An LEV system is an engineering control system that will control and remove harmful fumes, dust, oil mists, aerosols, gases and vapours from the working environment and is a highly effective way to prevent or reduce work force personnel exposure to hazardous substances.
Local Exhaust Ventilation system
A good, well designed Local Exhaust Ventilation system will:
- Collect the contaminated air from the working environment
- Control airborne contaminants and prevent workers exposure to contaminants
- Clean or filter the air to remove the contaminant
An LEV system will likely comprise collection hoods, ductwork, filtration systems, fans and a discharge. The discharge may be either internal or external to atmosphere, depending on the nature of the substance.
Components of a LEV system
These are potentially the most effective type of enclosure. Any contaminant is contained, extracted and filtered, thereby preventing it from entering the work environment. Examples include:
- Full enclosure – gloveboxes, shot blast booths.
- Partial enclosure – fume cupboards, spray booths
Receiving hoods: these utilise the directional velocity of contaminants from a process to control exposure, for example, grinding, cutting and hot processes.
Capture hoods: are used when the process, source and contaminant field would be outside of a receiving hood area. These must be sufficient to ‘capture’ and draw in the contaminant within an effective ‘capture zone’.
Cyclones: Centrifugal action spins captured particles to side walls where they drop into a collection hopper allowing for safe removal. A cyclone system is suitable for larger particles.
Wet Scrubbers: Water is used to wet captured particles to remove and contain them from a contaminant cloud. A wet scrubber will likely be used to control hot gases, sticky particles, explosive dusts and vapours.
Electrostatic Precipitator: A high voltage charge is given to captured dust or fume and collects particles against oppositely charged electrode plates. These are suitable for fine dusts but unsuitable for heavy contaminant or high temperature and corrosive applications.
Fabric Filters: Contaminated air is passed through a woven or felted medium and the captured contaminant is physically trapped. There are varying classes of filter depending on the nature of the substance and these can be a highly efficient medium for control of a hazardous substance.
HEPA (High Efficiency Particulate Absorbing) Filters: Filter small particles and finely clean air to return into the work environment, with filtration to 99.95% or above of contaminated air.
Adsorption/Absorption: These systems are usually implemented to remove gases and vapours using activated carbon or charcoal filtration and can be used in conjunction with particle filters.
Supplementary or Complimentary Systems
Push-Pull systems are commonly used over wider surface areas, for example, plating baths or large machine tool beds. Push jets will maintain velocities over large distances and direct a contaminant to a receptor hood, which provides extraction.
Any contaminant which is not captured at source will risk exposure to workers outside the capture zone and within the working environment. Filter towers compliment LEV systems by capturing and filtering dust, fumes or aerosols outside of the capture zone, cleaning and returning contaminated air to the work environment.
What needs to be considered when developing an LEV system
A good understanding of the process and containment required, the hazards, sources to be controlled and exposure benchmarks which classify the hazard of the emitted material, along with information on other processes and activities that occur adjacent to the process the LEV system will control.
An LEV system needs to be easy to use, check, maintain and clean, whilst also taking account of any other risks, for example accessibility and potential for contamination from waste removal or filter changing.
Whilst the employer holds obligations to ensure the safety of its workforce, the employee also has an obligation to fully utilise the control measures that are put in place, so you will need to consider any operator training that is required.
An LEV system will be accompanied by an appropriate manual that describes and explains:
- The LEV system
- How to use, check, maintain and test the system
- Performance benchmarks
- Maintenance schedules for replacement of filters and parts
- A logbook to record results of checks, maintenance and LEV testing
At this point you may need to take advice concerning the type of local exhaust ventilation system to be used, its effectiveness at controlling exposure and, of course, the costs involved.
Developing the LEV system
Once a specification has been established, a suitably qualified engineering design team would then look to carry out any necessary benchmarking exercises to assess the effectiveness of a proposed local exhaust ventilation system and its capability of controlling exposure. This will include an exposure benchmark, i.e., the level of exposure that may result once the control is in place. A contaminant must be controlled to a level as low as reasonably practical and within any maximum worker exposure levels (WEL) dictated by HSE guide EH40/2005 and COSHH regulations 2002 (as amended). At this point, it may be necessary to carry out air quality testing and exposure monitoring to establish current levels of contaminant within the workplace.
An engineering team managing design, installation and commissioning of an LEV system should be:
- Suitably qualified, eg to BOHS P602 and P604 certificate proficiency
- Able to provide operator training as part of the commissioning process
- Can carry out ongoing servicing and maintenance
- Suitably qualified with BOHS P601 certificate to carry out LEV testing and reporting to HSE and COSHH guidelines