DustExplosion Protection

D u s t E x p l o s i o n P r o t e c t i o n

Dust Explosion Protection by H.-J. Sussann Edition: 23.05.2003

Content

1. General .................................................................................................................................................1

1.1 Explosion triangle...............................................................................................................................2

1.2 Efficiency triangle...............................................................................................................................3

1.3 Guideline 94/9/EG (ATEX 95) – Guideline 1999/92/EG (ATEX 137) ................................................4

2. Protection concepts..........................................................................................................................14

2.1 Primary explosion protection............................................................................................................14

2.2 Suppression .....................................................................................................................................16

2.3 Explosions pressure release............................................................................................................18

3. Explosion disengagement................................................................................................................19

4. Reimelt Rules for the prevention of dust explosions ...................................................................20

5. Design recommendations for individual aggregates ....................................................................21

6. Photos showing dust explosions ....................................................................................................29

7. References .........................................................................................................................................31

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Dust explosion protection This document shall familiarize our employees and customers with the subject of dust explosion protec-tion as well as to show REIMELT safety policy on dust explosion protection. Furthermore we want to de-scribe by this breviary the execution of the guideline 94/9/EG as it is executed at Reimelt. We also give hints for execution of the guideline 1999/92/EG (a user guideline). 1. General Production and processing methods which create organic or chemical dusts are often closely tied to fire and explosion hazards. The knowledge that dust can cause explosions has been known for more than 200 years. The first dust explosion which was defined as one happened on the 14th December 1785 ac-cording to records of the Turin academy of sciences in a flour storage facility in Turin/Italy. In Germany for instance, every year approx. 350 to 400 explosions are counted, approx. 1 to 2 incidents per day. The cause for this rapid increase is certainly related to an increasing humanization of the work-place and the corresponding clean-air measures. The companies providing social insurance against occupational accidents and federal environmental agencies ensure that work areas are equipped with suitable exhaust systems, this also inevitably in-creases the quantity of dust collection in filters and associated equipment. In addition, the share of fines has strongly increased due to high-speed machinery and automated con-veying systems. Especially the volume of fine dust drastically increases the risk of explosions. With the reduced dust particle size, the surface area, hence, the impact of the explosion rises ex-ponentially. The disastrous effects of dust explosions can be consid-erably reduced or entirely prevented by primary explosion protection measures as well as by protective measures regarding the design.

Fig. 1: Feed mill in Belgium Destroyed by a dust explosion in 1994

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1.1 Explosion triangle In general, which factors must interact to cause a dust explosion?

Fig. 2: Explosion triangle As shown in Fig. 2, three factors are necessary to cause a dust explosion:

• Fuel • Oxygen • Ignition source

must be available in sufficient quantities and interact at the right point of time. If only one of these three factors is absent, a dust explosion will not occur. One effective measure is primary explosion protection for prevention of dust explosions. This explosion protection concept is designed to prevent all three factors from coming together. In particular, special care is taken to prevent ignition sources. Potential ignition sources could be, for ex-ample, mechanically or electrically generated sparks, frictional heat, static electricity, welding, or other thermal influences. Another protective measure is prevention through inertisation. The objective is to displace the ambient air required for oxidization of the fuel with inert gas, e.g. nitrogen. In this case, all system parts must be filled and purged with the selected inert gas. Experience shows that inertisation is a complex and expensive method, since the maximum permissible oxygen concentration must not be exceeded at any time. In most cases, it is impossible to prevent the presence of fuel, since the product itself or a byproduct of the production process serves as fuel.

Ignition Source

Fuel Oxygen

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1.2 Efficiency triangle Analog to the explosion triangle, up-to-date protection concepts must meet three major requirements:

Fig. 3: Efficiency triangle • efficient in running operation, requiring no maintenance • high availability, no false alarms, no restrictions on the production process • safe process, protection for man and machine

Efficiency

Availability Safety

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1.3 Guideline 94/9/EC – Guideline 1999/92/EC 1.3.1 EC-contract

Deadlines for the introduction of the guidelines

EC-contract For alignment of the statutory orders of the member states for equipment and protection

systems which are designated for use in potentially explosive atmosphere.

article 95 (94/9/EG) guideline with basic demands on safe

products

article 137 (99/92/EG) guideline with minimum regulation for safety

and protection of the employees.

EC-guidelines e.g.

guideline for machines

EC-guidelines as

frame guideline safety at work

equipment safety laws as used

in Germany

law for the protection of labour

as used in Germany

ATEX 95

The guideline 94/9/EG was passed on 23rd

March 1994 and was transfered into nationalright with coming into effect on 12th December1996 and with the 11th regulation of the devicesafety law.

End of the transitional period is the 30th June2003, afterwards it becomes binding!

ATEX 137

The guideline 99/92/EG came into force on 01st

January 2003. Material and workplace which are already in usehave a transitional period of 3 years and mustcorrespond to the guideline on 30th June 2006.

