FREQUENTLY ASKED QUESTIONS

Dust explosibility classification (Group A/B) test using the Vertical Tube apparatus to determine whether the material falls within the scope of the necessary requirements for explosion protection of plant, buildings, etc.

Group A (hazardous) – the dust cloud propagates flame from the ignition source and is classed as a combustible and potentially explosive material

Group B (non-hazardous) – the dust cloud does not propagate flame from the ignition source and is classed as a non-combustible (non-hazardous) material.

If you are unfortunate, enough to need further tests give us a call to discuss the range of tests appropriate to your needs.  Many clients have tests carried out which were not necessary for their project.  We have 50 years of experience in what’s really needed.

Whenever possible, the employer should prevent the occurrence of explosive atmospheres. Explosion risks must be assessed. In the event of an explosion, workers are at risk from uncontrolled flame and pressure effects. If all prevention measures have been exhausted, then protection measures must be employed, to mitigate the consequences.

We have a large range of protection systems at our disposal, if we do not make it ourselves, we can source it from colleagues. With 50 years of experience satisfying our client’s needs, we know it’s best to sell solutions not just products to build client loyalty.

In a system consisting of inter-connected enclosures, a dust explosion ignited in one enclosure can propagate through the connection, generating increased turbulence, perhaps causing some pre-compression and then acting as a larger ignition source in a connected enclosure. This combination of effects can enhance the violence of the secondary explosion and the protection requirements of the system thus need to be increased, or the enclosures isolated.

Explosion isolation is achieved by a protective system, which prevents an explosion pressure wave and /or flame from propagating via connecting pipes or ducts into other parts of apparatus or plant areas. Systems providing complete isolation by operation of the isolation device(s) preventing the propagation of the flame as well as pressure effects. Systems providing partial isolation only isolate the flame propagation. This distinction is important for practical applications, because it is not necessary in all cases to achieve a complete isolation of flame and pressure. In some cases, it is sufficient to achieve only flame isolation.

ATEX Explosion Hazards Limited has worked in Dairy, Food, Chemical, Pharmaceutical, Cement, Power, Biomass and Waste.  We know our own products and we endeavour to know our clients processes and safety challenges.  We network, sit on standards committees, attend industry seminars and keep up to date with industry trends.

Accurate sizing of vents is the most important aspect of vent design. The size of the vent depends on the explosion characteristics of the dust, the state of the dust cloud (concentration, turbulence, distribution and filling), the geometry of the enclosure and the design of the venting device. The two principal explosion characteristics of the dust are the maximum overpressure Pmax in barg and the dust explosion constant Kst Bar m/s.

For cubical enclosures, Pmax and Kst are essentially independent of enclosure volume. The volume of the enclosure and the length-to-diameter ratio L/D relevant to the shape of the enclosure and the position of the explosion vent are required for sizing vents. The explosion resistance of the enclosure Pred, max is also required for vent sizing. All parts of the enclosure, e.g. valves, sight-glasses, manholes and ducts that are exposed to the explosion pressure shall be taken into account and the explosion resistance of the weakest part shall be taken as the explosion resistance for the enclosure.

The two principal vent device parameters are the static activation over pressure Pstat and the venting efficiency of the venting device.

Many times clients will decide to reduce the pressure resistance of their vessel because they do not know the strength of their vessel; this can have very expensive consequences. We know the most cost effective ways to get that information and whither it is worth it.

If the pressure venting device is activated and there is a vent duct downstream of the system, then such a duct can be filled with an explosive mixture before the flames exit the protected vessel. This will result in a secondary explosion in the vent duct, which in turn, hinders the venting process. Therefore, the maximum reduced explosion overpressure inside the vessel will increase with the increasing length of the vent duct.  The Vent duct should be equal to the Cross Sectional area of the vent, as straight as possible without a bend greater than 20 degrees or any other obstruction to hinder its venting. Many times, equipment is sold to a client by a third party not always the OEM.  The end-users contracts a fabricator who may be skilled in air-conditioning but not explosion protection. If you have any concerns please call us, we do not charge for simple advice.  It’s in our best interest that our countries industry is a safe place to work and that all explosion protection measures are properly designed.

For maximum vent efficiency, we don’t want anything on the end of a vent duct but this is not practical for most applications.  Therefore, we are allowed cover the end of a vent duct with a membrane, which is light enough (<0.5kg/m2) to open at ½ the burst pressure (typically 50 mbar) of the protective vent. Foil or Polystyrene  are extremely common.  If on a roof please ensure a safety cage is underneath to prevent falls. A slight angle to minimise water or snow retention is desirable.

