Innovative explosionprotection conceptsfor dedusting plants

Confident and safe with Scheuch!

“With the exception of components, protectivesystems are design units that are intended tohalt incipient explosions immediately or to limitthe area affected by an explosion and that areseparately placed on the market for use asautonomous systems.” (Directive ATEX94/9/EC Article 1)

In detail, these systems are relief devices (e.g.,rupture disks, explosion flaps and Q-pipes),suppression devices and decoupling devices.In the case of decoupling devices, a distinctionis made between devices intended for pres-sure and flame de-coupling (e.g., rotary valves,and quick-closing sluice valves) and devicesfor partial de-coupling. Examples of the latterwould be a device for pressure decoupling(e.g., pressure relief stack) or devices for theprevention of flame penetration (e.g., extin-

guisher barriers).

Decoupling Devices:

Crude gas line — Filter:

Explosion protection measures must be imple-mented using the best available technology.For most applications, the decoupling of thecrude gas line – filter is achieved using a nonreturn flap. The best available technology isdescribed in the publications of the VDMA andprofessional associations. Alternatively, quick-closing sluice valves, pressure relief stacks orextinguisher barriers, or a combination of both,can also be used for this purpose.

Non return flap:

Airflow keeps this flap open during operation.An explosive event in the filter shuts the flap

automatically when the pressure wave occurs.Advantages: Passive system, low cost.

Quick-closing sluice valve:

Sensors positioned on each side of the quick-closing sluice valve recognise an incipient ex-plosion and initiate closure of the valve slide.Closure times range from 50 to 150 ms.Advantages: Pipe is hermetically sealed, pres-sure- and flameproofDisadvantages: Active system, cost intensive,requires longer pipes, limited nominal width,the repulsive force generated by closing of theslide must be absorbed.

Pressure relief stack:

The pressure relief stack uses an explosiondoor or rupture disk to discharge the flamefront and the pressure surge of an explosiondirectly to the outside atmosphere. This pre-vents the much feared “flame jet ignition” withinthe filter. In any case, pressure decouplingtakes place via the pressure relief stack; flamedecoupling is no longer assured for nominalwidths > NW650. In such cases, the flame frontcan be liquidated by installing additional extin-guisher barriers.Advantages of the pressure relief stack:Passive system, works in both directions.Disadvantages of pressure relief stack: Pres-sure lost ranging from 500 to 1000 Pa, flamepenetration cannot be avoided in every appli-cation.

Extinguisher barrier:

A detector system recognises an incipient ex-plosion, whereupon extinguishing agent is in-jected into the pipeline at the proper moment.In active systems, the reaction time of the ex-tinguishing system must always be co-ordinated with the propagation speed of theexplosion in the pipeline.Advantage: No pressure loss.Disadvantage: Active system, consequentlycost-intensive, no decoupling of the pressurewave.

Filter discharge:

Decoupling takes place using a rotary valve.This valve must be certified with respect to itspressure surge strength and flame penetrationsafety. Scheuch’s type zss rotary valves arecertified for nominal widths and pressures upto NW 630 and 1 bar, respectively. Valves withsealing lips (type zsl) are certified for nominalwidths and pressures up to NW 630 and 0.4bar, respectively.Advantage: Combines discharge device andprotection system in a single device, effective

8scheuchemissionen

1/2004

PROVEN EXPLOSION

PROTECTION SYSTEMS

FROM SCHEUCH

Eighty percent (80%) of dust types encountered in industrial dedusting appli-

cations are capable of exploding. If flammable dust is present in dispersed

form and sufficient concentration, and if sufficient oxygen and an ignition source

are present, a dust explosion can occur. If ignition sources cannot be avoided

with one hundred percent certainty — that is, explosions are permitted — ex-

traction plants must feature “explosion protection design”.

This means that the effects of explosive events must be kept to a tolerable

level. There are a variety of options available for accomplishing this: decoupling,

explosion release, and explosion suppression devices, as well as combinations

thereof. Some of these devices fulfil the requirements demanded of so-called

protection systems.

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9 scheuchemissionen

in both directions, the rotary valve has a decou-pling effect with respect to both pressure andflame.

Filter clean gas line with return air system:

Decoupling takes place either by means of anextinguisher barrier or by means of a signifi-cantly less expensive 2 x 90°-deflection devicein combination with a rupture disk (Best avail-able technology, Source: Association of theWood Processing Industry).

Explosion Relief Devices:

Explosion relief of filter cases and contai-ners takes place using certified rupture disksor explosion doors. Dimensioning of the re-quired relief area takes place according to VDI3673 guidelines and the strength of the filtercasing must be known. At Scheuch, this isdetermined through complex pressure tests,the results of which are certified by the TÜV orDMT. The operator must consider the effectsof incipient flames and pressure on externalareas during the relief process. As a rule, therelief process should only be used as a protec-tive measure if dust and its incineration by-products are non-toxic.

If a filter is positioned in an interior space, orif the available relief area is too small for a filterpositioned out of doors, relief can be providedusing a so-called “Q-pipe”. In this case, con-ventional rupture disks are used to relieve theexplosive pressure and the incipient flame frontis extinguished by a quenching process.Advantage: Filter can be positioned in workarea.Disadvantage of Q-pipe: Cost-intensive

Explosion Suppression Devices:

Another alternative when filters and containersare located in interior spaces is so-called ex-plosion suppression. A sensor system recog-nises an incipient explosion based on pressurebehaviour over time. Powder or gaseous extin-

guishing agent is then quickly injected into thecontainer that requires protection and the ex-plosion is stopped. This method works verywell in combination with extinguisher barriersinside the pipes.Advantage: Filter can be located in interiorspaces.Disadvantage: Requires increased casingstrength, active system and therefore cost-intensive.