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1.3.2 Demand on the system operator according to ATEX 137: 1.3.2.1 Targets of the guideline: Decree – as foreseen in article 137 of the contract – of minimum regulations for health protection and safety of the employees which can be in danger because of explosive atmosphere. Determination of specific regulations for improvement of health protection and safety of the employees which can be in danger because of explosive atmosphere considering strictly the principles described in the guideline. Creation of an equivalent frame work in the EC referring to the explosion proof for the general industry as it already exists for the mineral industry. 1.3.2.2 Duties of the employer according to ATEX 137 The employer has the duty to draw up an explosion proof document and to maintain it up to date. The document must define the measures necessary for health protection and safety of the employees. Besides the preventive measures additional measures must be planned if necessary which will be effec-tive when an ignition has already happened. • Basic rules for avoiding of and protection against explosions. • Avoiding of the formation of an explosive atmosphere. • Avoiding of ignition of an explosive atmosphere. • Diminishing of the results of an explosion so that the employees are not in danger. All limitation/reduction of the explosion risks must be seen in the entirety. Especially important are: The systems, the materials used, the processes, and their possible interaction.

In case of ascertainment of a danger for employees during the planning stage suitable preventive meas-ures and protection systems must be planned against these dangers. Result: As guarantee for improvement of health care and the safety of employees which can be exposed to the dangers caused by an explosive atmosphere the employer has the juridical responsibility.

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1.3.2.3 Zone division according to ATEX 137 Until now (valid until 30.06.2003)

• Zone 10: Dangerous explosive atmosphere existing long-term and often.

• Zone 11:

Dangerous explosive atmosphere existing only short-term and occasionally. New (obligation from 01.07.2003 on)

• Zone 20: Area where explosive atmosphere is existing in the form of a cloud of inflammable dust in the air al-ways or long-term or often.

• Zone 21:

Area where explosive atmosphere is appearing in the form of a cloud of inflammable dust in the air sometimes during normal operation.

• Zone 22: Area where it is not probable that explosive atmosphere in form of a cloud of inflammable dust in the air forms during normal operation, but if it forms during normal operation but if it appears it appears only short-term.

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1.3.2.4 Proposals for zone division according to ATEX 137 According to guideline 1999/92/EG article 2, existance of an explosive atmosphere is a basic condition for a ATEX 137 consideration, independent from an existing ignition source that means that first of all the burning, ignition and explosion reaction of the material to be treated must be taken into consideration. If there is a burning, ignition and explosion reaction of the material the occurrence of explosive atmospheres should be examined. These explosive atmospheres are existing when there are material/air-concentrations in the area between the upper and lower explo-sion limit of the material. If this is the case, ATEX 137 must be applied. ADVICE: The zone division proposals are based on the assumption that the material to be treated is a cus-tomary wheat or rye flour. Other material demands another zone division!

system part division

referring to EX-RL up to

30.06.03

zone division referring to ATEX

137 effective 01.07.03

remarks

system part outdoor no no

Zone division may not be necessary outdoor because of the weather compared to similar situations in rooms. Detail check eventually necessary.

Rooms where there are dust leading systems. no no Devices dustproof in the long term.

Rooms where there are dust leading systems. no no Dustproof of the device is not guaranteed in the long term but already small leaks are discovered soon and removed. (see VDI 2262).

Silo interior (tangential feeding; discharge by vibratory bottom). 10 21

If the silos is only fed and/or discharged some-times during operation period it can be presup-posed based on a risk examination that explo-sive atmosphere does not appear often

Silo interior (tangential feeding; discharge by fluidising bed or vibratory bottom). 10 20

Explosive atmosphere appears often caused by bulk material discharge by means of air. If the silo is filled or discharged often explosive at-mosphere appears often also independent of the kind of silo discharge.

Silo penthouse with installed cloth filters. 11 21 Dustproof of the device is not guaranteed in the long term. Dust deposit is possible. Dust is settling in the whole room.

Silo penthouse with installed automatic filters. 11 21 Dustproof of the device is not guaranteed in the long term. Dust deposit is possible. Dust is setting in the whole room.

Silo penthouse with installed automatic filters. Filters are supervised for filter break on the venting side.

no no Dustproof of the device is not guaranteed in the long term but already small leaks are discovered soon and removed (see VDI 2262).

Silo penthouse with installed automatic filters. Venting of the filter is lead outside. no no Device dustproof in the long term.

Silo with closed silo skirt with installed devices (e.g. fluidising bed with equipment) 11 no Device dustproof in the long term.

Silo with closed silo skirt with installed devices e.g. vibratory bottom with discharge hopper and automatically supervised filter).

11 no Dustproof of the device is not guaranteed in the long term but already small leaks are discovered soon and removed (see VDI 2262).

Silo with closed silo skirt with installed devices (e.g. vibratory bottom with discharge hopper and cloth filter)

11 22 Dustproof of the device is not guaranteed in the long term. Dust deposit is possible. Dust is settling in the whole room

Discharge hopper interior 10 20 Explosive atmosphere is probable caused by frequent feeding and discharge.

Discharge hopper interior with equipment such as slow running (ν < 1 m/s) agitator a.s.o. Driv-ing power HP < 4 kW.

10 20 Explosive atmosphere is probable caused by frequent feeding and discharge.

Interior of stationary filters. 10 20

Explosive atmosphere is always probable caused by intake of very fine dust and the frequent automatic cleaning of the filter ele-ments.

