Transcript WB 10.5.05 - FINS

Forschungsinstitut Futtermitteltechnik
Research Institute of Feed Technology
DUST EXPLOSION PROTECTION IN FEED MILLS
ACCORDING TO THE EUROPEAN ATEX DIRECTIVES –
RISK ASSESSMENT AND NEW FINDINGS
Dipl.-Ing. Alexandra Kirchner
IFF-Forschungsinstitut Futtermitteltechnik
Braunschweig-Thune
Cases of damage
Film
Pre-conditions of a dust explosion
high degree of dispersion
of flammable substances
concentration of the flammable
substances in air within their
combined explosions limits
effective ignition source
dust explosion
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
27.1 %
22.9 %
21.4 %
8.6 %
2.9 %
2.9 %
2.9 %
11.4 %
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
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
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
Zoned areas of hazard –
examples for a feed mill
raw material
silos
premix
silos
sieves
mills
conditioner
expander
pellet mill
scale
fat,
molasses,
etc.
cooler
main mixer
roller mill
Zone 20
Zone 21
Zone 22
no Zone
continuous mixer
fat
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.
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
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 dustformation behaviour?
 Do diffuse dusts occur in the explosive concentration
range at the reception area of raw materials?
 ....
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
Test methods used
sample funnel
dust valve
downpipe
dusting unit (drum)
V = 2.5 l
dust reservoir
receiver
laser beam
control unit
measuring and control unit
with drive and vacuum pump
air intake
baffle plates
separator (coarse particles)
V=1l
vacuum pump
100 g sample rapper
air filtration unit
schematic drawing DustView
Single-drop method
Palas DustView
Rotation-drum method
Stauber-Heubach (standard)
Findings on the dusting behaviour –
classification of all results
Klassenhäufigkeit
Staubzahlen
class frequency dust
indexes
1
1
100
3
80
Klassengrenzen
class limits
Worst case?
Very strong dust release?
1
1
60
Strong dust release and significantly increased risk respectively?
2
3
40
4
Increased dust release and increased risk respectively?
36
20
Low to moderate dust release and risk respectively?
0
5
10
15
20
25
30
37
35
Häufigkeitsverteilung
frequency
distribution
dust indexes SFSF
Staubzahlen
dust indexes SRSR
Staubzahlen
http://www.iff-braunschweig.de/staubdb.de
40
Concentration determination of diffuse
dusts – schematic build-up
depth of measuring room x
z
y
I0
I0
I
pixel f (x,y)
camera
I
bulk dropping,
material with extinction coefficient ε
reflexion
wall
lamp
PC
dropping
Dimensions of bulk chute and build-up of
measuring
rolling gate
camera
side-dump truck
side-dump truck
bulk chute
5000
1500
4200
material flow
reflection wall
grid
Measurements at work – concentration profiles
during the dropping process
bulk chute without aspiration, with dust blockers
45
30 s
40
dust concentration [g/m³]
35
UEG = 30 g/m³
30
25
20
15
10
5
droping process
total 38 s )
0
977
978
conc. in MP3
pre-cleaned wheat
moisture: 12.4 %
SF: 2.06, SR: 1.11
979
980
image number
conc. in MP5
981
conc. in MP6
982
conc. in MP7
983
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
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