What happens during a dust explosion in-

side a closed contained (e.g., filter casing)?

Once the mixture of dust and air ignites, pres-sure inside the container increases accordingto the following characteristics:The maximum occurring explosion pressurePmax has an average value of 7 to 10 bar. Thiscontrasts with the container strength, whichranges only from 130 to 420 mbar (typicalstrength for filter casings or silos). As a protec-tive measure, explosion relief prevents the ex-plosive pressure in the filter from exceeding thecontainer strength through the targeted re-lease of pressure using relief openings. Themaximum occurring pressure in the casing isdesignated Pred,max. The limiting of pressureinside the filter can also be achieved througha protective measure called explosion sup-pression. In this measure, the explosion is ex-tinguished in its initial phase.

Description of pressure profile:

The upper curve describes the pressure profileof a dust explosion inside a closed container.The maximum pressure increase along the ris-ing flank of the curve serves as a measure ofthe intensity of the dust explosion. Multiplyingthis value by the cubed root of the containervolume results in the so-called KST value.In explosion suppression, the explosion is

stopped during its initial phase. The maximumoccurring pressure corresponds to the so-called Pred,max value. The container strengthmust be somewhat greater to prevent destruc-tion of the container. The pressure increaseover time behaves in a similar way in the pres-sure relief method.

Customer consulting:

Because of technical and process-related dif-ferences, the selection of the suitable explo-sion protection measures varies for individualbusiness segments.

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Wood Processing Industry Alois Burgstaller, Ext. 196Particle and Fibre Board Industry Gerhard Wiesner, Ext. 173Industrial MineralsChristian Straif, Ext. 261Metals Industry Johann Desch, Ext. 142Powder coating plantsChristian Justl, Ext. 471Explosion protection (general) Helmut Gallhammer, Ext. 240

The following employees

provide consulting

services during concept

development

Offprint from the customer newsletter emissionen 1/2004Offprint from the customer newsletter emissionen 1/2004

“With the exception of components, protectivesystems are design units that are intended tohalt incipient explosions immediately or to limitthe area affected by an explosion and that areseparately placed on the market for use asautonomous systems.” (Directive ATEX94/9/EC Article 1)

In detail, these systems are relief devices (e.g.,rupture disks, explosion flaps and Q-pipes),suppression devices and decoupling devices.In the case of decoupling devices, a distinctionis made between devices intended for pres-sure and flame de-coupling (e.g., rotary valves,and quick-closing sluice valves) and devicesfor partial de-coupling. Examples of the latterwould be a device for pressure decoupling(e.g., pressure relief stack) or devices for theprevention of flame penetration (e.g., extin-

guisher barriers).

Decoupling Devices:

Crude gas line — Filter:

Explosion protection measures must be imple-mented using the best available technology.For most applications, the decoupling of thecrude gas line – filter is achieved using a nonreturn flap. The best available technology isdescribed in the publications of the VDMA andprofessional associations. Alternatively, quick-closing sluice valves, pressure relief stacks orextinguisher barriers, or a combination of both,can also be used for this purpose.

Non return flap:

Airflow keeps this flap open during operation.An explosive event in the filter shuts the flap

automatically when the pressure wave occurs.Advantages: Passive system, low cost.

Quick-closing sluice valve:

Sensors positioned on each side of the quick-closing sluice valve recognise an incipient ex-plosion and initiate closure of the valve slide.Closure times range from 50 to 150 ms.Advantages: Pipe is hermetically sealed, pres-sure- and flameproofDisadvantages: Active system, cost intensive,requires longer pipes, limited nominal width,the repulsive force generated by closing of theslide must be absorbed.

Pressure relief stack:

The pressure relief stack uses an explosiondoor or rupture disk to discharge the flamefront and the pressure surge of an explosiondirectly to the outside atmosphere. This pre-vents the much feared “flame jet ignition” withinthe filter. In any case, pressure decouplingtakes place via the pressure relief stack; flamedecoupling is no longer assured for nominalwidths > NW650. In such cases, the flame frontcan be liquidated by installing additional extin-guisher barriers.Advantages of the pressure relief stack:Passive system, works in both directions.Disadvantages of pressure relief stack: Pres-sure lost ranging from 500 to 1000 Pa, flamepenetration cannot be avoided in every appli-cation.

Extinguisher barrier:

A detector system recognises an incipient ex-plosion, whereupon extinguishing agent is in-jected into the pipeline at the proper moment.In active systems, the reaction time of the ex-tinguishing system must always be co-ordinated with the propagation speed of theexplosion in the pipeline.Advantage: No pressure loss.Disadvantage: Active system, consequentlycost-intensive, no decoupling of the pressurewave.

Filter discharge:

Decoupling takes place using a rotary valve.This valve must be certified with respect to itspressure surge strength and flame penetrationsafety. Scheuch’s type zss rotary valves arecertified for nominal widths and pressures upto NW 630 and 1 bar, respectively. Valves withsealing lips (type zsl) are certified for nominalwidths and pressures up to NW 630 and 0.4bar, respectively.Advantage: Combines discharge device andprotection system in a single device, effective

8scheuchemissionen

1/2004

PROVEN EXPLOSION

PROTECTION SYSTEMS

FROM SCHEUCH

Eighty percent (80%) of dust types encountered in industrial dedusting appli-

cations are capable of exploding. If flammable dust is present in dispersed

form and sufficient concentration, and if sufficient oxygen and an ignition source

are present, a dust explosion can occur. If ignition sources cannot be avoided

with one hundred percent certainty — that is, explosions are permitted — ex-

traction plants must feature “explosion protection design”.