Interior of automatic top mounted filters. 10 20

Explosive atmosphere is always probable caused by intake of very fine dust and the frequent automatic cleaning of the filter ele-ments

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system part division refer-ring to EX-RL

up to now

zone division referring to ATEX 137

remarks

Devices and system parts which are oper-ated with low pressure. 1-14.7 psi or under 1 bar.

no no

There is no oxydation mean such as oxygen if the devices and systems parts are process depending operated under vacuum during the main process time. Contact an expert if necessary!

Devices and system parts where the oxygen concentration has fallen below the limit because of inertization.

no no

There is no oxydation means such as oxy-gen if the devices and systems parts are process depending operated under vacuum during the main process time. Contact an expert if necessary!

Conveying screw interior. Slow running screw (ν < 1 m/s). No fluidising bed up-stream.

11 22 Explosive atmosphere is not often probable because of the low peripheral speed.

Conveying screw interior. Fast running screw (ν > 1 m/s). No fluidising bed upstream. 10 20

Explosive atmosphere is probable because of the high peripheral speed. Consider ignition source.

Conveying screw interior. Fluidising bed upstream. 10 20 Explosive atmosphere is always probable

because of the upstream fluidising bed.

Bag dumping stations, bagging station (area around these stations). no no

There is no dust release around open places as low pressure process, dust ex-hausts existing. Clean regularly.

Bag dumping stations, bagging station ( area around these stations). 11 22

Dust deposits are possible. Also small dust deposits are removed. Zone 22 in the area of 1 m around the outflow position to the floor.

Bag dumping (side area, discharge by fluid-ising bed) 10 20

Explosive atmosphere is always probable during the feeding process because of the existing fluidising bed.

Bag dumping (side area, discharge by vibra-tory bottom) 10 21 Explosive atmosphere is sometimes prob-

able during the feeding process..

Grinding systems (e.g. powdered sugar grinding system) 10 20

Explosive atmosphere is always probable. Avoiding of effective ignition sources is not possible as a rule. Contact an expert!

Pneumatic conveying systems: Dilute-phase conveying. 10 20 Explosive atmosphere is always probable.

Pneumatic conveying systems: Dense-phase conveying 10 21 (20)

Explosive atmosphere is only probable during start and shutdown processes if these processes are only effected some-times according to the operation duration.

Pneumatic mixer (side area, without move-able equipment) 10 20

Explosive atmosphere is always probable during the mixing and discharge process because of the existing fluidising bed.

Mechanical mixer (side area, low filling level < 70%) 10 20 Explosive atmosphere is always probable

during the mixing process.

Mechanical Mixer (side area, high filling level > 70%) 10 21

Explosive atmosphere is only sometimes probable during the mixing process because of the high filling level.

Interior of sifters (rotary sifters) 11 20 Explosive atmosphere is probable during sifting process.

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system part division refer-ring to EX-RL

up to now

zone division referring to ATEX 137

remarks

Interior of sifters 11 21 Explosive atmosphere is only sometimes probable during the sifting.

Interior of vibratory discharge systems (vi-bratory bottom, MIC-DOS-discharge etc.) 11 22

Explosive atmosphere is rarely probable during discharge (before each filling and during discharge of the upstream hopper etc.)

Interior of fluidising beds 11 22

Explosive atmosphere is rarely probable during discharge (before each filling and during discharge of the upstream hopper etc.)

Interior of rotary discharge valves. 11 22

Explosive atmosphere is rarely probable during discharge (before each filling and during discharge of the upstream hopper etc.)

1.3.2.5 Further information for the introduction of the guideline ATEX 137. Systems put into operation before 01st July 2003 can be operated until the 30st June 2006. Important: Systems which correspond to the minimum regulations “BetrSichV attachement 4A” and which are oper-ated safely can be operated safely also after the 30st July 2003! (e.g. Reimelt systems which were put into operation after 12/1996 but which are used according to the regulations!) If new systems are installed or put into operation after the 1st July 2003 only ATEX authorized devices can be used. The zone division must be executed according to the new zone division concept. 1.3.3 Demand on the construction and use of devices for the determined use in areas in danger of explosion (ATEX 95) Devices are machines, equipment, stationary or moveable installations, control and equipment parts as well as warning and preventive systems which by themselves or in combination produce or transfer, store, measure, control, transform or consume energies or which are designated to work material and which have their own potential ignition sources and therefore can cause an explosion.

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1.3.3.1 Basic requirements for devices referring to ATEX 95 Appendix II:

• Principles of the integrated explosion safety (see EN 292-2).

• Devices and safety systems must be designed and produced considering all possible faults.

• Special examination and maintenance conditions.

• Environmental conditions.

• Marking of the devices.

• Instruction manual.

• Further requirements for the design/manufacturing, material selection, risks from software etc.

• Further requirements for devices, subdivided in device group and device category (Reimelt → device group II).

• Further requirements for safety systems.

1.3.4 Device group II per ATEX 95

group II (explosive atmosphere of gas/air –or dust/air mixtures, mist or vapors)

category 1 category 2 category 3 G

(gas) (zone 0)

D (dust)

(zone 20)

G (gas)

(zone 1)

D (dust)

(zone 21)

G (gas)

(zone 2)

D (dust)

(zone 22)

For devices which guarantee a very high level of safety.

