ATEX risk assessment
Powders, when they are put in suspension (cloud), can present a risk of explosion. That is why powders are covered by the ATEX directive and the risks analyzed similarly to explosive vapours.
Powder explosions can be very destructive. Many examples have been recorded accross process industries. Explosions of flour silos, for example, can be particularly powerful. Basically, any powder that can be oxidized (=burn) can present an ATEX risk under certain conditions. Due to their frequency, to the damage they can occur, and to the lack of awareness within industry, the European Union has set up the ATEX directive which is making it mandatory for industrial to define the zones where a dust explosive atmosphere can be present and put in place the measures necessary to prevent or mitgate the risk.
The conditions that can lead to an explosion are often represented in the shape of a triangle. Each corner represents one of the conditions. In order to get an explosion, it is necessary to have a fuel (the powder), oxygen and a source of ignition.
Figure 1 : Explosion triangle
In the case of the explosion of powders, 2 more conditions need to be fulfilled : the dispersion (to have a dust cloud) and the confinement. For this reason, ATEX explosions conditions are sometimes represented in the shape of a pentagon.
Figure 2 : Dust explosion pentagon
If these graphical representation help to understand how an explosion can be created, they also give the keys to avoid accidents : if one of the above conditions is not here, explosion cannot occur. All the purpose of the ATEX analysis will then be to determine if these 5 conditions can be met at the same time, and, if yes, find solutions to remove one of them (prevention. In some cases, this will not be possible, then the explosion if it happens will have to be managed (mitigation) and the way to do that will have to be detailed in the ATEX analysis.
Powder clouds can have different origins. They can be due to the process (reception of a pneumatic transport) in the equipment. They can be due to leakages outside the equipment. Or they can be due to poor housekeeping dust deposits put in suspension.
Since the sources are varied but often related to the maintenance of the installation, strict procedures will have to be implemented to control the ATEX risk.
The ignition sources can be of multiple natures. What is important is the energy that is liberated. If this energy is sufficient, and happens in the presence of a dust cloud, an explosion may occur. The minimum energy that must be applied to a powder cloud is called the Minimum Ignition Energy (MIE) and is expressed in mJ. It is important to note that the MIE will vary from one powder to another.
This parameter, as well as others, is usually determined by specialized institutes which will carry out controlled explosion and measure at which threshold of energy, the powder will ignite and explode.
Among the parameters determined are the Kst and Pmax of the explosions, also dependent on the powder. Pmax will be the maximum pressure to be expected from the explosion and Kst will represent its velocity
The powder particle size distribution is also an important factor to know, especially for explosions in silos
A summary of the powder properties to know for an ATEX study is given below
Table 1 : Powder ATEX properties
|Property||Unit||Determination method||Use in ATEX study|
|Pmax - Maximal pressure of explosion||bar||Explosion tests in explosion cells instrumented||Will allow to calculate the consequences of an explosion|
|dP/dt max - Maximal rate of increase of pressure||bar/s||Explosion tests in explosion cells instrumented||Will allow to calculate the consequences of an explosion|
|Kst - Constant of explosion||bar.m/s||(dP/dt)max.V0.33=Kst
The value dP/dt is dependent of the volume of explosion, however, it has been found that Kst is independent of the volume and is only a function of the powder used. Thus, it is usual to use Kst rather than dP/dt to express the explosion properties of powders
|Kst is used to calculate the safety venting - explosion panel for example - to mitigate an explosion.|
In order to carry out a proper ATEX analysis, and especially be able to assess properly the risks of an explosion, the determination of some physical properties of the powder is necessary. Those properties are described in the following table.
Table 2 : Powder properties influencing explosion risks
|Property||Unit||Determination method||Use in ATEX study|
|MIT (Minimum Ignition Temperature)||degree c||Godbert-Greenwald furnace||2 ignition temperatures are determined : in dust cloud and in layer 5 mm. The dust cloud MIT will be related to powder explosion while the layer will be related to the risk of fire. Basically, it indicates what is the maximum temperature that can be authorized in the area where the powder is present - for example it helps to determine the temperature class of electric motors|
|MIE (Minimum Ignition Energy)||mJ||Ignition tests of dust cloud at different energy||Allows to identify actual hazard from ignition sources. It will show the sensitivity of the powder to a punctul source of energy like a spark from electrical or mechanical origins. The MIE is usually taken as giving a good representativity of the sensitivity of powder to the risk of explosion. Powders with a MIE less than 3 mJ are considered as very hazardous.|
|PSD (Particle Size Distribution)||Microns||Diffractometers||Allows to identify risks in silos storage (max diameter allowable)|
|Maximum permissible oxygen concentration||Percent||Vertical tube or 20 l sphere||This information is particularly interesting to design a process preventing explosion risks by inerting. Nitrogen for example can be introduced in the system to replace air. For organic powder, the limit will be generally around 11% remaining oxygen. For metal powder, the limit is much lower. Specific tests must be carried out to determine the value and a safety margin or at least 2% should be considered.|
Besides these key parameters directly used in ATEX analysis, the Engineer must also be aware of the influence of the following properties on the likelihood of explosion.
