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  1. #1
    BobL is offline Member: Blue and white apron brigade
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    Default Plasma cutter potential dangerous fumes and gases

    Most of us use plasma cutters without really thinking about possible dangerous fumes and particles.

    A couple of days ago I was cutting some 6mm Al plate and unusually for me I forgot to turn on the extractor in my welding bay fume hood. Within 30 seconds of cutting an alarm went off and at first I thought it was my smoke alarm as it has done that before when I forgot to turn the extractor on while welding although usually I take the smoke alarm off the ceiling when welding. However as I approached the alarm I realised it was not the smoke alarm but my new CO alarm.

    That's strange I thought it has never gone off before but then I realised I recently reset the threshold alarm level to 50 ppm after I learned that long term exposure to 35ppm was considered dangerous (I used to have it set at 100 ppm). The reason for me setting it higher than 35ppm is that the sensor is at ceiling level and it would be lower than this around my head.

    Anyway I just let it sit to see how high it got and it eventually got to 300 ppb! Nothing was on fire and I could not see where the carbon was coming from but then I thought the atmosphere has plenty of CO2 in it, and indoors, especially sheds with stale air, can have several thousand ppm of CO2. My guess is that plasma cutters probably dissociate CO2 into CO as this is common process in plasmas. FWIW I have worked on an analytical instrument called an ICPMS ( Inductively Coupled Plasma Mass Spectrometer) where CO from CO2 in air is commonly detected and a damned analytical nuisance.

    There is also the possibility that the MQ7 sensor used to detect the CO is also sensitive to other gasses so I looked into this and found it was sensitive to molecular Hydrogen (H2), Alcohol, methane and LPG. Of these, teh last 3 are not likely and while dissociation of H2O could produce hydrogen it would be fully ionised H+ and not H2.

    I then got to thinking what else could the plasma make so I checked out what research had been done, and found this excellent article for 2017 .
    https://academic.oup.com/annweh/arti.../3/311/2877141

    Extract from the conclusions.
    In the present study, we found that stainless steel plasma cutting fume contained high amounts of toxic metals, such as Cr6+.
    The fume generation rates and oxidation levels were directly associated with the arc current.
    Higher arc current introduced more vaporization and fume formation, as well as oxidation.
    These findings underscore the need for workers to exercise caution when selecting arc current to balance the job duty with fume exposure.
    It was worth noting that the high positive correlation between NO
    x and Cr6+emission under the same arc current provides an alternative way to quickly determine Cr6+ concentration using a low-cost NOx sensor and monitor in the workplace.
    Particulate concentrations of both fine and coarse fractions were impacted differentially by arc currents.
    The high current arc created more fine particles, although it shifted the GMD to a larger one that is less favorable for respiratory deposition.
    [insert by OP]Around 1 billion fine particles per cubic metre are produced using a 50A current[ends insert]
    In the coarse particle fraction, medium arc current produced fewer particles, since the excessive energy was avoided.
    So why haven't we heard about much this before?
    I know the instruction manuals suggest the possibilities of dangerous fumes but it's nice to know whats going on.

    One of the next things I will do is build an NOx gas sensor since NOx levels were shown to be proportional to Cr6+ levels when cutting SS. I already have an NOx sensor (MQ135) in my stash so no waiting on the slow boat from China.

    In a related way I was TIG welding some short strips of mild steel recently without the extractor on. Ten minutes or so after I finished I could see a fine haze in the air and I walked past my particle counter located about 5m away and noticed the dust levels were 50X higher than usual. Remember this is TiG - no flux and therefore one would think no dust - but it was clearly making plenty of fine dust especially to be able to reach 5m away. Next time I run the TIG I will bring the sensor much closer to see what it does.

    Meanwhile I guess everyone that doesn't have any should seriously think about setting up some extraction or ventilation for this sort of thing. Don't rely on just opening a door or window or a whirly bird get something that is forced at doesn't rely on the vagaries of the weather.

  2. #2
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    Thanks for this interesting information, Bob. For those of us that didn't do science at school, could you please advise us as to what NOx and Cr6+ is PLEASE?
    Thanks,
    Kryn
    To grow old is mandatory, growing up is optional.

  3. #3
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    Hi Bob,
    Some good information there.
    Back in the late eighties to early nineties, I was involved marginally with plasma cutting than in its very early days. The information we were able to obtain came from the manufacturers and places like the Australian Welding Institute, Welding Technology international and the American Welding Institute via Manufactuers Notes and Journals.

    Even then we were warned of the pulmonary hazards of the heavy metals and gases released during the process and respiratory protection recommended on steels containing nickel and chrome,ie. stainless steels.


    Nearly some 30 years later we have god knows how many, using industrial and DIY and often paying about as much attention to the safety warnings as users do with, say, PPE protection warnings with the ubiquitous angle grinder.

