrical equipment inside the silos like level indicators.

5. producers of equipment
Regulation
standards for
producers of equipment
standards for
plant operator
European regulation
ATEX 100 a (95)
94/9/EC
ATEX 137
1999/92/EC
national regulation
equipment
safety
worker
protection
Because of the latent explosion risk for example at grain processing and lots of cases of damage, the Europeen Commission enforced a standard legislative regulation on explosion protection. For the safety and health protection of employees there were two europeen directives established in the middle of the nineties of the last century dealing with the subject explosion protection related on the one hand to the suppliers of equipment to be used in areas with combustible dust and on the other hand for plant operators and empoyers using equipment in areas with combustible dust.
On the side of machinery and equipment suppliers there is the ATEX 95 that gives concrete attributes and requirements of construction related to the avoidance of ignition sources of electrical and non-electrical equipment.
On the other side there is the ATEX directive 137 setting standards for work safety and hazard assessment. Aim is the workers protection.

6. Assessment of explosion risks
Assessment of explosion risks focuses on
The likelihood that an explosive atmosphere will occur
The likelihood that ignition sources will be present and become effective
First lets see what employers and plant operators ought to do to comply with the ATEX directive: They have to assess the explosion risks in their plant and have to formulate measures for explosion protection. This risk assessment process must always be related to the individual case and circumstances and cannot be generalised.
The specific consideration according to the ATEX directive are
the likelihood and duration of the occurence of a hazardous explosive atmosphere,
the likelihood that ignition sources will be present and become active and effective,
the installations, substances used and processes,
their possible interaction and
the dimension of the expected effects of an possible explosion.
The assessment procedure must be carried out for every work or production process and every operational state and change of state.

7. Classifaction of hazardous areas (1999/92/EC)
Zone 20 a place in which an explosive atmosphere in form of a cloud of combustible dust in air is present continuously, or for long periods
Zone 21 a place in which an explosive atmosphere in form of a cloud of combustible dust in air is likely to occur in normal operation occasionally
Zone 22 a place in which an explosive atmosphere in form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only
As basis for determining the extent of protective measures, any remaining hazardous place must be classified in terms of zones according to the likelihood of occurence of such atmospheres. The classification of hazardous places according to the ATEX directive 1999/92/EC has to be follow these definitions:

8. Zoned areas of hazard – examples for a feed mill
raw material silos
sieves
premix silos
mills
conditioner
expander
fat
pellet mill
scale
cooler
Strains at milling, mixing, sieving and conveying lead to air movements and generation of dust-air mixes.
Depending on the handled materials, their relevant material properties especially the part of fine particles < 500 µm as well as the process and plant technique, the occurrence of explosive dust-air mixes in the interior of production plants as well as in their surrounding can be expected with a different likelihood.
This is an general example of how to classify areas with combustible dust in a feed-mill.
fat, molasses, etc.
main mixer
roller mill
Zone 20
Zone 21
continuous mixer
Zone 22
no Zone

9. Requirements for equipment in hazardous areas (94/9/EC)
In accordance with the three zones, three device categories are defined which are subdivided according to the relevant ignition-source reliability or absence:
Category 1: Ignition sources are avoided by two independent protection measures or safety despite two independent errors also at rare breakdowns
Category 2: Ignition sources are avoided in the normal operation and at frequent breakdowns
Category 3: Anticipated ignition sources are avoided in the normal operation.
These are requirements for electrical and non-electrical equipment.
The requirements of the ATEX-Directive forces suppliers of equipment to be used in hazardous palces to do a risk assessment of their equipment, too, for avoidance of ignition sources. Therefore one must know the various types of ignition sources and the way in which they operate. Overall there are 13 different ignition sources know, of which
·                    electrical equipment,
·                    hot surfaces,
·                    flames and hot gases,
·                    mechanically generated sparks,
·                    stray electrical currents,
·                    static electricity,
·                    lightning,
·                    chemical reactions.
May be available and effective in feed mills and grain processing plants.
The comprehensive hazard analysis, the zoned places as well as the necessary measures for achieving the aims of explosion protection have to be compiled in an explosion-protection document. This document has to be kept up-to-date.

10. Assessement of hazardous areas – state of knowledge
The dust concentrations in the interior of the plant components and in their surrounding, the frequency of explosive dust-air atmosphere and their changing in concentration over the duration are not always known
The current state of knowledge is mainly based on estimations so far
The results are uncertainties in the risk assessment as well as necessary safety equipment
State-of-the-art of science and technology is that knowledge on the dusting tendency can be taken as reference for an improved risk assessment concerning dust explosions
Part of my daily work are safety related inspections and risk assessments of feed mills. Very often, I have to discuss the findings of the risks assessment with employers because the actual dust concentrations in the interior of the plant components and in their surrounding and the frequency of explosive dust-air atmosphere are not always known. For this reason, experienced data and estimations are consulted. This can result in uncertainties in the risk assessment as well as necessary safety equipment. Depending on the ATEX-category of equipment there can be high differences in the prices.
State-of-the-art of science and technology is that knowledge on the dusting tendency can be taken as reference for an improved risk assessment concerning dust explosions when dust concentrations in the interior of the plant components and in their surrounding are not known safetily.
For improving the state of knowledge of explosion hazards at grain and feed plants, the IFF Research Institute initialised a research project.
The target results shall enable the mainly small and medium-sized enterprises (SME) of the grain processing and feed industry to carry out a scientifically assured analysis of dust explosion hazards and to conduct the required zoning on basis of reliable and objective results.

