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Welding safety

  1. Aug 29, 2010 #1
    So, my welding book has this big ole warning that says this:

    If oxygen under pressure comes into contact with oil or grease. A violent reaction will occur.

    When they say "comes into contact" does this mean the outside of the O2 cylinder or just the actual pressurized O2?

    Second, Why does it cause such an explosion? What is causing the ignition if no spark or flame is present?

    thanx
     
  2. jcsd
  3. Aug 30, 2010 #2
    I think it means if you have oil and pressurized O2 then it will combust.

    I'm not chemist however I believe the process happens due to oxidation. At regular pressures O2 and most oils react giving off a small amount of heat, which will not do anything because oil oxidizes so slowly that the heat given off is dissipated really fast into the surroundings.

    Now if we add pressure, oxidation occurs much faster meaning A LOT more heat is given off in the reaction. In fact it's enough heat to catch oil on fire and well you can use your imagination for what that might look like.
     
  4. Aug 30, 2010 #3
    So oxidation causes heat? never thought of that. I was thinking it was something to do with so much pressure causing energy which would cause the explosion. Hmm. maybe a chemist will chime in.

    thanx
     
  5. Aug 30, 2010 #4

    Astronuc

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    Staff: Mentor

    Hydrocarbons (grease or oil) will more easily ignite (and burn) when in contact with oxygen, especially if heated - hence the caution. Air is about 21% oxygen and 78% nitrogen, and in most cases, the concentration of oxygen is too low to initiate combustion.

    Think about what is required to light a candle (wax). Wax doesn't burn effectively until it melts, and in fact volatizes/vaporizes, and is heated and mixes with air.
     
  6. Aug 30, 2010 #5
    But if there is no flame or spark to begin with, what is in the pressurized O2 that cause it to explode when mixed with oil?

    thanx
     
  7. Aug 30, 2010 #6
  8. Aug 31, 2010 #7
    You can think of a flame or spark as just heat energy. If you have enough heat energy you can ignite the oil.
     
  9. Sep 1, 2010 #8
    So somehow heat energy is being created when O2 is mixed with a oil based product? How is that heat created? Is it the chemicals that dont like eachother so they create heat?

    thanx
     
  10. Sep 1, 2010 #9
    See this recommendation in the DOE guidelines:

    http://www.tpub.com/content/doe/h1015v2/css/h1015v2_120.htm

    "Oxygen

    Oxygen (O2) supports combustion, but does not burn. Even so, it must be considered a potentially hazardous element from a fire hazard standpoint. The results of an enriched oxygen atmosphere include a lowered ignition temperature, an increased flammable range, and an acceleration of the burning rate. Oxygen readily combines with other elements and compounds, with spontaneous ignition in some cases. When oxygen comes in contact with oil, grease, or fueloils, it may ignite violently. Every possible precaution must be taken to prevent this combination.
    "

    Bob S
     
  11. Sep 1, 2010 #10
    interesting. So with petrolium based fluids, it can cause spontaneous combustion. I wonder why? Its gotta be the chemical reaction causes alot of energy thus providing a low ignition and then boom

    thanx
     
  12. Sep 2, 2010 #11
    The energy is stored in the chemical bonds of O2 and Oil. These compounds react and when they react the original bonds are broken new ones are formed and some of that energy is released in the form of heat.

    O2 + Oil -> Some chemicals + heat

    When you subject the two to a high pressure the two will react a lot faster which means more heat energy over a shorter amount of time.
     
  13. Sep 2, 2010 #12
    The pressure of the O2 is not significant in the ignition process, because the combustion reaction with a given hydrocarbon in oxygen (or any chemical reaction for that matter) has a calculable and constant http://en.wikipedia.org/wiki/Activation_energy" [Broken].

    This energy is independent of physical factors such as pressures. The pure & pressurized O2 tank simply makes any combustion that happens go at a far greater rate. I think that the warning in your handbook is simply there because hydrocarbons in pure O2 are a violent fire waiting to happen. It's also because lawyers exist.
     
    Last edited by a moderator: May 4, 2017
  14. Sep 4, 2010 #13
    damn lawyers.. haha.

    I finally got a hold of my uncle who is a chem engineer. He said what you said about the rate is increased due to pressure and its purity. He says its also because O2 is one of the best oxidizers used to break apart other chemicals. So when O2 is induced into a flammable substance, it starts to oxidize and break apart the polymer chains thus causing a reaction which causes energy which will be in the form of heat..That small amount of heat is enough to cause the boom. My welding professor said that he had a student use 99.9% O2 to blow his clothes off from the dust and then he stepped outside into the 110*F sun light and said that his clothes caught fire. Could be true could be a myth. who knows...

    thanx
     
    Last edited by a moderator: May 4, 2017
  15. Sep 6, 2010 #14

    Ranger Mike

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    Science Advisor
    Gold Member

    this warning is regarding care when changing the regulator onto a new tank of ox. make sure the threads are clean...saw a lot of guys with very greasy hands hurrying to change out a ox tank on cutting torch..bad news..keep it clean!
     
  16. Sep 6, 2010 #15

    Q_Goest

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    There are a few mechanisms that create sufficient heat energy to initiate burning in oxygen systems.
    - Adiabatic compression: Sudden opening of a valve with high pressure oxygen behind it can result in the compression of oxygen downstream of that valve where the system is dead headed. Oxygen at low pressure in the lines will be pushed towards the dead end in the system which creates heat due to compression. Temperature in the dead section can be aproximated by assuming the gas is isentropically compressed.
    - Impingement: Like adiabatic compression, gas at high velocity impinging on a stationary wall or other change in direction of flow, can heat up due to the conversion of kinetic energy to pressure.
    - Particulate impingement: Similar to impingment above but due to particles, especially hydrocarbons. Where flow changes direction, the impingment of particles on internal piping walls can provide sufficient energy to ignite.

    High pressure oxygen is especially dangerous because things that don't burn in air or pure oxygen at low pressure, can burn in oxygen at high pressure. Various metals such as aluminum, stainless steel and steel all burn in pure oxygen at a few hundreds of psi. Also, various materials commonly used in oxygen systems such as Teflon, are not just flammable (they are less flamable than other plastics) but they create poisonous gasses when burned.

    There are methods and materials used in oxygen systems that help reduce the risk of fire. The best industrial reference for oxygen system design is provided by the Compressed Gas Association (CGA) pamphlet http://www.cganet.com/customer/publication_detail.aspx?id=G-4.4"
     
    Last edited by a moderator: Apr 25, 2017
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