Designated for the case that an

atmosphere in risk of explosion is often or always probable.

For devices which guarantee a

high level of safety.

Designated for the case that an atmosphere in risk of explosion is

probable.

For devices which guarantee a

normal level of safety.

Designated for the case that an atmosphere in risk of explosion is rather rare or happens only short-

term.

1.3.4.1 Detailed information about these device types: Category 1 – devices and protective systems: Devices of this category must guarantee the necessary very high level also if two faults occur independ-ent from each other or 1 of minimum 2 protective measures which are independent from each other fail. Certification by a named authority!

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Category 2 - devices: Devices of this category must guarantee the necessary high level of safety also if repeatedly faults of the device or other faults occur. Documentation must be deposited at a named authority! Category 3 - devices: The devices of category 3 meet the necessary level of safety, if foreseeable probable ignition sources which can occur during normal operation will be avoided. Self-certification without involvement of a named authority! 1.3.5 Classification proposal of devices in categories according to ATEX 95

device according to ATEX 95 in case of ATEX, which category

device or aggregate yes no category 1 category 2 category 3

advices, remarks a.s.o.

rotary sifter with plastic sifter basket x

The machine is designed according to the proposal from IVSS. It must be checked if temperatures arise which can serve as ignition sources caused by shaft glands or by defective bearings.

rotary sifter with plastic sifter basket and screen-d-tect x x

The machine is designed according to the proposal from IVSS. It must be checked if temperatures arise which can serve as ignition sources caused by shaft glands or by defective bearings. In addition the electrical installation in the dust leading area of the machine could become ignition source. This must be checked.

rotary sifter with metal sifter basket x x

The machine is not designed according to the proposal of the IVSS. Because of the material combination stainless steel beater and stainless steel sifter basket , ignition danger is arising caused by the peripheral speed of the paddles (ν > 1 m/s). There is danger of ignition.

Conveying screws ν < 1 m/s x

The conveying screw is designed according to the proposal of the IVSS. It must be checked if temperatures arise which can serve as ignition sources caused by shaft glands or by defective bearings

Conveying screws ν > 1 m/s x x There is danger of ignition because of the peripheral speed of the screw shaft.

Rotary discharge valve (different types of construction) x x

Rotary discharge valves are used as a protection element (as decoupling factor) and are therefore automatically subject to ATEX95! Existing certificates which are subject to Ex-RL must be rewritten!

Two-way diverter valve (PS-line) x Diverter valves are not subject to ATEX 95 as they have no ignition source in the dust leading areas.!

Two-way diverter valve (PS-line) x x

Diverter valves could be used as protection element (as decoupling, analogue to the rotary discharge valves) and are therefore automatically subject to ATEX 95! It could be useful to have such valves in the program. Check!

Vibratory discharge devices without equipment in the dust leading area x

These devices are not subject to ATEX 95, as they have no ignition source in the dust leading area.

Fluidising beds with Siperm mem-brane and Conidur membrane of different material properties

x Fluidising beds are not subject to ATEX 95, as they have no ignitions source in the dust leading area. Attention! Electrostatics must be considered!

Pneumatic mixer with equipment (fast running lump breakers) x x

There is danger of ignition because of the peripheral speed of the lump breakers. It must also be checked if temperatures arise which can serve as ignition sources caused by shaft glands or by defective bearings

Hopper agitator ν < 1 m/s and/or HP < 4 kW x

These agitators are designed according to the proposal of the IVSS. It must be checked if temperatures arise which can serve as ignition sources caused by shaft glands or by defective bearings

Hopper agitator ν > 1 m/s and/or HP > 4 kW x x

There is danger of ignition because of the peripheral speed and/or high driving power of the agitators. It must be checked if temperatures arise which can serve as ignition sources caused by shaft glands or by defective bearings.

Jet-Filter x Jet-Filters are not subject to ATEX 95 as they have no ignition source in the dust leading area. Attention! Electrostatics must be considered!

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device according to

ATEX 95 in case of ATEX, which category device or aggregate

yes no Kategorie 1 Kategorie 2 Kategorie 3 advices, remarks a.s.o.

Mechanical mixer without equipment x The machine is designed according to the proposal from the IVSS.

Mechanical mixer with equipment; mixer tools ν > 1 m/s and/or HP > 4 kW.

x x x

There is danger of ignition because of the high peripheral speed of the mixer tools and/or high driving power of the machine . Category 1 or 2 is subject to the zone division of the mixing room (free mixing volume!)

Mechanical mixer with equipment; mixer tools ν < 1 m/s and/or HP < 4 kW.

x

There is no danger of ignition because of the low periph-eral speed and/or high driving power of the machine. It must be checked if temperatures arise which can serve as ignition sources caused by shaft glands or by defective bearings

Fast running mechanical grinders (e.g. powdered sugar grinders) x x

There is danger of ignition because of the high peripheral speed of the grinding tools and/or high driving power of the machine. Category 2 because of the normally low volume of the grinding rooms and the high level of turbulences during the grinding process.

Fittings, hand valves, ball valves, slide gate valves, valves

These devices are not subject to ATEX 95 as they have no ignition source in the dust leading area. Attention! Electrostatics must be considered.

