Busted NRV... How does that happen?

Discussion in 'Fettling Forum' started by Barrett, Nov 7, 2018.

  1. Barrett New Zealand

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    Northern New Zealand
    Hi All

    New learning experience/issue for me today.

    Since trading/obtaining the Haller, Solus and Monitor I hadn't done much with them other than starting to clean and a quick test burn before bringing them back to the city and sitting them on the garage bench, for later.
    (well the Haller cleaned up so pretty that it has an "approved" spot in the kitchen)

    In the garage today and noticed that the Monitor nearly had a full spirit cup of kero....damn... did I leave the air screw nipped up.... or has the now 9 year old boy been playing stovie?

    Better deal with it now, and may as well replace the perished rubber pump cup with the leather one I had made whilst at it.
    All back together, but not happy with pump flow to tank, and having not pulled the NRV as of yet, decided to have a look.

    It started undoing pleasantly easy, having soaked the pump tube in kero and forcing some through it a week or so ago.

    The NRV was wanting to bind a bit as it threaded out, so it was a case of a little turn out, back in a bit and out a bit etc till it freed up and wound out.

    Hmmmmmm something not right here..... Where's my reading eyes?

    A 2 piece.... A NVR head and separate sealing flange?.... wouldn't think so? ....is that a machined joint/surface or a split?..... (need stronger reading eyes again it appears)
    20181107_121133.jpg

    Bottom of pump tube had something the colour of lead spread around the NRV hole and I managed to retrieve a loose thin curly sliver of what was obviously lead that had been down amongst the threads, so clearly the remains of a lead washer in there. (pic was unusable due to forgetting to use flash)

    This lead really didn't want to come out with gentle prodding and ended up requiring the use of my NRV tool to slowly and carefully wear/grind it away from the NRV hole and tube end.

    So what had happened?
    Is this a common occurrence?

    (pure Barrett speculation now)

    But I think someone has been in here since it left the factory.... :) ....

    and it appears they may have had trouble getting a seal again.......
    (maybe during or shortly after this surgery it underwent at some previous point in time)
    20181107_172445.jpg

    So have they cranked on the NRV so tightly that lead has been forced outwards and down around the NRV threads up until the point the flange could take no more and split, came detached from the NRV head?

    Looking at the split flange (which is totally detached and can be spun when its riding down just above the threads where a washer would sit, but is captive between the head break area and threads) it is slightly dished now showing the strain it had taken before letting go, and you can see the jaggered edges of the break around the body, the internal faces of the break and split look aged and old damage rather than shiney and being of recent time?.
    (worked the flange off the NRV head for last pic)
    20181107_152835.jpg 20181107_153206.jpg 20181107_153438.jpg

    But even broken it had achieved a seal as evident by my test burn a week or so ago, and again whilst sitting on the bench as it had self pressurised enough to start filling the spirit cup.

    How did the pump tube end or tube to tank joins handle the torque during such a wrenching, or has there been an expansion event with the lead washer?, guess the flange was luckily the weak point?


    Lead washer remains finally gave in and cleaned away from the tube end, still needs a bit more tidying up, but whats left of the NRV threads in and out freely, so threads are still ok.
    20181107_180150.jpg

    Guess I'll be looking for a new NRV head as I don't think I have the skills to repair that damage.....but I may get the solder out for a crack at some stage just for the punishment of frustration.....lol

    Cheers
    Barrett
     
  2. kerophile

    kerophile United Kingdom Subscriber

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    Hi @Barrett , you have an interesting failure there.

    Your description and photos indicate that the failure likely occurred some time ago, when the NRV was being fitted, or re-fitted.

    The head of the NRV looks in good condition, whereas heads on stuck NRV tend to get severely damaged, on the flats, when extraction is attempted.

    If the lead washer became displaced and got into the threads on insertion, you can understand that tightening would have been more difficult, and one might not have felt back-pressure when the flange contacted the base of the pump-tube. If tightening continues there are three possibilities:

    1. The flats on the head of the NRV round and the tool “cams off”

    2. The solder joint between the walls of the pump cylinder and end-cap fails.

    3. Stesses become high enough for the shaped head of the NRV to become detached from its flange.

    I would have put money on the first two possibilities, but it looks like No.3 occurred in your case!

    Such a failure might not be detected at the time it occurred as the NRV could still function as intended, and there is no leak path with the threads being well sealed with the lead.

    This post provides some images of the parts of an NRV/pump tube assembly:

    https://classiccampstoves.com/threads/pump-tube-nrv-assembly.14028/#post-138233

    Perhaps the failure occurred in the Factory during first assembly. It seems likely that they would be using air-tools for tightening the NRVs so the operator would have had no manual feed-back of the loads incurred. Obviously the clutch/ load limiter was set a bit high!

    If you wish, it is possible to calculate the torque required to cause NRV head detachment from the dimensions of the head, and the mechanical propertiees of brass....

    Best Regards,
    Kerophile.
     
  3. Barrett New Zealand

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    Northern New Zealand
    I hadn't considered the use of air tools at the factory, yes a torque setting slightly too high combined with the higher speed of the air tool vs manual feel and speed could easily produce the quick shock load to shear the flange before the tool "cams off" on a new/perfect head.
    And the curly slivers of lead that came out today would explain the threads having formed and held a seal with the lead... for how many years I will wonder now.

    Thank you for the NRV link, I have seen/been inside there on the Hasag, once I had the pump tube out I realised I could cheat.... and wound the NRV out in reverse using the internal/tank end, thus side stepping the tube end removal for now/then thankfully.

    Thank you for another in depth and thoughtful reply @kerophile

    Cheers
    Barrett