Explosion venting is always accompanied by flame propagation plus pressure consequences in the surrounding areas. The reason for this is the unburned product, which is pushed outside once the vent system actuates. A flame jet subsequently exits the vent area, then ignites the fuel-air-mixture generated externally. Dependent on the volume of the equipment it can reach up to 60 m. The venting process should not endanger personnel, the operation of any equipment and should not be restricted. This shall be considered when designing the plant and may be accomplished by releasing the pressure upwards, which is effected by gravity, i.e. length of flame = 8 x V 1/3 in m or horizontally length of flame = 10 Volume 1/3 in m, which usually causes bigger problems in a congested complex. Larger fireballs have been observed during a vented dust explosion in cases where additional dust deposits accumulated in the vicinity of the vent opening.

We can offer a full range of options from flame diverters, flameless venting or suppression, which may save you a lot of financial trouble in the short or long term.

ATEX certified Rotary valves shall only be used as “explosion barriers” for enclosures. The effectiveness against flame propagation (minimum explosion safe Gap) and their pressure rating shall be ATEX certified. After an explosion the rotary valve shall be stopped automatically to prevent transfer of glowing material.

If the Rotary valve is not ATEX certified or fails to fulfil the ignition breakthrough requirements due to wear and tear, a product plug having an adequate height, is suitable in combination, as an Isolation system.   The product layer height must be ensured by a level indicator, that under the pressure stress of the explosion, no flames can pass through the product. Level indication must be ensured as fail safe and as a double knock system by operation.

Many clients have replaced rotary valves for explosion barriers, to save costs in industries where wear and tear costs more than originally desired. Competency is more about experience when it comes to cost effective safer design.  Many things are complaint when installed new but 6 to 12 months down the road, the issues come to the fore.  Please call us first to save you this painful experience.

When an explosion vent opens because of a dust explosion, a fireball or jet of flame must be expected. This can carry out a mass of burning and unburnt dust. In addition, there will be a pressure wave associated with the explosion. If the vent opens inside the building the burning dust may start further fires, and the blast may damage nearby plant. Anyone inside the room or building may be at serious risk. For these reasons explosion vents which discharge inside a building will give people inside the building little protection from the explosion. The usual solution is to fit a duct to divert the explosion to a safe place in the open air. You will need to keep personnel away from an area around the end of a vent duct.

We think with 50 years of experience we have seen it all.  A client had a cyclone in the middle of a large room. The vented flame was calculated to be 15m long, 5m short of the roof and 6m wide, when the nearest walkway was 10m away.  The vent would only cost 500 euro but the pressure from the secondary explosion in the room would have been from 168 mbar at the epicentre, to 32 mbar 20 meters away! At 30 mbar you will break glass and cause minor structural damage, never mind the damage to personnel.  The client has now purchased a flameless vent for €5,000 euro.

Proprietary flameless venting devices, which quench flames and catch burning dust, are certified for use inside a building. The flameless vents do have restrictions on their use due to the backpressure they cause in quenching the flame and filtering the majority of the burning dust. There will be room volume restrictions for exhausting the unburnt gases and a safe area, which should not be populated by a permanent work station and loose flammable material. The supplier’s’ advice concerning installation must be followed carefully.

We have been installing flameless vents for many years now and have listened to our clients and seen how they survive various environments.  Check out our new revisions with improved features, which open flameless vents to a wider range of applications, especially those requiring higher hygiene requirements and serviceability.

You have to be very careful about ATEX compliance as there are 2 forms

The ATEX 114 directive or the product Directive 2014/34/EU is for equipment and protective systems for use in potentially explosive atmospheres. This qualifies you to place your products on the market by different categories suitable for specified Explosion Zones. If not self-certified for the lower risk categories, you will need Notified body approval. One must be very careful about the difference between component approval and system approval. Having a component such as a sensor on a simple assembly such as a silo or a vent panel does not automatically give you system approval. Many suppliers have added the certificate of a component(s) in an assembly and are claiming system approval.

The ATEX 153 directive or Users directive 99/92/EC is for the introduction of measures to encourage improvements in the safety and health of workers at work in explosive atmospheres.

  • preventing the formulation of explosive atmospheres where this is possible
  • preventing the ignition of explosive atmospheres
  • mitigating the detrimental effects of an explosion so as to ensure the health and safety of workers and where necessary
  • preventing propagation of explosions

The end user is responsible to ensure the overall explosion safety of their facility, which must be verified by competent person before use. We always recommend that you employ the services of a consultant with a track record in explosion technology but also with experience in your particular industry. Always remember people and their actions, cause explosions. Zoning and system approvals does not make you any safer, if your workers have not been properly trained.