11. Selection of open questions
Coherence between dusting behaviour and the occurrence of explosive atmospheres?
Possibilities of predicting the occurrence of explosive concentrations of diffuse dusts on basis of the dust-formation behaviour?
Do diffuse dusts occur in the explosive concentration range at the reception area of raw materials?
....
Some opent question to be answered within the project.

12. Designation of the dusting behaviour on the laboratorial scale
The requirements basically to be placed on a procedure for characterising the dusting behaviour of bulk materials result from the processes during which dust is set free and dispersed
For the processing of agro raw materials, this are mainly impact and shear stresses due to conveying and transporting
First, we wanted to find out more about the dusting behaviour of raw materials and feed stuffs. For the investigation of the dusting behaviour of bulk materials there are several methods available. To find the right method one has to know the process and the strain during which dust is set free.
For the grain processing, strains are mainly impact and shear stresses due to conveying and transport processes.

13. Stauber-Heubach (standard)
Test methods used
Rotation-drum method
Stauber-Heubach (standard)
Single-drop method
Palas DustView
schematic drawing DustView
receiver
sample funnel
dust valve
downpipe
dust reservoir
laser beam
control unit
For characterising the dust generation by impact stresses, a single-drop process (design Palas DustView) was used in the frame of the research project. The sample (sample size 30 g normally) drops through a downpipe (length 500 mm) into a dust chamber. Lasers and detectors measure the reduction of the initial intensity of the laser (opacity). The measured result is given as dimensionless index from the values 0.5 s and 30 s [Palas].
The rotation-drum method according to Heubach (Heubach Dustmeter) is used for determining the dust generation from powder by rotating under mechanical stress. Here shear stresses are mostly efficient. The material sample (sample size 100 g normally) is added into the rotation drum which rotates for 5 min at a speed of 30 min-1. An air throughput of 20 l/min is necessary. Dispersible dust contents are released by the rotation movement. They are captured by the air flow and deposited on a filter with defined porosity. The amount of the deposited dust is determined gravimetrically. The declaration of the dusting behaviour is given as dimensionless dust index SR which is calculated by subtracting the starting from the ending weight

14. Findings on the dusting behaviour – classification of all results
class frequency dust indexes
Worst case?
Very strong dust release?
class limits
Strong dust release and significantly increased risk respectively?
Increased dust release and increased risk respectively?
Over all there were results from more than 50 different materials. To have an overview a statistical classification of the results can be usefull which is shown here. We made 5 different classes of dusting behaviour and thought about 5 different risk classes for the risk assessment, what has to be checked with further investigations. Most of the materials showed a low or moderate dusting behaviour.
Low to moderate dust release and risk respectively?
frequency distribution
dust indexes SF
dust indexes SR

15. Concentration determination of diffuse dusts – schematic build-up
To find out more about actual dust concentrations especially of diffuse dusts, we developed a a measuring system which can measure disperse diffuse dust concentrations spatially (two-dimensional) and temporally. Base is a photometrical emission measurement. The measured variable is the light attenuation by particles (extinction).
The knowledge that a standard photo consists of 256 grey-value levels was used for evaluating the photo captures. The change of grey-value levels by a dispersed material amount is equivalent to the light attenuation or the opacity respectively. By means of this measured variable and by knowing the material specific extinction coefficients, the concentration of a dispersed material can be determined by the Lambert-Beer law.

16. Dimensions of bulk chute and build-up of measuring
dropping
camera
rolling gate
side-dump truck
side-dump truck
material flow
4200
bulk chute
1500
5000
reflection wall
Measurements on diffuse dust concentrations by use of this system were done at a grain bulk chute for side-dump trucks. The bulk chute had a dust barrier. Dust-separation equipment was not available.
When recording precleaned wheat was delivered..
grid

17. Measurements at work – concentration profiles during the dropping process
bulk chute without aspiration, with dust blockers
The current general risk assessment emanates from a distinct minimisation of the dust releases and explosion hazard if grain is available in precleaned form. The concentration profiles illustrate that during the dropping process there are both measuring points with explosive dust concentrations that are above the lower explosion limit, i.e. > 30 g/m³, and measuring points with dust concentrations that are below the lower explosion limit. The lower explosion limit is expected to be exceeded permanently in areas above the grid at intervals of up to approx. 1.5 m during the duration of the considered dropping process.
This result is contradictory to the current hazard assessment when handling cleaned or precleaned grain as well as recommendations for the classification of zones in the raw-material receipt of the grain and feed industry and to the findings of the dusting behaviour at laboratory scale. This shows a further need of research for the assessment of explosion risks.
pre-cleaned wheat
moisture: 12.4 %
SF: 2.06, SR: 1.11

18. Research findings
The measured results show that explosive dust concentrations can occur in the area above the grid at intervals of up to approx. 1.5 m
The present risk assessment when handling cleaned and pre-cleaned grain respectively as well as recommendations for the classification to zones in reception area for raw materials have to be revised

19. Research findings (2)
The evaluation of the measured results by means of the dust indexes shows for the considered application that this is alone not sufficient for assessing the frequency of the occurrence of explosive atmosphere
Besides machine and process parameters
amounts of bulk materials
dropping height and
the involved dispersion effect by displaced air masses
have to be considered