DUST EXPLOSION PROTECTION IN FEED MILLS DUST EXPLOSION PROTECTION IN FEED MILLS ACCORDING TO THE EUROPEAN ATEX DIRECTIVES –ACCORDING TO THE EUROPEAN ATEX DIRECTIVES –
RISK ASSESSMENT AND NEW FINDINGSRISK ASSESSMENT AND NEW FINDINGS
Dipl.-Ing. Alexandra Kirchner IFF-Forschungsinstitut
Futtermitteltechnik
Braunschweig-Thune
Forschungsinstitut FuttermitteltechnikResearch Institute of Feed Technology
Cases of damageCases of damage
Film
Pre-conditions of a dust explosionPre-conditions of a dust explosion
high degree of dispersion of flammable substances
concentration of the flammablesubstances in air within their combined explosions limits
effective ignition source
dust explosiondust explosion
StatisticsStatistics
Dust explosions of food and feed powder, damaged equipment:
conveyor 27.1 % mills 22.9 % silos 21.4 % dryer 8.6 % combustion plants 2.9 % sieves 2.9 % dust-extractions systems 2.9 % other 11.4 %
reference: BG Nahrungsmittel und Gaststätten
RegulationRegulation
standards for producers of equipment
standards for producers of equipment
standards forplant operator
standards forplant operator
ATEX 100 a (95)94/9/EC
ATEX 100 a (95)94/9/EC
ATEX 1371999/92/EC
ATEX 1371999/92/EC
equipmentsafety
equipmentsafety
worker protection
worker protection
European regulation
national regulation
Assessment of explosion risksAssessment 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
Classifaction of hazardous areas Classifaction of hazardous areas (1999/92/EC)(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
Zoned areas of hazard – Zoned areas of hazard – examples for a feed millexamples for a feed mill
fat, molasses, etc.
premixsilos
raw material silos
sieves
mills
main mixer
continuous mixer
conditioner
cooler
roller mill
fat
pellet mill
expander
scale
Zone 20
Zone 21
no Zone
Zone 22
Requirements for equipment Requirements for equipment in hazardous areas (94/9/EC)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.
Assessement of hazardous areas – Assessement of hazardous areas – state of knowledge 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
Selection of open questionsSelection 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?
....
Designation of the dusting behaviour Designation of the dusting behaviour on the laboratorial scaleon 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
Test methods usedTest methods used
Rotation-drum methodStauber-Heubach (standard)
measuring and control unitwith drive and vacuum pump
baffle plates
dusting unit (drum)V = 2.5 l
air intake
100 g sample rapper
separator (coarse particles)V = 1 l
air filtration unit
vacuum pump
Single-drop methodPalas DustView
schematic drawing DustView
receiver
sample funnel
dust valve
downpipe
dust reservoir
laser beam
control unit
Klassenhäufigkeit Staubzahlen
37
4
2
1
1
36
3
1
3
1
0 5 10 15 20 25 30 35 40
20
40
60
80
100
Kla
ssen
gren
zen
Häufigkeitsverteilung
Staubzahlen SF Staubzahlen SR
Findings on the dusting behaviour – Findings on the dusting behaviour – classification of all resultsclassification of all results
Low to moderate dust release and risk respectively?
Increased dust release and increased risk respectively?
Strong dust release and significantly increased risk respectively?
Very strong dust release?
Worst case?
http://www.iff-braunschweig.de/staubdb.de
class frequency dust indexes
dust indexes SRdust indexes SF
frequency distribution
Concentration determination of Concentration determination of diffuse dusts – schematic build-updiffuse dusts – schematic build-up
reflexionwall
camera
lamp
bulk dropping,material with extinction coefficient ε
PC
depth of measuring room x
y
z
I0 I
I0 I
pixel f (x,y)
rolling gate
side-dump truck
bulk chute
15
00
5000
camera
42
00
side-dump truck
reflection wall
Dimensions of bulk chute and build-Dimensions of bulk chute and build-up of measuring up of measuring
grid
material flow
Measurements at work – concentration Measurements at work – concentration profiles during the dropping processprofiles during the dropping process
pre-cleaned wheatmoisture: 12.4 %SF: 2.06, SR: 1.11
bulk chute without aspiration, with dust blockers
0
5
10
15
20
25
30
35
40
45
977 978 979 980 981 982 983
dust
con
cent
ratio
n [g
/m³]
image number
conc. in MP3 conc. in MP5 conc. in MP6 conc. in MP7
droping process
UEG = 30 g/m³
30 s
total 38 s)
Research findingsResearch 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
Research findings (2)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