At an SAIW evening meeting earlier this year, Jacques Cato talked about welding fume, filtration technology and the solutions available from Donaldson Filtration Solutions in South Africa. African Fusion attends and reports.
“The fume given off by welding and hot cutting processes is a varying mixture of airborne gases and very fine particles, which, if inhaled in sufficient quantities, will cause ill health,” says Cato, while displaying a slide on the facts about welding fume.
The gases present in welding fume include nitrous oxide (NOx), carbon dioxide (CO2), carbon monoxide (CO), inert shielding gases such as argon and helium, and ozone (O3), which is produced by the high temperatures associated with welding arc plasmas. “The visible welding fume is mainly particles of metal, metal oxide and/or flux, if it is being used, but it is not only the visible fume that is dangerous,” he points out. “The exact level of risk from the fume will depend on three factors, how toxic the fume is; the concentration of fume; and how long the welder breathes it in for,” he adds.
Displaying a diagram of the amount of fume produced by different welding and cutting processes, he points out that the submerged arc welding process generates the least fume, while arc-air gouging generates the most. The SMAW (MMA) and flux-cored welding processes (FCAW) are on the high generation side, closely followed by the solid wire GMAW (MIG/MAG) processes.
From a size perspective, Cato notes that fine sand with particles larger than 0.1 mm (100 µm) cannot get through a person’s natural filtration systems and into the lungs. This particle size is said to be at the limit of inhalability. “The finer the particle, the more damaging it can be to the lungs,” he says, pointing out that smoke and fume fit into the ultrafine particle size range of 0.1 µm or less and are therefore dangerous.
“A typical welder can inhale around 500 mg/min of welding fume in this size range,” Cato warns. “Even if we assume a duty cycle of 30%, 150 mg/min of fume can be entering that welders lungs if no filtration or fume extraction system is being used.”
To prevent this, global standards such as BGW (Belgische Grenswaarde) and MAC (Nederland) set maximum fume concentrations in an enclosed area in the vicinity of any person to 5.0 mg/m3 and 3.5 mg/m3 respectively.
Pointing towards the South Africa SABS Health and Safety standards for welding and thermal cutting processes, Cato notes that different welding fume constituents are allocated different threshold limit values (TLVs) and threshold weighted averages (TWAs) in the standard. The thresholds for aluminium and iron, for example are set at 5.0 mg/m3, while more dangerous constituents, such as mercury and silver are set significantly lower (0.01 to 0.1 mg/m3).
The specific illnesses linked to welding fume? “Pneumonia and lung Infections are at the top of the list. Health and Safety statistics from the UK report 40 to 50 hospitalisations per year for occupation-related pneumonia, of which two to three are fatal,” Cato says.
Occupational asthma is strongly associated with the fumes from stainless steel welding, which contain chromium oxide (CrO3) and Nickel Oxide (Ni2O3). “Both of these constituents are known to cause asthma. Welding fume is also classified as ‘possibly carcinogenic’ to humans, although the system of classifying substances does not consider the by-products of a process, which means that welding fume is not currently assigned a hazard classification,” he says.
Other known conditions are metal fume fever, which presents as flu-like symptoms after welding but does not usually have any lasting ill effects – “and this condition cannot be prevented by drinking milk before welding,” Cato says, debunking a common myth.
Dryness, irritation and ‘tickling’ of the throat and lungs; coughing or a tight chest are common. “Ozone is a particular cause of this, particularly when TIG welding stainless steels and aluminium. High exposures to nitrous oxides, generated during most arc welding operations, can also cause this health effect.
Extreme exposure to ozone can also cause pulmonary oedema (fluid on the lungs). “Temporary reduced lung function is relatively common. It affects the ease at which you can breathe out (peak flow). It tend to get worse through the working week but gradually improve shortly after exposure is reduced – following a weekend, for example.”
Donaldson’s Ultra-Web® nanofibre technology is at the core of its filtration technology. Nanofibres are scientifically proven to give an advantage with respect to filtration efficiency and pressure drop reduction, which relates directly to energy efficiency. “Ultra-Web patented nanofibres are made using an electro spinning process that produces a very fine, continuous fibre of between 0.2 and 0.3 µm in diameter” Cato reveals.
Using nanofibre, a permanent web-like net is formed with very fine interfibre spaces. Traditional cellulose or cellulose/synthetic media blends are made from fibres in the 10 to 20 µm range, which makes it impossible to create a net with sub-micron spaces. These media, therefore, rely on the use of larger spaces and thicker volumes to trap particles inside the medium.
In contrast, the use of Ultra-Web nanofibre enables sub-micron particles to be captured on the surface. This promotes much better pulse cleaning and it significantly reduces the operating pressure drop across the filter media. “The nanofibre net results is cleaner air, longer filter life, better energy efficiency and greater cost savings,” Cato explains.
Ultra-Web nanofibre is used by Donaldson in its cartridge filter units, which, according to Cato, are the modern alternative to bag filters. “Cartridges offer a smaller foot print, higher filtration efficiencies – 99.997% at 0.5 µm – and lower emissions in the clean air – 5.0 mg/m3 or less. In addition, they are cheaper than bag filters, operate at lower pressure and consume less energy,” he reveals.
Cartridges can be used in any of Donaldson’s filtration solutions, from portable fume extraction systems to its Torit dust collectors. A typical dust collector draws the fume in from the top of the collector. The cartridges are arranged inside the collector at an angle so that the dirty air passes through the filter media on the outside of the cartridge and into the cartridge’s core. The clean air then flows up the incline to the outlet of the collector.
Periodically, a reverse pulse of compressed air is sent through the cartridge cores, which shakes the dust off the outside surface of the cartridge. The dust falls into a collecting bin below, for disposal when full. This process keeps the cartridges from clogging and the system’s operating pressure low.
“The use of cartridges in dust collectors offers very easy maintenance. Cartridges can be removed and replaced by one technician without the need for any tools. We have also adopted a modular approach, which enables collector units to be combined in various configurations to make up larger units with air volumes ranging from 2 500 m3/hr to 200 000 m3/hr,” Cato says.
For welding applications, Cato lifts out the Donaldson Environmental Control Booth or ECB workstation. ‘The ECB is specially designed to remove fine airborne dust from multiple sources without interfering with workers’ movements or visibility. The booth comes complete with lighting, sound-proofing, built-in fume extraction and dust collection. It requires no ductwork, can easily be moved around the shop floor and re-circulates the cleaned air for reduced energy costs.
“We at Donaldson offer solutions for every conceivable dust or fume extraction application, and our experience and range ensures that the sub-micron particles and dangerous constituents in weld fume are efficiently collected before they can reach a welder’s breathing zone,” Cato concludes.