DUST EXPLOSION PROTECTION IN FEED MILLS ACCORDING TO THE EUROPEAN ATEX DIRECTIVES – RISK ASSESSMENT AND NEW FINDINGS Dipl.-Ing. Alexandra Kirchner IFF-Forschungsinstitut.

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