This means that the effects of explosive events must be kept to a tolerable

level. There are a variety of options available for accomplishing this: decoupling,

explosion release, and explosion suppression devices, as well as combinations

thereof. Some of these devices fulfil the requirements demanded of so-called

protection systems.

1

4

3

2

5

9 scheuchemissionen

in both directions, the rotary valve has a decou-pling effect with respect to both pressure andflame.

Filter clean gas line with return air system:

Decoupling takes place either by means of anextinguisher barrier or by means of a signifi-cantly less expensive 2 x 90°-deflection devicein combination with a rupture disk (Best avail-able technology, Source: Association of theWood Processing Industry).

Explosion Relief Devices:

Explosion relief of filter cases and contai-ners takes place using certified rupture disksor explosion doors. Dimensioning of the re-quired relief area takes place according to VDI3673 guidelines and the strength of the filtercasing must be known. At Scheuch, this isdetermined through complex pressure tests,the results of which are certified by the TÜV orDMT. The operator must consider the effectsof incipient flames and pressure on externalareas during the relief process. As a rule, therelief process should only be used as a protec-tive measure if dust and its incineration by-products are non-toxic.

If a filter is positioned in an interior space, orif the available relief area is too small for a filterpositioned out of doors, relief can be providedusing a so-called “Q-pipe”. In this case, con-ventional rupture disks are used to relieve theexplosive pressure and the incipient flame frontis extinguished by a quenching process.Advantage: Filter can be positioned in workarea.Disadvantage of Q-pipe: Cost-intensive

Explosion Suppression Devices:

Another alternative when filters and containersare located in interior spaces is so-called ex-plosion suppression. A sensor system recog-nises an incipient explosion based on pressurebehaviour over time. Powder or gaseous extin-

guishing agent is then quickly injected into thecontainer that requires protection and the ex-plosion is stopped. This method works verywell in combination with extinguisher barriersinside the pipes.Advantage: Filter can be located in interiorspaces.Disadvantage: Requires increased casingstrength, active system and therefore cost-intensive.

What happens during a dust explosion in-

side a closed contained (e.g., filter casing)?

Once the mixture of dust and air ignites, pres-sure inside the container increases accordingto the following characteristics:The maximum occurring explosion pressurePmax has an average value of 7 to 10 bar. Thiscontrasts with the container strength, whichranges only from 130 to 420 mbar (typicalstrength for filter casings or silos). As a protec-tive measure, explosion relief prevents the ex-plosive pressure in the filter from exceeding thecontainer strength through the targeted re-lease of pressure using relief openings. Themaximum occurring pressure in the casing isdesignated Pred,max. The limiting of pressureinside the filter can also be achieved througha protective measure called explosion sup-pression. In this measure, the explosion is ex-tinguished in its initial phase.

Description of pressure profile:

The upper curve describes the pressure profileof a dust explosion inside a closed container.The maximum pressure increase along the ris-ing flank of the curve serves as a measure ofthe intensity of the dust explosion. Multiplyingthis value by the cubed root of the containervolume results in the so-called KST value.In explosion suppression, the explosion is

stopped during its initial phase. The maximumoccurring pressure corresponds to the so-called Pred,max value. The container strengthmust be somewhat greater to prevent destruc-tion of the container. The pressure increaseover time behaves in a similar way in the pres-sure relief method.

Customer consulting:

Because of technical and process-related dif-ferences, the selection of the suitable explo-sion protection measures varies for individualbusiness segments.

7

4

3

2

1

6

7

5

6

Wood Processing Industry Alois Burgstaller, Ext. 196Particle and Fibre Board Industry Gerhard Wiesner, Ext. 173Industrial MineralsChristian Straif, Ext. 261Metals Industry Johann Desch, Ext. 142Powder coating plantsChristian Justl, Ext. 471Explosion protection (general) Helmut Gallhammer, Ext. 240

The following employees

provide consulting

services during concept

development

Offprint from the customer newsletter emissionen 1/2004Offprint from the customer newsletter emissionen 1/2004

8scheuchemissionen

1/2006

9 scheuchemissionen1/2006

Offprint from the customer newsletter emissionen 1/2006Offprint from the customer newsletter emissionen 1/2006

Scheuch determined and compared the crudegas volumes, clean gas volumes, the filter baseopen surface area, the filter surface area, max-imum air volume, the rupture disk surface areaand other factors. In order to carry out testingunder practical conditions, a fan with extrac-tion line and a material feeder with meteringstation were erected. Testing occurred atcrude gas dust loads of 50 g/m3, 80 g/m3, 100g/m3 and 150 g/m3. Dust concentrations in thefilter were determined during filter operationand during the cleaning process. The so-called“test shots” occurred optionally with an emptybunker or with material bunkering.

Prior to actual explosive testing, Scheuch co-operated with the Holz-BG in determining fromthe manufacturer's information the expectedmaximum dust concentration that can occurin practical operation prior to filtration in filtersof the type studied. In addition, we determinedthe distributions of particle sizes in three repre-sentative dust samples taken from wood dustfilters located at plants in the wood processingindustry.

Results / Findings

Decoupling measuresIn all tests with relief on the crude gas sidein combination with a non return flap, 90° de-flection with upward relief and a rotary valve,there was no propagation of flames or pressurefrom the filter into the extraction pipe or fromthe filter discharge and into the clean air duct.Thus, the plant as tested in its entirety - usingScheuch's construction method - is a suitabledevice with integrated decoupling whenoperated in a correspondingly approvedmanner.

Installation position of rupture disks -Flame rangeThe series of practical tests confirmed the ex-pulsion of unburned dust from the openedpressure relief outlets. There is a danger of se-condary fires and secondary explosions. Therange and intensity of the dust expulsion de-pends on the explosion pressure pred of theevent and the reaction pressure of the reliefdevice.