Level indicator for bulk material x x x x

Depends on the zone of the dust leading area. If the device is installed in a zone separation wall, the device must be suitable for both zones. These devices should generally be designed by Reimelt for being installed in a zone separation wall and should be suitable for the zones 1/2/3.

Sensors and switches for pressures and temperatures for bulk material x x x x

Depends on the zone of the dust leading area. If the device is installed in a zone separation wall so the device must be suitable for both zones. These devices should generally be designed by Reimelt for being installed in a zone separation wall and should be suitable for the zones 1/2/3.

Special advice This proposal only describes the dust leading area of a device. If the device is put in a dust leading envi-ronment or the customer defines the position of the device as Ex-Zone (e.g. zone 22), so the above mentioned assessment must be executed again under this point of view. If necessary motors, switches, sensors, actors and initiators must be chosen again referring to the zone (e.g. zone 22 → cate-gory 3 devices). 1.3.6 Temporary regulation of ATEX 95 Devices and protection systems which correspond to the regulations valid on 23rd March 1994 may be put into operation until the 30st June 2003. The authorities which care about the evaluation of the conformity of the equipment which has been put into operation before 1st July 2003 must take into account the results of the already executed examina-tions and controls which are subject to the regulation (valid on 23rd March 1994) about electrical systems which are located in hazardous rooms. 1.3.7 Necessary coordination between system operator and system designer The system operator must define the Ex-zone based on the zone definition. ATEX 137! Based on the zone definition by the systems operator the system designer must define/select the devices (machines, protection systems a.s.o.) ATEX 95!

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1.3.8 Classification of the device categories to the Ex-zones.

device group

categories

II 1 2 3

demands redundant protective measures

simple protective measures

safe during normal operation

gas-Ex-zones

zone 0 zone 1 zone 2

dust-Ex-zones

zone 20 zone 21 zone 22

1.3.9 Useful references • Internet: http://www.europa.eu.int/comm/enterprise/atex/guide/guide_de.pdf

http://www.bgn.de/Fachartikel/Akzente06_01/Bindend/Atex.html http://www.de.osha.eu.int/legislation/verord/GSGV-11.html etc.

• VDMA: Positionspapier zu ATEX • DIN EN-Normen: DIN EN 1050 / DIN EN 1127-1 / DIN EN 13463-1 / DIN EN 13463-5 / etc. • VDI-Richtlinien: VDI 2263 Blatt 1-5 / VDI 3673 / etc. • Explosion safety regulations: BGR 104 • Suppliers: E&H-Broschüre „Anlagensicherheit durch Explosionsschutz“.

etc.

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2. Protection concepts To meet these requirements, several protection concepts are available:

• Prevention of ignition sources as primary explosion protection measures • Suppression systems • Explosion pressure relief as protection for man and machine through design measures

2.1 Primary explosion protection The concept of preventing ignition sources should be given the highest priority in all applicable cases. Therefore, design measures or secondary measures, as listed under 2.2 and 2.3 are a must when the product frequently changes or the minimum ignition energy of the products is <10mJ . Also, if the product has a tendency towards self-ignition, the protection concept of “Prevention of Effective Ignitions Sources “ is by no means sufficient. REIMELT systems are standard designed according to the concept of „Prevention of Effective Ignition Sources”. REIMELT gives a lot of thought to this standard, but also spends a lot of capital to realize the measures prescribed in the relevant technical codes and standards. Currently, these codes and standards list 13 potential ignition sources. Hot Surfaces Maximum ambient temperature = outside temperature on hot summer days Minimum glow temperature = e.g. 450° C (explosion characteristics) Minimum ignition temperature = e.g. 370° C (explosion characteristics) 2/3 * ignition temperature = 246° C Glow temperature - 75° C = 375° C according to VDI 2263 2/3 x ignition temperature - 75° C must not be reached at any point in the system. Electrical equipment in zone 11 type of protection IP 54 Electrical equipment in zone 10 to include type examination certificate plus observation of the relevant regulations for zone 11. Conveying air temperature system related <120° C. Dust layers are prevented through operational meas-ures as far as possible in accordance with VDI Guideline 2262 „Dust control at the workplace“. Please inform our customers about this important point. Shut-down procedures initiated by the control system will not increase surface temperatures. Due to operational measures, welding and cutting operations are not executed, unless the permissibility of such operations has been examined in detail first. Flames and Hot Gases Due to operational measures, flames are excluded. The conveying air temperature is limited to 120° C due to the system. The temperature of the dehumidifiers must be monitored. Mechanically Generated Sparks Effective ignition sources due to mechanically generated sparks during pneumatic conveying of e.g. steel screws are excluded in compliance with E 2.3.3 of Ex RL. Due to operational measures cutting operations are not performed, unless the permissibility of such operations has been examined in detail first. Screw compressors with suction filters are not equipped with rotating parts in the conveying pipe or silo. The circumferential speed of rotating equipment < 1 m/s.