    OK, maybe everyone has an angle grinder and not a plasma cutter but thought does need to be given about whatever metal is to be cut with a plasma.

    Grahame

  4. #4
    BobL is offline Member: Blue and white apron brigade
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    NOx are Nitrous Oxides gasses. The x means there are many forms, The main forms are NO and NO2 but there are many more including N20 (laughing gas), all of which are no laughing matter. Considerable NOx is produced naturally in the atmosphere by lightning plasmas. Motor vehicle emissions, fertilisers and industrial process production outweighs natural production by a factor of ~3 and it is on the increase. Long term NOx is considered 300 times worse than CO2 as a greenhouse gas

    In addition, in the air NOx reacts with water to produce nitrous and nitric acid thus contributing to acid rain, and is a major contributor to the generation of fine dust particles, none of which you want to be breathing in. When you breathe in NOx directly it reacts with the wet linings of your lungs producing nitric acid. It is considered particularly nasty for young kids and seniors, anyone with a preexisting respiratory or cardiovascular issue, and smokers.

    NOx production rates by a plasma cutter running at 50A is ~500 micrograms (ug) per minute. The current recommended exposure over one hour is 0.12 ug/g.
    This means a 100 cubic meter shed (~6 x 6 x 2.7m which is about 120kg of air) would if the air were well mixed reach 0.004 ppm in 1 minute so it would take ~30 minutes of cutting to reach the limit. The NOX would also hang around the shed for considerable time unless the shed were vented. The current levels of NOx exposure are recognised as being too high and are being revised downward with long term target to get the levels down by over a factor of 300. If we use the long term target a the maximum exposure levels then it would only take 17 seconds of cutting to be considered dangerous.

    Cr6+ is hexavalent Chromium (Cr) gasses or compounds. There are two types of Cr compounds, Cr3+ and Cr6+ with the Cr6+ being far more toxic and a recognised carcinogen for nearly 40 years. In workshops it is formed during some electrolysis process and when high heat is involved in dealing with alloys like SS.

    Cr6+ has recommended levels in air of 0.005 micrograms per gram. A 50A arc cutting SS produces around 150 ug of Cr6+ per minute so in a 100 m^ shed this is 0.0013 ug/g after just one minute of cutting. This means only 4 minutes of cutting would exceed the OHS limit. This of course assumes an even distribution around the shed but in practice you would be exposed to greater levels much more quickly than 4 minutes.

    I could do some more numbers about long term exposure but this is probably enough for now and either way it looks like plasma cutters perhaps need to be taken a bit more seriously than in the past and some form of extraction/ventilation would appear to be advisable.

    All this is why industrial setups plasma cut under water.

  5. #5
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    Thanks for some very interesting reading there Bob.

    I have been very seriously looking at setting up a CNC plasma table as soon as I have funds available, and have been debating the various methods of fume/dust extraction available for such a setup.

    Whilst my workshop is a lot more than 100m^3 (more like 3000m^3) the airflow through it is quite poor, so forced ventilation is my only option for fume extraction. As such I was looking at using a downdraft table for simplicity's sake, but your info suggests that a water table would be far superior even if it is a bit more hassle in the first place.

    With all of that in mind though, what are your thoughts on the hazardous nature or otherwise of the water that has been used in the water table? Does that present a hazard over time?

    My knowledge of chemistry is fairly rudimentary, but I did know enough to know that nitrous oxide bubbled through water can make nitric acid (which I know to be particularly dangerous in high concentrations). Are we talking high enough concentrations to be dangerous here? To the point where the water table should be drained at frequent intervals to avoid it becoming hazardous?

    Also what about heavy metals building up in the water (the hexavalent chromium etc.) Are they likely to build up to the point where they create a disposal problem? Or am I imaging a potential problem where there isn't one?

    Appreciate your feedback on these thoughts.

    Cheers,... Jon.

    Sent from my Lenovo YT3-X50F using Tapatalk

  6. #6
    BobL is offline Member: Blue and white apron brigade
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    Max permissible Cr6+ in discharge water seems to be tricky to track down but I have seen 1ppm in a couple of reports but am uncertain what it applies to.

    If a 50A cutter makes 500 micrograms of Cr6+ per minute and assuming the starting water has negligible Cr6+ in it. then 500 ml of water will reach the limit in one minute - after that it depends on how many litres of water you have. A setup that uses a 500L tank would nominally 1000 minutes (16 hours) of cutting before you would have to dispose of the water. That does not sound too unreasonable to me. On bigger tables with higher current PCs they would use comparably more water so it might work out close to the same.

    The NOx is handled by an additive to the water neutralises it. It also seems like the down draft tables do work but there are no specs so I have no idea what flows they use.

    Again, like I said read my sig - get pro advice - don't rely just on what I say.

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