Previous assumptions, whereby rupture diskspositioned as high as possible, with installationheights starting at 8 meters, were thought toeliminate the danger near ground level, couldnot be confirmed based on the dust dischargeand subsequent ignition.

With respect to the range of flames outside ourfiltration plant, the operator can consider forhis risk assessment either the previous calcu-lations according to VDI or alternately our ownflame profile, which is the result of many prac-tical tests.

In summary, it can be said that in the case ofmany of the “test shots”, the flame range wassubstantially less than expected and that thepressure development was not so intense asassumed. Despite our new reduced flame pro-file, filter locations are still often problematicalwith respect to the range of flames in an explo-sion. This is especially true when roadways orproperty borders are located within the flameprofile. For this reason, we tested new optionsfor pressure relief during these tests and willsoon be able to offer customers who requirethem filter plants that exhibit even smaller flameprofiles with our new protective system.

Lower explosion limit (German: UEG) forWood DustBecause the first series of tests with crude gasdust loads of 50 g/m3 was unable to produceignition despite ignition energies as high as10,000 joules, we examined the lower explo-sion limit (UEG) more closely. It was found to

be approximately 60 g/m3 as measured withthe addition of the most unfavourable dust type(original) in a container with a capacity of 1 m3.A UEG of 30 g/m3, as mentioned in the perti-nent literature, could only be achieved by re-screening the dust (impossible in actual prac-tice). Accordingly, a UEG of 30 g/m3 alreadyincludes a safety margin of 50%. With respectto the best available technology, referenceshould be made here to the VexAT, the Austrianimplementation of the operator version of ATEXRL 1999/9 EG, and its 50% shortfall of theUEG. A twofold safety margin of 50% is un-necessary. Only the dust content is relevantwhen considering an explosive atmosphere.Additional coarse material such as chips orshavings reduces the ignition propensity.

The probability of an eventDespite an unfavourable dust concentrationand a correspondingly large ignition source, adust explosion was not always possible. A cor-responding concentration of dust is necessarybecause of turbulence within the filter.Filter fires and a slight pressure increase onlyoccurred when crude gas dust loadingreached levels of 80 - 100 g/m3. An explosiveeffect according to VDI calculations is ex-tremely unlikely, but not impossible.

Bag bottom and filter bags as pressurebarriersA smaller pressure increase was measured inthe clean gas region of the filter than in thecrude gas chamber. This led to the realisationthat compared with the crude gas volume, thefree surface area of the bag bottom plays akey role in determining the value of pred in theclean gas chamber.

Conclusion

We were able to prove that Scheuch's IMPULSfilter provides the wood processing and woodbased panel industries with a complete systemthat has now been certified by a recognisedGerman testing institute.The know-how acquired during the tests willbe used for the benefit of our customers withrespect to design, consultation and support inconnection with explosion zones, the avoid-ance of ignition sources, design-engineeringof explosion protection systems, filter location,decoupling measures, flame range, etc. In thiscase - explosion protection and fire protection- our expenditures for practical, basic researchhave also proven to be worthwhile and we haveyet another confirmation for our proposition:“Confident and safe with Scheuch.”

General

With respect to filtration plants, for example,there are many open questions:

Correct location of the filter plant based onthe existing danger of explosion (Zone re-quirement)Installation position of the rupture disks:Down below in the area where an explosionoriginates or as high as possible in order toprotect the filter's surroundings?Flame range: Under what conditions doflame ranges of 35 meters and more occuras calculated by VDI?Are the often proven decoupling measures- “check valve” in the crude gas area and“deflection with pressure relief” in theclean gas area - adequate according to AT-EX?In the case of relief on the clean gas side,by what factor must the pressure relief sur-face be increased?

So on and so forth.

Initial Situation / Objective forScheuch

In the wood processing and wood based panelindustries, stationary filtration plants are de-sign-engineered with explosion protection,

usually with explosion pressure relief and de-coupling measures. Currently, there are recog-nised testing options only for rotary valves andpipe non return flaps (Scheuch has already of-fered tested products for some time) but thereare currently no official testing options for otherequipment. Even previous testing of pipe nonreturn flaps as an autonomous protective sys-tem by the VDI raises doubts with respect totheir intended operation. While a flow rate of20-30 m/s with simultaneous transport of ma-terial occurs in actual practice, VDI testing ispurely static (without material transport).Questions and concerns about the permissi-bility of applying the testing process to the ac-tual application are thus quite valid.For this reason, we wanted to allow a completefiltration plant to be tested under practical con-ditions.

Testing Method

In cooperation with Germany's Holz BG, awood industry trade organisation, ScheuchGmbH of Aurolzmünster therefore commis-sioned a testing institute with practical testingof a wood dust filter. This institute was the FSAForschungsgesellschaft für angewandte Sys-temsichereheit und Arbeitsmedizin mbH,Prüfstelle für Systemsicherheit (EU-Kennnummer: 0588), Dynamostraße 7-11,D-68165 Mannheim. A suitable test dust forthese tests was selected in cooperation withthe Holz BG and the testing conditions wereset in such a way as to represent a worst-casescenario for the test filter.

To determine the representative test filter,Scheuch calculated proportionality ratios forall filter types and sizes. To calculate the ratios,

The requirements for the national implementation of the ATEX

guidelines have been redefined and tightened. Depending on

one's point of view, there are also various ambiguities

for the professional world that create uncertainty.

