Failing to adequately protect an employee from dust exposure can lead to criminal prosecutions for the company owners who are responsible for the protection of their employees. For EU member states, the level of protection required is based on the IOELV (Indicative Occupational Exposure Limit Values) for each dust type. Each EU country will base their own workplace exposure limits on the Chemical Agents Directive (98/24/EC) and the figures derived by The Scientific Committee on Occupational Exposure Limits (SCOEL). Flour dust in bakeries for example, causes a significant number of occupational asthma cases because of its high percentage of fine particles. As such, the OEL for flour dust in the U.K is set at 10 mg/m3 for long-term exposure, but there are calls for this figure to be reduced further. For more toxic dusts such as many pharmaceutical ingredients and metals, the OEL values are much lower and therefore require extra attention when considering the best methods of dust collection and containment to adequately protect the workers.
Exposure to airborne dusts can be prevented using the appropriate Personal Protective Equipment (PPE). Many production environments however are hot and humid so PPE can be uncomfortable to wear over long periods. It can also be a very expensive, ongoing cost which can be fully or partially eliminated by capturing the dust at source using a well-designed dust collection system. The airborne dust will be captured using a suitable hood or machine connection point, transported through a ducting system to the dust collector where the filters remove the dust from the air. The clean air can then be sent to atmosphere or recirculated back into the building to reduce the heatloss and help save energy.
2. DUST EXPLOSIONS
The second major challenge relates to dust explosions, a serious safety risk which is often not given the required consideration it demands. There are many examples of serious dust explosions that have caused significant loss of life and these are still occurring today.
A dust explosion occurs when aconcentrated explosive dust cloud comes into contact with an ignition source. Many food and beverage solid ingredients produce explosive dusts including sugar, starch, flour, spices, tea, grain and proteins. With good house-keeping and by installing a well-designed dust collection system, the airborne dust is prevented from building up in the work environment, on electrical equipment and areas where dust can accumulate, such as false ceilings. This will help to negate the risk of a primary or secondary explosion. The primary explosion is the first point where an explosion occurs and is usually an isolated incident. A secondary explosion occurs when the primary explosion pressure disturbs the dust collected in the areas mentioned above, creating a far more extensive and potentially deadly explosion.
In Europe, the ATEX Directives were developed to provide a legal framework to reduce the risk of explosions in the workplace and also provide safety measures to ensure that if an explosion did occur, it could be safely controlled to minimise the destructive potential. The 2 directives in particular that are relevant to dust collection are as follows:
1) Directive 99/92/EC (also known as ‘ATEX 137’ or the ‘ATEX Workplace Directive’) on minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres - defining ATEX zones.
2) Directive 2014/34/EU recently replacing 94/9/EC (also known as ‘the ATEX Equipment Directive’) on the approximation of the laws of Members States concerning equipment and protective systems intended for use in potentially explosive atmospheres. The dust collector must comply with this directive if it is located in an ATEX Zone and/or it is collecting potentially explosive dust. Compliance is achieved by incorporating the necessary safety features determined by the explosive potential of the dust.
In food and beverage manufacturing it is common for a dust collector to be extracting air mixed with potentially explosive dusts and/or gases. It is therefore critical for the manufacturer to know the explosive potential of the dusts, gases and dust/gas mixtures they are handling. It is relatively inexpensive to get the dusts tested to determine the required information as described below:
• Kst – the normalised maximum rate of explosion pressure rise (measured in bar m/s). The dusts can then be categorized as follows to help with the protective equipment selection.
• ST1 Dust – Kst 0 bar m/s up to 199 bar m/s
• ST2 Dust – Kst 200 up to 299
• ST3 Dust – Kst 300 +
• Pmax – the maximum explosion pressure of a dust cloud (measured in bar).
• MIE – The Minimum Ignition Energy (mJ) of a dust cloud by electrical and electrostatic discharges.
Once these values are realised, the internal and external ATEX Zones for the dust collector can be determined by undertaking thorough risk assessments of the process to which the dust collector is applied, as well as the area in which the dust collector is installed. The dust collector can then be designed with all the appropriate safety systems installed, based on ATEX zone identification and ATEX Category shown below:
This information will help in the correct specification of the safety features that need to be applied to the dust collector which include the following:
Antistatic filter cartridges and dust collector earthing ensures that any static charge that builds up can be dissipated, preventing the possibility of a spark as an ignition source.
A vent panel can be installed on a dust collector to safely dissipate an explosion pressure and flame. During an explosion the vent panel will rupture and the explosion pressure wave and flame will be directed towards a pre-determined safe area. The pressure wave and flame can be sent horizontally or vertically using vent ducts reinforrced to the same level as the dust collector.
These devices use the similar sizing criteria as the vent panels. They safely vent the explosion pressure and have the added benefit of stopping the flame propagation.
These systems are designed to detect and chemically extinguish an explosion before it can propagate.
There are a wide range of valves that are designed to prevent any flame front from an explosion travelling down the dirty air ducting and causing secondary explosions in the production facility.
3. DUST EMISSIONS
The third challenge relates to the dust emissions to atmosphere that must be controlled either to comply with legal limits set for a particular material, or if no legal limits are applicable, then the limits will be set by the company for hygiene purposes, pest prevention and environmental management standards such as ISO14001. Each food or beverage manufacturing application will have its own unique set of process conditions.
These process conditions, the characteristics of the dust being collected and the particle emission limits to atmosphere will be the determining factors for the choice of filter media/material. Filter materials will have a filtration efficiency which relates to their ability to capture a percentage of dust particles of a certain size. The correct filter material can then be selected to meet the emission levels set. The higher the efficiency of the filter material then the cleaner the air being released to atmosphere.
Dust collectors will have an automatic filter cleaning operation to maintain the efficiency of the filters. Some food and beverage dusts can also be oily and/or sticky, particularly when moisture is present, which can affect the performance of the filter materials and their ability to effectively release the dust during the cleaning cycle. Special filter materials can be employed to improve the ability of the filters to clean-down to ensure they achieve an acceptable service life.
To summarise; the effective control of dusts being generated in food and beverage manufacturing facilities is an essential, legal requirement. Dust can cause serious harm to human health, have an impact on the environment and cause devastating explosions with a huge potential to cause harm to workers, damage machinery and destroy buildings and reputations. There are a number of critical steps that therefore need to be employed when selecting the most appropriate dust collection system for each and every application:
1. Understand the dust characteristics
Ensure all the necessary information relating to each product and dust being generated has been determined, including the OEL, Kst, Pmax and MIE.
2. Employ the experts
Use a reputable dust collector company to ensure that the dust collection system is properly designed.
3. Total Cost of Ownership
By implementing the required safety features on a dust collection system, the capital cost will increase. It is therefore imperative that the running costs of the dust collection system are calculated to ensure the operational costs are kept to a minimum.