When we are discussing whether a facility needs to conduct a Combustible Dust Hazard Analysis (DHA) or if dust explosibility testing is needed, it is common for material like aluminum dust, phenolic resin, iron, powder coat, etc. to be included in the discussion as it is relatively known that these materials are combustible dusts. However, it regularly occurs that the dusts and fumes from welding activities are not brought up by the facility. A widely held view in general industry is that dust from welding operations contains only oxidized metal and is therefore not combustible. Recent studies and CTI’s own investigations have found this to be incorrect.
CTI engineers are working with several facilities and industry experts to better understand and classify the combustibility of welding fume dust. It can be difficult to characterize welding fume dusts. These dusts are often a mixture of partially oxidized metals, whose characteristics are influenced by the oxide content and particle size, as well as a wide array of particle size distributions. The presence of an oxide content is due to the chemical processes that take place on the metal surface as part of the heating and exposure to oxygen in the air that occurs during welding. Recent testing data has shown that the dust deflagration index or Kst (an indicator of a dust’s explosive power compared to other dusts) to be in the range of 90-200 bar-m/sec with a high percentage of smaller particles sizes that consist of iron and iron oxide structures. It seems that, despite the high temperature that occurs during welding, the dust that was produced was not sufficiently oxidized to become inert.
Some notions on the explosibility of welding fumes is that the welding fume dust, while containing iron and iron oxide, also contains material from the welding wire or electrode, surface oils and lubricants, metals from sublimation (the transition of a substance directly from the solid to the gas state, without passing through the liquid state), other surface contaminants, etc.
When the base metal is iron or steel and the welding wire used is made of similar materials, the fume will contain iron oxide and magnesium. Welding on stainless steel can generate fumes containing chromium and nickel. Welding on plated, galvanized, or painted metals may produce fumes containing lead, zinc oxide, or cadmium. With these combinations of material making up the welding fume dust collected in dust collectors that pull off welding cells, it makes sense that we have continuously encountered welding fume dust that is combustible.
You may be thinking “Ok, so I may have combustible dust from welding operations at my facility. Now what?” Well, the first step would be to get your dust tested to determine if it is combustible. If the dust comes back as combustible, then you should have a DHA completed in order to identify the risks and ignition sources in the equipment and facility. The DHA should be done in accordance with National Fire Protection Association (NFPA) Standards 652 and 484. With a DHA done for all equipment, processes, and area that handle or generate the combustible dust done, and ignition sources identified, next would be to come up with an action plan and timeline of how to mitigate or abate the issues identified in the DHA.
CTI’s experience conducting the DHA for welding processes has resulted in findings that combustible welding fumes were present throughout the dust collection systems. While the dust itself was found to be combustible it is important to understand how the dust moves and collects in the system and equipment. For example, a DHA was completed on a series of dust collectors that collect the welding fume generated from enclosed robotic welding cells. The dust came back as moderately combustible. The dust collectors were programed to only filter pulse after production, which was 20 hours of operation. It was determined that there was not a hazardous or explosive atmosphere of dust in the dust collector or ducting during normal operation. However, it was determined that sufficient dust could be entrained on the filters at the end of a 20-hour workday to produce a hazardous atmosphere during the filter pulse; Therefore, allowing for a simple and straight forward solution. Instead of only filter pulsing after production, increase the frequency to include filter pulsing during production time, thereby minimizing the amount of dust accumulated on the filters and lowering the likelihood of a hazardous or explosive atmosphere.
From the example above, while welding fume dust can be considered combustible, the amount and concentration of dust within the system, primarily dust collection systems, may present an easily mitigated combustible dust hazard. Generally, there is only a minimal amount of dust that is captured from welding activities. Therefore, it is important to consider the dust explosibility data and material properties, and to calculate the potential for an explosive atmosphere in the ducting and dust collection equipment. One thing to keep in mind is that while the concentration in the dust collection lines may be minimal, welding fume dust can easily deposit in exhaust systems and ignite, causing pipe fires that are difficult to control or extinguish or, more likely, causing a fire inside the dust collector.
Please feel free and contact us if you are concerned that you may be storing or generating combustible dust from welding activities within your process lines and equipment, or if you have any questions regarding the dust hazards present at your facility.