•

•

•

•

•

Scheuch NEWScheuchtodayacc. VDI-calculation

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2

1

1

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ATEX-COMPLIANT THROUGH

INTEGRATED SAFETY

THE SCHEUCH FILTRATION

PLANT AS COMPLETE SYSTEM

8scheuchemissionen

1/2006

9 scheuchemissionen1/2006

Offprint from the customer newsletter emissionen 1/2006Offprint from the customer newsletter emissionen 1/2006

Scheuch determined and compared the crudegas volumes, clean gas volumes, the filter baseopen surface area, the filter surface area, max-imum air volume, the rupture disk surface areaand other factors. In order to carry out testingunder practical conditions, a fan with extrac-tion line and a material feeder with meteringstation were erected. Testing occurred atcrude gas dust loads of 50 g/m3, 80 g/m3, 100g/m3 and 150 g/m3. Dust concentrations in thefilter were determined during filter operationand during the cleaning process. The so-called“test shots” occurred optionally with an emptybunker or with material bunkering.

Prior to actual explosive testing, Scheuch co-operated with the Holz-BG in determining fromthe manufacturer's information the expectedmaximum dust concentration that can occurin practical operation prior to filtration in filtersof the type studied. In addition, we determinedthe distributions of particle sizes in three repre-sentative dust samples taken from wood dustfilters located at plants in the wood processingindustry.

Results / Findings

Decoupling measuresIn all tests with relief on the crude gas sidein combination with a non return flap, 90° de-flection with upward relief and a rotary valve,there was no propagation of flames or pressurefrom the filter into the extraction pipe or fromthe filter discharge and into the clean air duct.Thus, the plant as tested in its entirety - usingScheuch's construction method - is a suitabledevice with integrated decoupling whenoperated in a correspondingly approvedmanner.

Installation position of rupture disks -Flame rangeThe series of practical tests confirmed the ex-pulsion of unburned dust from the openedpressure relief outlets. There is a danger of se-condary fires and secondary explosions. Therange and intensity of the dust expulsion de-pends on the explosion pressure pred of theevent and the reaction pressure of the reliefdevice.

Previous assumptions, whereby rupture diskspositioned as high as possible, with installationheights starting at 8 meters, were thought toeliminate the danger near ground level, couldnot be confirmed based on the dust dischargeand subsequent ignition.

With respect to the range of flames outside ourfiltration plant, the operator can consider forhis risk assessment either the previous calcu-lations according to VDI or alternately our ownflame profile, which is the result of many prac-tical tests.

In summary, it can be said that in the case ofmany of the “test shots”, the flame range wassubstantially less than expected and that thepressure development was not so intense asassumed. Despite our new reduced flame pro-file, filter locations are still often problematicalwith respect to the range of flames in an explo-sion. This is especially true when roadways orproperty borders are located within the flameprofile. For this reason, we tested new optionsfor pressure relief during these tests and willsoon be able to offer customers who requirethem filter plants that exhibit even smaller flameprofiles with our new protective system.

Lower explosion limit (German: UEG) forWood DustBecause the first series of tests with crude gasdust loads of 50 g/m3 was unable to produceignition despite ignition energies as high as10,000 joules, we examined the lower explo-sion limit (UEG) more closely. It was found to

be approximately 60 g/m3 as measured withthe addition of the most unfavourable dust type(original) in a container with a capacity of 1 m3.A UEG of 30 g/m3, as mentioned in the perti-nent literature, could only be achieved by re-screening the dust (impossible in actual prac-tice). Accordingly, a UEG of 30 g/m3 alreadyincludes a safety margin of 50%. With respectto the best available technology, referenceshould be made here to the VexAT, the Austrianimplementation of the operator version of ATEXRL 1999/9 EG, and its 50% shortfall of theUEG. A twofold safety margin of 50% is un-necessary. Only the dust content is relevantwhen considering an explosive atmosphere.Additional coarse material such as chips orshavings reduces the ignition propensity.

The probability of an eventDespite an unfavourable dust concentrationand a correspondingly large ignition source, adust explosion was not always possible. A cor-responding concentration of dust is necessarybecause of turbulence within the filter.Filter fires and a slight pressure increase onlyoccurred when crude gas dust loadingreached levels of 80 - 100 g/m3. An explosiveeffect according to VDI calculations is ex-tremely unlikely, but not impossible.

Bag bottom and filter bags as pressurebarriersA smaller pressure increase was measured inthe clean gas region of the filter than in thecrude gas chamber. This led to the realisationthat compared with the crude gas volume, thefree surface area of the bag bottom plays akey role in determining the value of pred in theclean gas chamber.

Conclusion

We were able to prove that Scheuch's IMPULSfilter provides the wood processing and woodbased panel industries with a complete systemthat has now been certified by a recognisedGerman testing institute.The know-how acquired during the tests willbe used for the benefit of our customers withrespect to design, consultation and support inconnection with explosion zones, the avoid-ance of ignition sources, design-engineeringof explosion protection systems, filter location,decoupling measures, flame range, etc. In thiscase - explosion protection and fire protection- our expenditures for practical, basic researchhave also proven to be worthwhile and we haveyet another confirmation for our proposition:“Confident and safe with Scheuch.”

General

With respect to filtration plants, for example,there are many open questions:

Correct location of the filter plant based onthe existing danger of explosion (Zone re-quirement)Installation position of the rupture disks:Down below in the area where an explosionoriginates or as high as possible in order toprotect the filter's surroundings?Flame range: Under what conditions doflame ranges of 35 meters and more occuras calculated by VDI?Are the often proven decoupling measures- “check valve” in the crude gas area and“deflection with pressure relief” in theclean gas area - adequate according to AT-EX?In the case of relief on the clean gas side,by what factor must the pressure relief sur-face be increased?

So on and so forth.