15

Electrical Apparatus Electrical apparatus are designed according to VDE 0165/9.83, Part Z. Operational measures ensure that the proper state function is regularly monitored. Electrical Compensation Circuits Equipotential bonding is designed in compliance with VDE 0165/9.83. Static Electricity According to ZH 1/200 Guideline for the Prevention of the Danger of Ignition caused by Electrostatic Charges, all conductive parts are grounded to prevent spark discharges. Based on experience, brush discharges will not ignite dust/air mixtures. Lightening-like discharges in silos are not to be expected ac-cording to VDI 2263. Lightning High-voltage protection equipment is to be provided by the customer in accordance with the general pro-visions governing the protection of equipment against high voltage/lightening, especially the paragraph on explosion hazardous operational areas and storage areas. Electromagnetic waves 104 Hz to 3.1012 Hz N/A Electromagnetic waves 3.1011 Hz to 3.1015 Hz N/A Ionizing radiation N/A Ultrasound N/A Adiabatic compression, shock waves, flowing gas N/A Chemical reactions N/A Should the concept of „Prevention of Effective Ignition Sources“ not be sufficient, design measures are required which we will now review in more detail. Since inertisation is usually a very expensive and complex protective measure, it shall not be discussed in detail.

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2.2 Suppression Explosion suppression utilizes a recognition system which detects an explosion in the early stages and interrupts its propagation through the rapid dispersion of an extinguishing medium (powder or water). The extinguishing medium is dispersed in the equipment to be protected via several high-pressure extinguish-ers. Once the extinguishers have been activated, they must be refilled by the service personnel. The entire system, including the connected pipelines, must be cleaned before production can be started up again. This means downtimes and, therefore, loss of production, because explosion protection is not effective anymore until the suppression system is ready for operation again. The advantage of explosion suppression systems is that they can also be used for dust explosions with toxic dusts and independently from the place where the equipment to be protected is located. Further-more, the equipment must only be designed for a maximum gauge pressure of 1 bar (VDI 2263). There-fore, this protective measure is also well suited for existing systems which were not designed for higher pressures!

Fig. 4: Rate of pressure rise in the course of an explosion with/without suppression in a vessel (optimum concentration)

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Explosion suppression systems (Fig. 2) consist of a sensor unit which registers a starting explosion, pres-surized extinguishers and a control/monitoring unit. Via this control/monitoring unit, the valves of the ex-tinguishers are activated when the sensor unit registers a starting explosion and the extinguishing me-dium is dispersed in the vessel.

Fig. 5: Elements of an explosion suppression system

The contents of the extinguisher is discharged into the equipment to be protected in as short a time as possible (Fig. 3) and evenly distributed. By this action, the explosion flames will be extinguished and the max. explosion pressure to be expected in the area of Pmax = 7 to 10 bar will be reduced to Pred ≤ 1 bar (see Fig. 1).

Fig. 6: Effectiveness of an explosion suppression system (schematic representation)

Emergency shut-down of the system

Activation of shut-off

AlarmExtinguishers Sensors

Control unit

End of sup-pression

SuppressionSuppressionStart of suppression

Sensor registers starting explosion

Ignition

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2.3 Explosion pressure relief Explosion pressure relief is a safe and economical measure for the protection of all types of dust process-ing systems. This safety concept allows our customers to protect even large volume vessels, e.g. silos, at a reasonable price and is virtually maintenance-free. It is the objective of explosion pressure relief to release the maximum expected explosion pressure (Pmax) via a relief area into the open air so that the reduced pressure (Pred) is below the pressure resistance of the vessel. The size of the relief opening is determined by the shock-pressure tightness of the vessel. For the calculation of this variable, refer to VDI 3673! Explosion pressure relief also allows the protection of very large vessels or systems with very low pres-sure resistance values. For the realization of explosion pressure relief measures, several different possibilities are available. For systems where the process or manufacturing requires that they be set up inside production halls, it must be ensured that the propagating flames are released into nonhazardous open-air areas. This meas-ure, however, results in costs for exhaust ducts in addition to the loss of valuable operating space. The quench pipe allows the explosion pressure to be released into the room with no spreading of dust or flames. Burnt and unburnt particles are retained by the integrated dust filter, thus avoiding an explosion. Systems can be protected without complex and expensive exhaust ducts. Thus, system parts may be set up to meet optimum process criteria. With the quench pipe, the required relief area is much smaller than the required area for using an exhaust duct.

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3. Explosion disengagement In all applications where dust explosion hazardous vessels and equipment in systems are connected via pipes, there is the risk of propagation of a dust explosion through these pipes to other locations. During this propagation of an explosion, turbulence, displacement and supercharging effects could either cause excessive explosion pressures or even detonations. To prevent the propagation of an explosion through connecting pipes, it may make sense to separate specific system parts from the effects of a pos-sible explosion by using suitable devices, i.e. explosion disengagement. The use of such disengagement equipment is always required in the following cases: − When the unprotected system section which could be, for example, depressurized must be safely

separated from the protected section of the system where effective ignition sources and, thus, explo-sions must be anticipated.

or − When vessels are connected through longer pipes so that flame jet ignition or high pressure peaks

must be anticipated. This is especially problematic, if a large vessel releases pressure into a small one, or when a vessel with a higher pressure resistance is connected to vessels of smaller pressure resistance.

Depending on the area of application, system parts can be separated by using e.g. rotary valves, sup-pression barriers, rapid action valves or diverters. To establish the connection between above described measures and reality, the following pages list the measures recommended by REIMELT for application with its systems.

