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  1. #1
    Join Date
    Jan 2004
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    Mackay North Qld
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    6,446

    Default Opening a used LPG Cylinder

    I have started this thread as a response to the discussion about the LPG cylinder in the Wanted to Buy Forum posts.

    Preparing a used cylinder to be opened up.

    Certainly no one in their right mind would suggest cutting open a cylinder that has had LPG in it with out insuring that the cylinder was empty and had no residual gas present.

    To ensure this I remove the brass valve. These valves are Left Hand thread and sealed with a loctite type sealant .The sealant makes for difficult removal so I place a 150 watt soldering iron on the brass valve for a time and find invariably that the brass valve will screw out with out much effort. Remember its LH thread. Because I have done a few I make it easy by use of a purpose made tool with a 1 meter leverage to grab the valve .

    Fortunately LPG is heavier than air so when the brass valve is screwed out and the cylinder is inverted any gas will drain out. I do this in a well ventilated area so the gas dissipates.

    At this point I fill the cylinder up with water until it is filled completely. Due to this cylinder shape with hemispherical ends, there is no place for a gas pocket. I have opened several 9kg cylinders to make camp stoves.

    If mercaptan smell is a real problem, to anyone considering a camp stove , just buy a brand new one.

    Moving on to the 45kg cylinder I would use the same procedure before and drilling cutting or welding. I’ll discuss that in my next post.

    Of course any body doing this work does it at their own risk.Common sense, attitude and ability have more to do with it anything else.

    Grahame

  2. #2
    Join Date
    Nov 2018
    Location
    Tasmania
    Posts
    97

    Default

    All good advice
    and to drain the water out of the cylinder, a trick I've seen is to lay it on its side and poke a straw into the air pocket at the top
    This stops the water / air gurgling and fighting to get in/out, and the water will just flow out

  3. #3
    Join Date
    Jan 2004
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    Mackay North Qld
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    Default Discussion on welding on an LPG Cylinder

    I have thought long and hard about and writing on this topic here in the forum. Snapatap was correct to raise his concern.

    As my metalwork teaching HOD used to intone as we went off to classes.

    ”Another day of spreading the ink of knowledge on the blotting paper of ignorance”

    No, I’ am not trying to insult anyone, but feel free, if you want to feel insulted, go right ahead be, my guest. What you do not know may kill you.

    Seriously, I am offering my thoughts on welding on equipment which has the capacity to kill or maim you if you get it wrong or simply don’t understand all the pertinent factors.
    An air receiver-compressed air tank can explode, violently and powerfully. It has the energy, potentially to blow the wall of your garage down.

    Now that I have your attention let us consider a few factors.

    An air receiver could be compared to a set of lungs. As internal air pressure increases, the skin on the vessel expands and decreases as the pressure reduces. Through out its service life the skin, the cylinder wall of an air receiver pressure vessel cycles to stretch and shrink, a huge number of times.

    Any defect in the skin of the vessel could cause the pressure to act upon in it and cause a crack which can progress into a tear and disaster follows.

    Corrosion decreases the SAFE wall thickness of the air receiver wall and should be monitored. This is where certification comes in and commercial vessels are checked periodically.
    In commercial air receiver’s inspection is mandated. How many thirty or forty year old unchecked BOMBs are in OUR sheds and workshops, particularly ones that have not been drained after each use?

    The LPG cylinders are subject to date inspection every time you have them refilled, for the same reason. After ten years they must be tested and re certified or removed from service.

    Moving on to welding on to what will now be a pressure vessel, the welding that takes place on it must be carried out in such a way as to not induce any defects that could induce catastrophic failure.

    Also thought should be given to the pressure this new receiver is expected to take and compare it to the design pressure. In this case I would not care to work on a cylinder with a pressure load above say 120 PSI.

    The flowing links have information on LPG cylinder pressures in both design load and working load.

    https://gashosesandregulators.com/pr...latorfacts.php

    https://www.elgas.com.au/blog/1969-h...-in-what-state

    https://en.wikipedia.org/wiki/Liquefied_petroleum_gas



    Getting back to welding the bush in the air receiver. In a previous post I mentioned a competent welder.

    Ok ! Picture this.
    The hole is drilled in the cylinder and the bung is placed ready for tacking in position.

    Basically what is required in a 360° circumferential fillet weld. If striking the arc outside of the area cover the arc strikes themselves can cause micro fractures which in turn can result. Remember the cylinder/air receiver flexes in a cyclic manner.

    Not now, not tomorrow but somewhere way down the track after half a million flexes, a crack starts and the whole lets go.

    So what is needed for a defect free 360° fillet?