Table 3 : Other powder properties of interest in ATEX studies
|Property||Influence on explosion|
|Granulometry||In general, the explosivity of a dust cloud will increase if the particles get smaller. Indeed, the available surface for burning will increase if the solid is more divided.
On the contrary, from 200-500 microns, the explosion will not be possible anymore. It must however be noted that this limit is usually not used in ATEX study since anyway the fines present in a product with a PSD around 500 microns can be sufficient to trigger an explosion in some cases.
|Explosion concentration||There is a limit of concentration below which the explosion will not occur. Looking at litterature, this value varies from 0.010 to 0.100 kg/m3. It is highly dependent on the product.
On the other side, there is a maximum concentration limit above which the explosion cannot occur neither, It ranges from 1 to 10 kg/m3.
|Humidity||The humidity of powder is in general good to reduce the risk of explosion. The more it is humid, the less the explosion is likely. It could be interesting during ATEX analysis to know the humidity level of the powder being processed. A limit of 30 percent can be found in the litterature as a the liit above which explosion is not probable.|
The prevention of explosion will consist in AVOIDING the explosion by taking some measures to act on some of the parameters allowing an explosion - see the explosion pentagon
Table 4 : Prevention of explosion
|Housekeeping||It is the most basic but also a very efficient measure : dust must not accumulate where it is not supposed to be ! Dust in the working environment can be put back in suspension and trigger an explosion, for example. Every dust leakage must be cleaned up and root cause identified and corrected.|
|Inerting||The use of nitrogen or carbon dioxide in the process allows to go below the maximum permissible oxygen concentration to trigger an explosion. To do so, the inerting gas must be available, the process must be designed to control the concentration of oxygen and govern the inerting accordingly, in order to be always in the safe zone. The factory operator will also have to assess the risks induced by the inerting itself|
|Removing sources of inflammation||The process and its operation must be designed to remove the sources of inflammation :
Thermal : the MIT measurement must be used to design the process so that the sources of heat are always below this value. The plant operation must not authorize any intervention on the process if the process has not been properly cleaned and released through a work permit procedure.
Mechanical : sparks can be created by mechanical impact, the sensitivity of the powder is directly related to its MIE. In order to prevent such sparks, a rigourous control and maintenance plan must be put in place for each rotating equipment susceptible to create such spak. The process must be designed so that, if possible, the tip speed of rotating part is less than 1 m/s, a limit generally admitted for stainless steel as being the threshold below which spark are not anymore created. For equipment turning quicker, some means of detection that pieces are touching each other - vibration for example - must be put in place. Finally, foreign bodies must be avoided through proper sieving and metal checking of each material entering the process.
Electrical : as well related to the MIE of the powder, even small sparks from static electricity suddenly discharging can be hazardous. As a general rule, the process must be grounded, isolated parts that can accumulate static electricity must be avoided, people must wear antistatic protective equipment, silos muzst be designed so that there diameter is small enough to avoid cone discharge risks. All electric equipment must also be certified ATEX
The protection of explosion will consist in MANAGING the CONSEQUENCES of an explosion
Table 5 : Mitigation of explosion
|Explosion resistance||If it is considered that, despite the prevention measures, the risks of explosion is still too high, then the installation can be made resistance to explosion - 10 bar g is a usual value for most of the powders. Equipment must be certified.|
|Explosion venting||The use of bursting discs or explosion panels can be used to release the pressure of the explosion. Their design is very important and must be let to a professional company, although some shortcut methods exist. The point of venting must also be particularly studied to avoid any risks due to the explosion blow, or flames. Some flame arresters exist on the market if the release must be done in a building.|
|Explosion suppression||It is possible to detect the sudden increase of pressure characteristic of an explosion and trigger the injection of an extinguisher. Such system, efficient present however the disadvantages of asking particular care of the suppression system to make sure it works when needed, it will also inject some product in the process, which must be thoroughly cleaned after. It should also be install by a specialized company.|
|Isolation of process||These measures must be coupled with the previous items. Some equipment can basically isolate the part of the process submitted to an explosion. Typical equipment are the following :
Quick acting valve : often couple with a suppression system. It is an active valve that is triggered by the safety PLC when an increase of pressure is detected
Ventex valve : it is a passive valve which basically closed if pressure increases
Star valve : star valve with a minimum of 8 alveoles
One outcome of an ATEX study is to define the zoning of the installation. The zoning will help to identify which area of the installation are submitted to an ATEX risk, and to quote the level of this risk. The different ATEX risk zone are defined below
Table 6 : ATEX zoning definition
|Zone 20||Location where an explosive atmosphere is permanently present or during long periods or frequently
Dust in explosive concentration is present 1000 h or more / y (= normal process conditions)
|Zone 21||Location where an explosive atmosphere is likely to be present occasionnaly in normal working conditions
Dust in explosive concentration is present 10 h 1000 h / y (= in certain process conditions)
|Zone 22||Location where an explosive atmosphere is not likely to be present in normal working conditions or, if it is present, it is only for short duration
Dust in explosive concentration is present from 1h to 10h / y (= very rare process conditions or abnormal conditions - leaks)
The possibility to have an ATEX environment is to be analysed case by case, all along the process. The experience of the staff performing the ATEX study is key to determine when an explosive atmosphere can occur, and along which frequency. The knowledge of the process is key to be able to identify where powders are processed, what is their nature... For this reason, ATEX study should be performed by a multi-disciplinary team typically gathering Process Engineers, Production Technician and Managers, Safety Officer, as well as a moderator who will help to carry out the study in time and in a structured way.