The first part of good explosion prevention is to have proper and detailed data on the Flammability and Ignition characteristics of your hazard, be it gas, powder or vapours. Once you know your product, you need to consider appropriate safety measures. Prevention measures such as avoidance of the identified ignition sources in your plant, the selection of the proper equipment for your designated explosion zones. You may still decide that you need prevention safety equipment to reinforce your safety measures, such as Inert gas blanketing to reduce O2 levels or spark detection and extinguishing, temperature monitoring or gas monitoring such as the ATEX CO monitoring system, which is very popular in the Dairy powder drying industry. Ultimately, a well-trained workforce is your ultimate prevention measure.

A well-trained employee is as important as a well-informed employer.  We can offer presentations on the basics of explosion safety, application driven, suited to your industry.  We want to share our experience with you to make a safer industry.

We have 50 years of explosion safety experience in a wide range of industries. We can send out the appropriate consultant verified by us to meet your industry needs.

During the 1970’s and 80’s the Dairy industry had a very big problem with existing protection options. Hygiene and personnel safety was becoming a bigger issue, especially for their large volume dryers up to 1000m3. Explosion venting inside the building was common, even if venting was not possible especially on Fluidbed and cyclones. Chemical suppression with 60 bar powder bottles opened by explosively actuated rupture discs was unacceptable. Pressure Hot Water Explosion suppression used 10 bar hot water bottles at 184C releasing through pyrotechnically actuated lockable valves. When released the water instantly dropped to less than 100C, flashing off to 1600 times its original volume. This is due to excess pressure and temperate, producing16% steam and 84% micromist water droplets small enough to instantly quench and wet the milk powder fireball. After system activation, the bottles did not need to be removed only valves reset, refilled and the heat turned back on.

Although explosion doors were the main type of explosion venting device, they have now been replaced with more sophisticated vent panels.  These panels in their most basic form are relatively cheap flat single profiled stainless steel discs, stitch cut on 3 sides and release at typically 100mbar with virtually 100% efficiency. These panels get more expensive, if you have higher-pressure fluctuations, vacuum resistance, temperature and possible personnel safety, as you cannot walk on a vent panel. If they activate, they stay open, allowing oxygen into the subsequent fire and are replaced by spares. Explosion doors can be self-reclosing, endure all the conditions described above and are reclosed after the event. They are still very common on the cement and steel industries, where such problems are unacceptable.

Ownership is initially about what’s best for your maximum protection.  This may be a passive system like vents and float valves or active like slide valves, suppression. Either way they need to be inspected at least annually by a competent Engineer.

“Competent persons are persons with comprehensive expertise in explosion protection as a result of their professional training, relevant experience and current professional activity”

Passive requires less maintenance over active, as there are moving parts plus sensors and a control system. Most of our systems have up to 10 years life span, with a good service record and most important good service back up.  We also train all our clients to be able to carry out all maintenance needs between inspections, so that with the appropriate spares, they only need to see us once a year.

The end user does not need to certify the product under ATEX 114, that’s the OEM’s responsibility.  If it is Cat 3 for Zone 2 or 22 then its self-certification.  If its Cat 2 or 3 then it will involve a notified body, depending on the nature of the product; Mechanical or Electrical. The End user under ATEX 153 only needs to purchase the appropriate category for the relevant Zone.

It was assumed that dust explosions could not propagate through <100mm diameter due to the surface to volume ratio of the pipe. Now explosions have been recorded propagating down less that 50mm diameter! Therefore, we now have to produce protection isolation for DN50.

There have been old rules of thumb; that less than 1m diameter and less than 1m3 did not require protection, but this has all been disproved. Similar to minimum pipe diameters the Pmax reduces with volume to surface ratio and turbulence influences.  Mills are a good example. Small mills with a lot of rollers or balls produce about Pmax 2 to 3 bar even though they are normally designed for 10 barg. This is why some mills are built to 3.5 barg based on NFPA 85 Boiler and combustion systems @ 50PSI.  Filters may produce 10 barg if they did not have the filter elements inside but due to the large surface area of the filter elements the surface to volume area is huge and becomes more relevant the smaller the filter. This may mean Pmax <1barg but without a proper range of type testing or a peer reviewed programme of testing this cannot be confirmed.

Yes this can be done but be careful.  Flat panels curved to small diameters can distort Pstat. Therefore, we offer pre-curved panels, tested in the factory before dispatch.

Vent panels rupture because there has been a pressure greater than 100mbar typically from an explosion event.  This vents the pressure at this point only.  It does not stop secondary explosions nor does it stop a potential fire.  You need to remove the fuel and the oxygen; this can only be done by installing a vent sensor, to initiate an Emergency stop.