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4. Rules for the prevention of dust explosions at REIMELT : 4.1 General rule:

• Consequential prevention of effective ignition sources wherever possible!

4.2. Additional protective measures are required in the following cases:

• If the minimum ignition energy of the handled dust is < 10 mJ (CAUTION: increased temperatures reduce the minimum ignition energy!).

• If the products show a tendency towards selfignition.

• If the prevention of effective ignition sources is impossible, e.g. high-speed machinery as sugar

grinders, rotary sifters, etc.

• If all 13 published ignition sources cannot be safely excluded.

example: Crystal sugar: Crystal sugar has a high specific resistance ⇒ elektrostatic charging is theoretically possible ⇒ electrical discharges could occur in conical piles ⇒ Constructive explosion protection is necessary

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5. Design recommendations for individual aggregates (in case of protection systems and/or devices with own potential ignition source these de-vices must have a corresponding authorization referring to ATEX 95=. 5.1 For outdoor silos and indoor silo systems if the explosion pressure can be released into the open air: Maintenance-free three-piece rupture disks The protection of systems with rupture disks is a very economical possibility of explosion pres-sure relief. The advantage of the dished three-piece rupture disks is their accurate activation pressure. Their low-mass design allows fast explosion pressure relief and, thus, reduced explosion end pressures (Pred). Due to their special design features (dished, three-piece), three-piece rupture disks are very rugged pressure relief devices. Their nearly unlimited service life offers the highest degree of efficiency, availability and safety. This allows the pressure resistance of vessels or silos to be very low due to the reduced explo-sion end pressures, making the design cost-effective.

Three-piece rupture disk − rugged − maintenance-free − accurate activation pressure − unlimited service life − economical

Fig 7: Three-piece rupture disk

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5.2 For indoor silo systems (< 25m3 volume each silo) or other large units if pressure relief into the open air is economically not feasible: Our customers’ increasing demand for a pressure relief system with no flame propagation can be met by our quench pipe. The quench pipe allows the explosion pressure to be released into the room with no spreading of dust or flames. Burnt and undamaged particles are retained by the integrated dust filter, thus keeping the material from supplying the explosion. The quench pipe guarantees safety from secondary explosions in the outer room. Extensive testing at international test facilities has proven safety under the most extreme conditions. Systems can be protected without complex and expensive exhaust ducts. Thus, system parts may be set up to meet optimum process criteria. With the quench pipe, the required relief area is much smaller than the required area for using an exhaust duct.

Fig.:8

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5.3 Continuous pneumatic conveying equipment The probability that explosive dust/air mixtures occur depends on the applied conveying me-thod, e.g. dilute-phase conveying, strand conveying, slug conveying. With continuous pneumatic conveying systems, the protective measure „Prevention of Effective Ignition Sources“ is generally considered to be sufficient. When applying the concept of „Prevention of Effective Ignition Sources“, it must be ensured that − no hazardous electrostatic discharge processes occur, e.g. by using conductive materials

and by grounding conductive components, − pressure generators, e.g. fans, are located in areas without product (clean-air side), − the temperature of the conveying air shall not create ignition hazards,

(The characteristic values selfignition temperature and ignition temperature should be con-sidered in particular. Since the minimum ignition energy is temperature dependent, a high conveying air temperature can be an additional ignition hazard.)

− no hazardous friction and impact processes are created by impurities carried along by the

conveying system, (In general, friction and impact generated sparks do not pose an ignition hazard. This is es-pecially applicable if the minimum ignition energy of the conveyed dust is >10 mJ).

− no effective ignition sources can enter continuous pneumatic conveying equipment.

(Experience shows that in many cases there is no immediate danger of igniting explosive dust/air mixtures through glowing particle nests inside continuous pneumatic conveying sys-tems. However, the possibility of ignition sources entering and transferring via the equipment should be considered.)

In most conveying phases, continuous pneumatic conveying equipment has a very low dust explosion risk. In particular, this can be attributed to the reduced probability of occurrence of explosive dust/air mixtures and the increased minimum ignition energy through turbulence.

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5.4 Oscillating conveyors With oscillating conveyors, the generation of explosive dust/air mixtures is usually not antici-pated. In general, only electrostatic charges may present effective ignition sources. Hazardous electrostatic discharge processes must therefore be prevented. This is ensured, for instance, by using conductive materials and grounding all conductive parts. . 5.5 Screw conveyors The occurrence of explosive dust/air mixtures inside screw conveyors cannot be excluded. Possible ignition sources in particular are: − hot surfaces through friction and grinding processes, − increase in temperature due to blockage of product, − electrostatic discharge processes. The occurrence of effective ignition sources increases as

the circumferential speed increases. Possible protective measures to prevent effective ignition sources are as follows: − reduce circumferential speed, (based on experience, no hazardous friction or grinding proc-

esses are to be expected if the circumferential speed is ≤1 m/s.) − avoid indoor storage − prevent impurities from entering the equipment − prevent dangerous blockage of product, e.g. through overload protection, stop valves − avoid dangerous electrostatic charges, e.g. by using conductive materials and grounding all

conductive parts. 5.6 High-speed mills High-speed mills, such as pin mills, hammer mills, impact grinding mills, must always be con-sidered to be possible ignition sources. Therefore, suitable protective measures (inertisation, design measures) against dust explosions as well as their propagation must be applied inside the mill as well as in the upstream and downstream equipment. In any case, impurities should be prevented from entering the equipment to minimize the risk of ignition. Only in special exceptional cases can the measure of „Prevention of Effective Ignition Sources“ be applied (e.g. if minimum ignition energy and ignition temperature of the processed dust are extremely high).