Initial Situation / Objective forScheuch

In the wood processing and wood based panelindustries, stationary filtration plants are de-sign-engineered with explosion protection,

usually with explosion pressure relief and de-coupling measures. Currently, there are recog-nised testing options only for rotary valves andpipe non return flaps (Scheuch has already of-fered tested products for some time) but thereare currently no official testing options for otherequipment. Even previous testing of pipe nonreturn flaps as an autonomous protective sys-tem by the VDI raises doubts with respect totheir intended operation. While a flow rate of20-30 m/s with simultaneous transport of ma-terial occurs in actual practice, VDI testing ispurely static (without material transport).Questions and concerns about the permissi-bility of applying the testing process to the ac-tual application are thus quite valid.For this reason, we wanted to allow a completefiltration plant to be tested under practical con-ditions.

Testing Method

In cooperation with Germany's Holz BG, awood industry trade organisation, ScheuchGmbH of Aurolzmünster therefore commis-sioned a testing institute with practical testingof a wood dust filter. This institute was the FSAForschungsgesellschaft für angewandte Sys-temsichereheit und Arbeitsmedizin mbH,Prüfstelle für Systemsicherheit (EU-Kennnummer: 0588), Dynamostraße 7-11,D-68165 Mannheim. A suitable test dust forthese tests was selected in cooperation withthe Holz BG and the testing conditions wereset in such a way as to represent a worst-casescenario for the test filter.

To determine the representative test filter,Scheuch calculated proportionality ratios forall filter types and sizes. To calculate the ratios,

The requirements for the national implementation of the ATEX

guidelines have been redefined and tightened. Depending on

one's point of view, there are also various ambiguities

for the professional world that create uncertainty.

•

•

•

•

•

Scheuch NEWScheuchtodayacc. VDI-calculation

3

2

1

1

2

3

ATEX-COMPLIANT THROUGH

INTEGRATED SAFETY

THE SCHEUCH FILTRATION

PLANT AS COMPLETE SYSTEM

Pneumatic Filling Free-Fall Filling

12scheuchemissionen

2/2008

13 scheuchemissionen2/2008

Offprint from the customer newsletter emissionen 2/2008 Offprint from the customer newsletter emissionen 2/2008

Initial Situation

The operator is usually also the first person orentity to place silos on the market and accordingto the ATEX Directive RL 1999/92 EG is respon-sible amongst other things for the interface“Silo/Filling”. This resulted in deficiencies withrespect to planning coordination and imple-mentation between the manufac-

turers of extraction equipment, who are respon-sible for the decoupling measures of their com-ponents, and the silo builder, who undertakes cal-culations and structural implementation of thevessel design and its pressure venting system.

With respect to the insertion of material intothe silo, one differentiates between “pneumaticfilling” - used primarily in the furniture industry- and so-called “free-fall filling”, which is usedprimarily in the solid wood and wood basedpanel industry. The basis for the calculation of

pressure vent-ing in silos using

rupture disks is EN 14491:2006. This allowsunder special loading conditions for a deviationfrom the normal means of calculation such as,for example, the accepted reduction formulaaccording to VDI 3673/Appendix A, but also

in the form of proven and certified testing pre-formed by an accredited testing institute.

Objectives

The well-known and significant risk potentialresulting from high pressure and long flameranges of up to 60 m resulted in more frequentattempts to direct venting “upwards” insteadof relying on side venting using rupture disks.The limited available pressure venting area onthe silo roof makes calculations according toVDI 3673/Appendix A absolutely necessary.The use of this method of calculation neverthe-less resulted in such large pressure ventingareas in the silo roof that the risk-free installa-tion of separation equipment or maintenanceaccess on the silo roof was possible only toa limited extent.

For this reason, Scheuch began a test projectwith a respected silo manufacturer in order tostudy and verify the danger zones for pressureventing systems directed to the side and alsoupwards, much like Scheuch's well-known ex-plosion protection concept for filtration plants.

An additional goal was to expand the reductionformula according to VDI 3673/Appendix A,independently of the maximum silo inlet open-ing of Ø 300 mm, which proved impractical forthe wood processing industry. The sought-after criterion was a maximum quantity ofdust.

The accumulation of potentially explosive quantities of dust endangers silo in-

stallations because the introduction of an ignition source cannot be reliably

excluded despite the use of all available technical measures. An explosion cre-

ates extremely high pressures that the vessel structures cannot absorb and

withstand. For this reason, silos must be equipped with targeted and appropriate

venting devices or decoupled using explosion protection techniques.

Finally, tests were also to be made of the de-coupling devices for the pneumatic conveyorsystems.

Procedural Overview

Comprehensive testing was performed lastyear together with Wolf Systembau of Scharn-stein/Austria at the FSA Forschungsgesell-schaft für angewandte Systemsicherheit undArbeitsmedizin, a well-known research insti-tute based in Mannheim/Germany. Almost ayear was required for the planning, develop-ment, coordination, execution and analysis ofthe test explosions. A total of 40 individualtests, carried out under practical conditionsusing wood dust and different filling variations,were able to substantiate the improvements

at first only “suspected” by Scheuch with re-spect to venting surface area and flame range.

Results

On the one hand, Scheuch was able to findsolutions to the existing interface problems ofsilo systems and, on the other hand, was alsoable to demonstrate a smaller risk potential.The certified solutions presented now provideoperators with the desired level of safety.The advantage of Scheuch's filling variants forwood dust and related types of dust lies in thefact that the potentially explosive amount ofdust effectively introduced into the silo, andnot the limited silo inlet opening of Ø 300 mm,is now the standard for the calculation of pres-sure venting according to VDI 3673/Appendix

A. In the case of pneumatic filling, this permitsa maximum feed pipe diameter of NW 400, orin the case of free-fall filling a maximum dustamount of 5,400 kg/h.

The flame profile for these variants was alsospecially determined for a reduced explosivepressure in the silo of 300 and 500 mbar re-spectively.

