Risks unmasked01 February 2007

There has been a succession of advances in dust and fume control in recent times, resulting from better equipment, improved modelling packages - to ensure that the dust and fumes actually go where they are supposed to go and smart systems that are run only when needed.

Such progress is just as well, because the pressures on plant operators have never been greater. Crucially, there is pressure from the Health and Safety Executive to reduce the 4,000 deaths and
3.3 million lost working days resulting from chronic obstructive pulmonary disease (COPD), thought either to have been caused, or made worse, by work. Also, there is a need to conform to the dust and fume explosion hazard requirements of ATEX.

According to Leigh Bramall at Ansys Fluent, "CFD has been used to develop dust extraction systems for years. The EU ATEX Directive requires that, in an environment where there is a possible risk from dust and fumes in the atmosphere, installations must meet safety standards. The UK HSE recommends computational fluid dynamics (CFD) as the best method to show compliance. The deadline to prove compliance was July 2006, but a plant operator making any changes to their operations will probably need to carry out a subsequent study to show they still comply."

Ansys Fluent cites the example of the modelling of a gas turbine installation at a power plant site in the UK, following changes in the ventilation system to ensure conformance to ATEX-137A and ATEX100A Guidance Note PM84, 'Control of Safety Risks for Gas Turbines used for Power Generation'. The example concerns hazardous fuel air mixtures, but exactly the same process would have been followed had the problem been hazardous dust in air.

PM84 declares that the size of the 50% Lower Explosive Limit (LEL) gas cloud before detection should be no more than 0.1% of the net enclosure volume (100% LEL is the fuel air mixture just rich enough to be ignitable). Additionally, the note requires that the sensor alarm threshold should be set at "ideally less than 5 per cent and no more than 10 per cent LEL" and that these detectors should be positioned in the exhaust ducting at positions where the mixing is adequate.

CFD can be used to model conditions such as emergency shutdown, cold engine and worst-case situations (eg, largest leaks), as well as orientations which can all be modelled and examined in great detail, remotely, safely and at low cost relative to experimental testing. Furthermore, it is possible to use CFD to model situations that nobody would want to attempt to investigate experimentally. For example, the US Department of Energy contracted Sandia laboratory to develop 'Guidance on Risk Analysis and Safety Implications of a Large Liquefied Natural Gas (LNG) Spill Over Water', with the intention of improving marine gas terminal safety. This was based on CFD modelling of a scenario of a tanker hitting an obstacle and producing an LNG leak from a hole just below the water surface, leading to a vast pool fire.

In the case of the gas turbine in the power plant, a model was made that included the gas generator, intake, diffuser, pipework and enclosure walls. A series of outlet ventilation ports was positioned on the roof, incorporating extraction fans that would establish a negative pressure inside the enclosure. The supply of air was from a number of inlet vents located in the side walls. Leaks were modelled one at a time. The sizes of the 50% LEL gas clouds were predicted at 3, 5 and 10% LEL sensor detection levels. It was decided to install a detection system capable of responding to 3% LEL, so it would respond before the 50% LEL cloud reached 0.1% of the enclosure volume.

As regards smart dust and fume control equipment, Tony Ashall of PlymoVent, a Swedish company, but part of the Dutch-owned Clean Air Group, comments: "What we provide is intelligent process ventilation equipment that only works when you need it. In a welding environment, the system starts the fan or speeds it up, in response to detecting either welding current or the light from the welding arc. In the case of vehicle exhaust extraction systems for garages, these start up when engines start, in response to pressure sensors in ducting. Catalytic converters only start to work when they get warm. We are also offering a similar system for fire stations. Fire engines have big engines and they give off a lot of fumes when they start up from cold. Our system automatically disconnects when the doors open and waits for the fire trucks to return."

The HSE recommends several ways of reducing dust hazards - see our 'Top Tips' panel. Nilfisk Advance has brought out a range of IVB industrial vacuum cleaners that are fully approved to handle medium 'M' class and hazardous 'H' class dust and can be used for wet or dry filters and have superior sealing. Units are equipped with air monitoring, and an alarm sounds when the filter is clogged and airflow falls below 20m/s. They can be attached directly to hand tools to remove dust at source. The vacuum motor is remotely activated with the power tool switched on or off and a speed controller adjusts the suction performance accordingly. Specialist accessories include an exhaust air collector for more stringent dust control applications, disposal bags for different types of dust, plus a choice of hoses, tubes and nozzles.

An entirely different approach - encapsulating dust in air using water sprays - is used by Probe Industries of Tyne and Wear. Its product 'ProSorb' reduces the surface tension of water droplets and thus decreases the amount of water that is required to create a dust-free environment.
The formulation also includes a component to destroy any pathogens in the water supply. It is said to be 100% bio-degradable, and harmless to humans and animals.

Top Tips for Dust Control
According to the HSE, there are a number of ways in which employers and employees can ensure a healthier work environment. They advise that preventing dust, fume and irritant gases from getting into the air in the first place is usually more effective, simpler and cheaper than controlling dust once it is in the air

Employers can:
- Use water
- Put dusty machinery in a separate room or automate processes so humans do not have to be present
- Carry out risk assessments

Employees can:
- Vacuum clean, rather than use brushes or compressed air
- Handle materials gently: reduce the distance they fall or are thrown and reduce machine speed or power.
- Not drop material
- Avoid creating draughts
- Use closed bags or containers. Do not let wet waste dry out - and remove it frequently from your workplace
- Reduce grinding and sawing
- Keep machines clean
- Use effective local exhaust ventilation
- Use respiratory protective equipment when required

Do not forget that it is not only the dust you can see, but also the dust that you cannot see, that is a problem. The dust particles that can get deep into the lungs are not usually visible to the naked eye.

SOE

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