The process should be studied looking at 2 types of events : Powder is processed inside the equipments, as it should, or some loss of containment occur. When the powder is inside the process, the perimeter of the ATEX area is usually well defined, however it may be difficult to determine at which frequency the ATEX zone happens. It is more difficult to assess where, outside the equipment, an ATEX area cam occur, and at which frequency. The process should then be studied in details, selecting the places where leakages could occur. The extent of the dust cloud formed must then be assessed.
What are the consequence of the zoning on the process ?
Once the ATEX study done, the factory needs to ensure that all equipments that can be in the ATEX area are compatible with the rating defined. This concerns the process equipment in contact with the product in normal operations (mixers...) but all the equipment outside that may be in an ATEX zone (motors, electrical switch...)
If some equipment are not compatible, then specific procedures and/or equipment upgrade must be put in place. If the risk is high, it cam lead to putting in place mitigation measures (explosion disks, explosion suppression systems...)
STEP 1 - Collect physical data on powders processed
The following parameters need to be known in order to carry out a good ATEX analysis :
- PSD (d50 or d90)
- Explosion characteristics (Kst and Pmax)
STEP 2 - Define the zoning of the production area
According to the frequency at which a powder cloud can be formed, define the area classification. For areas outside the process, define which size to consider
STEP 3 - Assess the risk of having an ignition source
Having a zone 20 for example, does not mean necessary that the risk is high. To determine if there is a high risk of having an ignition source, additional calculations may be necessary. Some of the typical risks of ignition and the calculation associated are given in the toolbox below.
Mechanical spark risk
It is generally admitted that the energy dissipated by a metal / metal contact at a speed of less than 1 m/s is not sufficient to create a spark that can cause an explosion. It is important to note that this statement may be revised if the metal in contact are not common materials like Carbon Steel or Stainless Steel. In order to check if a rotating material is at risk, it is therefore necessary to calculate the tip speed of the moving part. Such tip speed can be calculated with the following way :
Equation 1 : tip speed
With D the diameter of the rotating equipment in m
n the rotation speed in rpm
Cone discharge risk
Cone discharge can happen in vessels containing powders, especially silos. It is a discharge between a charged heap of non conductive powder and the wall of a container. Hazardous situation can happen if :
The resistivity of the powder (bulk) is > 1010 ohm
d > 0.612*MIE0.297*M-0.435
With d=diameter of the container in m
MIE the Minimum Ignition Energy in mJ
M the mediam value of the solid granulometry in mm
Brush discharge risk
It happens that, in the process, a non conductive material is submitted to an action susceptible to make it grow some electric chargers (example the flow of particles in a pneumatic transport). If this item is sufficiently charged, an electric discharge can be triggered.
It is necessary for such items to check their size and make sure it does not exceed 500 cm2. The risk is however present for very sensitive powder having a very low MIE < 1mJ
STEP 4 - Assess the risk, define measures
The last step is to assess the risk. The risk is defined as a combination to have a dust cloud (=zoning) and to have an ignition source.
If the risk is high (let's say there is a zone 20 in which an equipment is rotating very fast with a risk of metal / metal contact) then measures will have to be taken either to reduce the frequency of appearance of the ignition source (or dust cloud), or to mitigate the consequences of an explosion (rupture disk, suppression system...)
In order to help the analysis and the classification of risk, the scenarios are usually presented in a table showing the zone and the likelihood to have an ignition. Te likelihood depends on the regulation and / or the company and is usually expressed in probability to have the event over 1 year, 2 years, 10 years... Then a matrix is created and scenario in "orange" and "red" area require immediate attention and actions to reduce the risk.
Figure 2 : example of ATEX risk assessment matrix
STEP 5 - Report
All conclusions must be documented, then implemented by the production (replacement of equipment, installation of mitigation measures, specific operator procedures...)
Principles of Powder Technology, Martin Rhodes et al., John Wiley and Sons, 1990
Securite des Procedes Chimiques, Andre Laurent, Lavoisier Tec et Doc, 2003
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