A subsequent expert opinion issued by Inburexdefined protection needs and specified pro-tection zones.

In addition to the rotary valves already certifiedas decoupling devices, this project was alsoable to demonstrate the suitability ofScheuch's non-return flaps as a decouplingdevice and the decoupling of the return line.

Flame profile:

Direct contact with the flame can occur in thisarea. This area was ascertained through theuse of video recordings and was assigned asafety factor. In this area, direct physiologicaleffects on persons in the area are to be feared.Flammable materials in the area can burst intoflame and components relevant to the techni-cal safety would be damaged.

Area 1:

In this area there are no direct effects from theflame, but thermal radiation will cause in-creased thermal stress for a short period oftime. However, experience shows that thisdoes not result in damage to persons or equip-ment.

Area 2:

In Area 2 there is exposure only to the flameradiation. However, this is so minimal that nodamage to persons or equipment is possible.

Conventional flame profile when pressure venting is dimensioned ac-cording to EN 14491:2006

NEW: Flame profiledemonstrated and verified byScheuch for special filling condi-tions according to VDI 3673, Ap-pendix A, for pred,max < 300 mbar

MAXIMUM SAFETY LEVELS

Advantages and Benefits for the Operator

Scheuch's new silo safety concept has fun-damental and wide-reaching advantages.Through application of the reduction formula,rupture disks can be reduced by 30 - 60%depending on the silo volume and the fillingtechnique. This makes targeted side ventingpossible.

In addition, this expert opinion can be used forrisk assessment in the explosion protectiondocument and provides legal certainty for theoperator, providing original Scheuch compo-nents are used.The dramatically reduced flame ranges and thecorrespondingly smaller danger areas result in

additional operating space that can be usedfor other purposes (buildings, equipmentspace, walkways and roadways, propertylines) and planners can in the future budgetfor and use less space in the erection of newsilo systems.

Pneumatic Filling Free-Fall Filling

12scheuchemissionen

2/2008

13 scheuchemissionen2/2008

Offprint from the customer newsletter emissionen 2/2008 Offprint from the customer newsletter emissionen 2/2008

Initial Situation

The operator is usually also the first person orentity to place silos on the market and accordingto the ATEX Directive RL 1999/92 EG is respon-sible amongst other things for the interface“Silo/Filling”. This resulted in deficiencies withrespect to planning coordination and imple-mentation between the manufac-

turers of extraction equipment, who are respon-sible for the decoupling measures of their com-ponents, and the silo builder, who undertakes cal-culations and structural implementation of thevessel design and its pressure venting system.

With respect to the insertion of material intothe silo, one differentiates between “pneumaticfilling” - used primarily in the furniture industry- and so-called “free-fall filling”, which is usedprimarily in the solid wood and wood basedpanel industry. The basis for the calculation of

pressure vent-ing in silos using

rupture disks is EN 14491:2006. This allowsunder special loading conditions for a deviationfrom the normal means of calculation such as,for example, the accepted reduction formulaaccording to VDI 3673/Appendix A, but also

in the form of proven and certified testing pre-formed by an accredited testing institute.

Objectives

The well-known and significant risk potentialresulting from high pressure and long flameranges of up to 60 m resulted in more frequentattempts to direct venting “upwards” insteadof relying on side venting using rupture disks.The limited available pressure venting area onthe silo roof makes calculations according toVDI 3673/Appendix A absolutely necessary.The use of this method of calculation neverthe-less resulted in such large pressure ventingareas in the silo roof that the risk-free installa-tion of separation equipment or maintenanceaccess on the silo roof was possible only toa limited extent.

For this reason, Scheuch began a test projectwith a respected silo manufacturer in order tostudy and verify the danger zones for pressureventing systems directed to the side and alsoupwards, much like Scheuch's well-known ex-plosion protection concept for filtration plants.

An additional goal was to expand the reductionformula according to VDI 3673/Appendix A,independently of the maximum silo inlet open-ing of Ø 300 mm, which proved impractical forthe wood processing industry. The sought-after criterion was a maximum quantity ofdust.

The accumulation of potentially explosive quantities of dust endangers silo in-

stallations because the introduction of an ignition source cannot be reliably

excluded despite the use of all available technical measures. An explosion cre-

ates extremely high pressures that the vessel structures cannot absorb and

withstand. For this reason, silos must be equipped with targeted and appropriate

venting devices or decoupled using explosion protection techniques.

Finally, tests were also to be made of the de-coupling devices for the pneumatic conveyorsystems.

Procedural Overview

Comprehensive testing was performed lastyear together with Wolf Systembau of Scharn-stein/Austria at the FSA Forschungsgesell-schaft für angewandte Systemsicherheit undArbeitsmedizin, a well-known research insti-tute based in Mannheim/Germany. Almost ayear was required for the planning, develop-ment, coordination, execution and analysis ofthe test explosions. A total of 40 individualtests, carried out under practical conditionsusing wood dust and different filling variations,were able to substantiate the improvements

at first only “suspected” by Scheuch with re-spect to venting surface area and flame range.

Results

On the one hand, Scheuch was able to findsolutions to the existing interface problems ofsilo systems and, on the other hand, was alsoable to demonstrate a smaller risk potential.The certified solutions presented now provideoperators with the desired level of safety.The advantage of Scheuch's filling variants forwood dust and related types of dust lies in thefact that the potentially explosive amount ofdust effectively introduced into the silo, andnot the limited silo inlet opening of Ø 300 mm,is now the standard for the calculation of pres-sure venting according to VDI 3673/Appendix

A. In the case of pneumatic filling, this permitsa maximum feed pipe diameter of NW 400, orin the case of free-fall filling a maximum dustamount of 5,400 kg/h.