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Exemplary design of a high-speed mill (schematic representation) for fluidizable prod-ucts protected through explosion pressure relief devices

Fig.:9

dust control

Filter socks on filter cages

Exhaust pipe; alternative: quench pipe

Mill

Rotary valve

Clean air duct Vent-ex valve, etc.

Rupture disk

Mill bunker

Rotary valve

Air

Product

Explosion pressure release

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5.7 Rotary sifters with interior moving components Contrary to rotary sifters without interior moving components, mechanically generated sparks and hot surfaces must be particularly considered when using sifters with interior moving compo-nents. With slow-moving components inside (v ≤1 m x s-1) and small motor sizes (HP ≤4 kW), experi-ence shows that there is no danger of ignition to be expected.

Fig.:10 With higher speed components inside and/or larger drive sizes, explosion protection through design measures or inertisation may be omitted in the following cases: − If design measures prevent effective ignition through mechanical sparks and hot surfaces,

e.g. by selecting suitable material combinations, such as screen structure made of plastic (REIMELT standard).

− If the distance between the moving parts is large enough to prevent impurities that may have

entered the equipment from initiating dangerous friction processes. By using impurities sepa-rators (e.g. prescreen), the size of the entering impurities can be limited.

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5.8 Mixer Due to process engineering technology generally used with mixers, e.g. design features or high filling levels, the occurrence of explosive dust/air mixtures is frequently limited. If this is the ca-se, mixers can be safely operated by applying measures of „Prevention of Effective Ignition Sources“. Otherwise, further measures must be applied (e.g. inertisation or explosion protection through design measures). Mixers without interior moving components Mixers without rotating components inside are, for instance, spiral, drum, double-cone, asym-metric moved and air mixers. Since this type of mixer design does not require mobile components, such ignition sources (e.g. sparks or hot surfaces) can be excluded. When applying the measure of „Prevention of Effective Ignition Sources“, it must be ensured that − no ignition sources, e.g. glowing particle nests, enter the equipment − dangerous electrostatic charges are prevented by grounding (shunting resistance against

earth < 106 Ω) and by using conductive materials for lining − bearings which might reach into the product area are protected from heating up, e.g. by us-

ing air purged bearings or monitoring the temperature − with heated mixers (e.g. heating jackets or hot air flows), the temperatures are limited so that

no dangerous reactions (ignition of layered or turbulent dust, selfignition processes, disinte-gration processes) might occur.

5.9 Mixers with interior moving components Contrary to mixers without moving components inside (item 8.1), mechanically generated sparks and hot surfaces must be considered for mixers with interior moving components. With slow-speed interior components (v ≤1 m x s-1) and lower driving capacities (w ≤4 kW), ex-perience shows that there is no danger of ignition to be expected. With higher speed components inside and/or higher driving capacities, explosion protection through design measures or inertisation may be omitted in the following cases: − If the high filling level (≤ 70%) limits the occurrence of explosive dust/air mixtures. − If the mixing velocity is reduced (circumferential speed ≤1 m x s-1) during the filling and dis-

charge process, and so-called choppers or disintegrators are not used. − If there is sufficient distance between the parts moving against each other so that they do not

touch under any circumstances, no matter what the operating mode. − If hot surfaces which could become effective ignition sources can be prevented through im-

purities separators.

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− If no products are used which, under the anticipated operating conditions, such as thermal and mechanical wear, tend towards selfignition.

− If dangerous product caking is prevented by design measures or, if applicable, by appropri-ate cleaning methods.

Fig 11.: example for mixer with mobile components inside

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6. Photos showing silos after dust explosions

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This publication should make it possible to design state-of-the-art systems that are safe with respect to dust explosions. Should you have any further questions, please do not hesitate to contact the experts at REIMELT any time. Rödermark, den 16.03.1998 Dipl. Ing. H.-J. Sussann

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7. References • Bia-Report 11/97. • VDI-Bericht 1717. • BGR 104 Band II. • BG Zucker Entscheidungs- und Maßnahmenkatalog 02/99. • Leitfaden „Explosionsfähige Staub/Luftgemische und Störfallverordnung, Teil 1 Anwendungsbereich,

Stand 01/1997. • IVSS-Broschüre des AK-2: „Staubexplosionen; Stetigförderer“. • IVSS-Broschüre des AK-6: « Beispielsammlung; Staubexplosionen an Maschinen und Apparaten“. • ZH1/200. • Broschüre der Fa. Rembe-Brilon. • Reimelt-Werknormen.

Reimelt GmbH · Messenhäuser Straße 37-45 D-63322 Rödermark, GermanyTel. (49) 0 60 74 / 691- 0Fax (49) 0 60 74 / 60 31email: info@reimelt.dewww.reimelt.de

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