    *No ARC strikes- ANYWHERE on the air receiver skin, bar under the weld bead.
    *No undercut on the fillet toes.
    *NO crater at stop or start interfaces of the bead.
    *A correct full fillet profile. a consistent leg length for the full circumference No bead overoll either(see mig comment)
    *No weld inclusion or porosities in the fillet bead.
    *The bead is welded with minimum drag angle for best penetration.

    I would use the electrode process and select a 3.25mm semi iron powder or a nice flowing 6012 electrode.
    If I was still unsure I would do a practice and macro cross section, polish and inspect the result.

    A low hydrogen electrode is OK but not necessary.
    To many sepos in the overseas welding columns advise using LHydrgen electrodes.Sure the weld metal is stronger but the base metal is plain carbon steel (mild steel to you) and that is the weakest link, so no advantage is gained
    Without High Freq start, Low Hydrogen electrodes can be bitches to start and a good way of stray arcing the plate surface.

    As an hobby welder I would avoid mig unless you intend to x-ray or ultra sound. Some MIG beads are notorious for being visually OK, but crap below. I would not risk MIG with out NON Destructive Inspection.

    The above should be pretty well cover it as to how I understand the meaning of a competent welder.

    If you cannot achieve the above DO NOT DO IT

    Do Not let ego overcome ability. The value is not in a paper document held, but the knowledge the bearer has gained and understood that influences their ability.

    As always, should you undertake this weld, the risk is on the person doing the job.

  4. #4
    Join Date
    Oct 2004
    Location
    Southern Highlands NSW
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    1,894

    Default

    Thanks Grahame, I got the info I needed from your short essay.
    I will not be attempting to weld anything to a pressure vessel.

    As a backyard welder who goes for months without doing any, I at times have brilliant results that made me think for a short time that I'd finally got the hang of it.
    Next day perhaps, for no reason I can fathom, the results were rubbish.
    Crestfallen, I had to admit that I am nothing like a consistently passable welder.

  5. #5
    Join Date
    Jan 2004
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    Mackay North Qld
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    Default

    Hey nadroj
    Don't feel bad. Its a horses for courses type of thing. I don't use my lathe much and consequently my machining is not what I think it should be.
    Yes it was a bit of a War and Peace tome,but I had to get all those points in.

    Grahame

  6. #6
    Join Date
    May 2011
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    Murray Bridge S Aust.
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    71
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    Default

    Grahame, that is excellent advice that you have written, concise and to the point without any waffling on.
    Thanks on behalf of the MEMBERS.
    Kryn
    To grow old is mandatory, growing up is optional.

  7. #7
    Join Date
    Oct 2001
    Location
    ACT
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    667

    Default

    Hi Grahame,

    You ever had any problems unscrewing the brass valve from cylinders? or just used a large spanner/extension?

    also found it a bit difficult to hold the darn cylinders being large and round.

    cheers

  8. #8
    Join Date
    Jan 2004
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    Mackay North Qld
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    Default

    Quote Originally Posted by Zsteve View Post
    Hi Grahame,

    You ever had any problems unscrewing the brass valve from cylinders? or just used a large spanner/extension?

    also found it a bit difficult to hold the darn cylinders being large and round.

    cheers
    Hi Steve,
    In the beginning, yes!

    There are two sorts of cylinder base rings ,ones with openings in them and ones that do not have them.

    Some cylinders have that opening in the bottom ring I just stuck a crowbar through those holes and levered against that when the cylinder was sideways on the ground.

    with the no opening rings I used a ratchet straps to anchor the cylinder to the rolled steel channel column of my BBQ area.


    Used the big soldering iron ( because there was no flame) and set it against the brass valve.

    Leave it there long enough to heat up the valve. Say too hot to touch.

    I used a big lump of gal pipe that I had the socket ( just a couple of bits of flat welded to form a open section that fitted over the tap)

    The long pipe welded at right angles to a short bit of pipe that acted like an extension bar on a socket set.

    That allowed me to reach down through the rolled ring and loosen the valve.

    Remember that it is left hand thread .Once you get the valve hot enough to make the Loctite compound ( or what ever it is ) it is relatively easy to screw the tap out.

    Try to do it cold and fight the Loctite and your eyes will bulge with the effort

    Remember, keep naked flames away or it might be the last we hear from you.

    Grahame

  9. #9
    BobL is offline Member: Blue and white apron brigade
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    Default

    If your straps are slightly oily like mine a bit of tyre tube between the strap and cylinder provides a really good grip.
    Hot air gun can substitute for soldering iron. Mine goes to 650ºC.
    Every now and then you might want to turn off the HAG and open the cylinder valve to release any pressure build up - you will really smell the mercaptan as it volatilised by the heat.

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