The flame profile for these variants was alsospecially determined for a reduced explosivepressure in the silo of 300 and 500 mbar re-spectively.

A subsequent expert opinion issued by Inburexdefined protection needs and specified pro-tection zones.

In addition to the rotary valves already certifiedas decoupling devices, this project was alsoable to demonstrate the suitability ofScheuch's non-return flaps as a decouplingdevice and the decoupling of the return line.

Flame profile:

Direct contact with the flame can occur in thisarea. This area was ascertained through theuse of video recordings and was assigned asafety factor. In this area, direct physiologicaleffects on persons in the area are to be feared.Flammable materials in the area can burst intoflame and components relevant to the techni-cal safety would be damaged.

Area 1:

In this area there are no direct effects from theflame, but thermal radiation will cause in-creased thermal stress for a short period oftime. However, experience shows that thisdoes not result in damage to persons or equip-ment.

Area 2:

In Area 2 there is exposure only to the flameradiation. However, this is so minimal that nodamage to persons or equipment is possible.

Conventional flame profile when pressure venting is dimensioned ac-cording to EN 14491:2006

NEW: Flame profiledemonstrated and verified byScheuch for special filling condi-tions according to VDI 3673, Ap-pendix A, for pred,max < 300 mbar

MAXIMUM SAFETY LEVELS

Advantages and Benefits for the Operator

Scheuch's new silo safety concept has fun-damental and wide-reaching advantages.Through application of the reduction formula,rupture disks can be reduced by 30 - 60%depending on the silo volume and the fillingtechnique. This makes targeted side ventingpossible.

In addition, this expert opinion can be used forrisk assessment in the explosion protectiondocument and provides legal certainty for theoperator, providing original Scheuch compo-nents are used.The dramatically reduced flame ranges and thecorrespondingly smaller danger areas result in

additional operating space that can be usedfor other purposes (buildings, equipmentspace, walkways and roadways, propertylines) and planners can in the future budgetfor and use less space in the erection of newsilo systems.

12scheuchemissionen

1/2007

Offprint from the customer newsletter emissionen 1/2007

The complete IMPULS bag filter program fromScheuch has been ATEX-certified by a recog-nised testing institute with respect to protec-tion against both explosion and fire; it has alsoreceived H3 certification from the Holz-Berufsgenossenschaft with respect to work-place safety issues. Both certifications pertainto the overall filtration system. Scheuch is thusthe first company to be H3 certified for its com-plete row filter program.

H3 Mark

Scheuch's goal was to en-sure levels of residual dustcontent below 0.1 mg/m3

while achieving the longestpossible service life for filterbags, low filter resistanceand while avoiding for theoperator the costs of ongo-ing testing.

In order to receive certification in the form ofthe H3 Test Certificate (BG-PRÜFZERT-Zeichen H3) issued by the Holz-BG, the BIA(Berufsgenossenschaftliches Institut für Ar-beitsschutz) performed the appropriate testson a fully implemented IMPULS filtration plant.Upon the conclusion of all measurements, thisfiltration plant became the first H3-certified sta-tionary filter. The measurements confirm thatresidual dust levels are reliably below 0.1mg/m3 and that these levels are constantlymonitored (as required in BGI 739 and EN12779 - Sicherheitstechnische Anforderungenfür Absauganlagen für Holzstaub und Späne).

This translates into the

following benefits for filteroperators: Through verifi-cation of this dedustingtechnology, (filter bag, fil-ter bag mounting and bagbase were certified), theusual costs incurred by

operators for first-time onsite measurementsno longer apply. The same is true for previouslyrequired recurring measurements, since thecontinuous monitoring of residual dust levelswas also certified. The assurance that residualdust levels are reliably below 0.1 mg/m3 meansthat the return air is free of Ex-zone require-ments.

New information about ATEXcertification for the entire system

Previously, extensive testing was performedunder actual operating conditions on a wooddust test filter. These tests were carried out incooperation and consultation with the GermanHolz-BG at the FSA Forschungsgesellschaftfür angewandte Systemsicherheit und Arbeits-medizin in Mannheim/Germany.

Roughly a year ago, it wassuccessfully demon-strated that the ScheuchIMPULS filter as a com-plete system - includingthe filtration plant and alldecoupling measuresand dust discharge sys-tems - is ATEX-compliantin the wood processingand wood based panelindustries. Because filterlocations remain prob-lematical despite the lowScheuch flame profile with respect tothe explosion flame range - especiallywhen roadways or property borders arelocated within the flame profile -Scheuch has tested additional new reliefoptions. The efficiency of four differentexplosion protection concepts witheven smaller flame profiles

has now been confirmed.

Based on the test results obtained by FSA,Scheuch commissioned a recognised expertto define the danger areas and to describe thelimitations. The expert opinion is divided intothree areas:

Direct contact with the flame can occur in thearea of the flame profile. This area was as-certained through the use of video recordingsand was assigned a safety factor. In this area,direct physiological effects on persons in thearea are to be feared. Flammable materials inthe area can burst into flame and componentsrelevant to the technical safety would be dam-aged.

In Area 1 there are no direct effects from theflame, but thermal radiation will cause in-creased thermal stress for a short period oftime. However, experience shows that thisdoes not result in damage to persons or equip-ment.

In Area 2 there is exposure only to the flameradiation. However, this is so minimal that nodamage to persons or equipment is possible.

This expert opinion can be used for risk ass-essment in the explosion protection docu-ment. The opinion also confirms that road-ways or walkways may be located within thearea.

Scheuch has placed a high value in the past few years on the topic of safety

in filtration plants and has successfully implemented numerous measures

in this regard. This has also been good news for operators of Scheuch

filtration plants in the wood processing and wood

based panel industries:

NEW INFORMATION

ABOUT SAFETY