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73096

Discussion in 'Steam Traction' started by domeyhead, Mar 16, 2009.

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  1. 73096

    73096 Member

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    To get an accurate answer 73129 it'd be best to have a word with someone experienced tomorrow if you're at ropley, I'll show you if you're down and I'm not busy

    Regards

    D.Pilton
     
  2. martin butler

    martin butler Part of the furniture

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    looking at the way the cylinder had shattered, i would guess that it and the piston took most of the force before it could be transmitted through the motion , would a cylinder blowing out bend the frames, or loosen stretchers ?
    and who ever has been saying that its priming that coursed the failure , that has been ruled out by the railway looking at the damage i would say that it looks more like a weekness in the cylinder rather than human error, ,might there had been a flaw in the casting? after all how long is a cylinder expected to last , i previously worked in the motor trade and we used to occationally get engine blocks break on brand new cars because of flaws in the casting that were not apparent when built
    after all the standards didnt survive long enough in traffic to have any long term problems found and rectified
     
  3. 34007

    34007 Part of the furniture

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    I too think the Cylinder was weak - Was this the original cylinder?
     
  4. Steve

    Steve Resident of Nat Pres Friend

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    Well, any force has an equal and opposite efffect so it has to be taken somewhere else. A close exam of the whole of the frames/running gear is the only possible outcome and, with a bit of luck, everythingh else will be up to the forces involved.

    As for how long a cylinder is expected to last; if it can be lined. I'd expect the answer to be close on 'forever'.... Engine blocks braking on brand new cars is all part of the bathtub theory.
     
  5. Steve

    Steve Resident of Nat Pres Friend

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    You do?
    Why?
     
  6. Fireman Dave

    Fireman Dave New Member

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    While you are right about Newtons law, I don't believe you have fully understood it. The forces (energy contained in the steam) are normally contained within the cylinder and are passed through the pistons and rods to the wheels where the energy is used to derive motion. In this case, once the cylinder failed the forces were dissipated to atmosphere as the steam expanded (hope this makes sense). Having said this though, I'm sure a full examination will be undertaken to eliminate a possibility of further damage.
     
  7. 61624

    61624 Part of the furniture

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    If you think back to the Blue Peter incident, the rods were bent even though the cylinders withstood the forces on them, and at least one of the drivers was shifted on its axle. In the case of 73096 there was quite obviously a jamming incident that was enough to severely bend the piston rod. I would have though there must be a very good chance that the shock of that impact has done damage elsewhere on the chassis.
     
  8. 73129

    73129 Part of the furniture

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    There must have been a lot of energy that was dispersed threw the axle box's and horn guides when the piston couldn’t reach its full travel in the cylinder.
     
  9. streuth

    streuth New Member

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    I have to say, I too (although we're not in a majority) have difficulty with the idea of a core plug falling out. I'm not an expert, and I have no authority.

    I just can't square the size of the plug, with the "bendage" on the piston rod.

    This is purely speculation, but clearly, it's also an idea and ideas have validity too, even if they are duff.

    O.K. so the cylinder must have had three states of existence. Normal, failed and in-between. In it's normal state, the piston rod wasn't bending.

    After the failure, there was nothing except the rear of the cylinder for the piston and rod to foul on. I think I'm fairly confident that the damage to the rear cover was caused as a result of the piston and rod retracting in a bent condition after the failure. The point to remember about this "after" state is that there was nothing to cause the piston rod to bend further from it's normal straight condition.

    In practice the gland and the additional strength of the surrounding boss on the rear cover would have caused the piston rod to straighten, if anything. As it seems the rear cover has pulled away from the flange at the rear of the cylinder, despite the studs securing it there, it is notable that the upper part of the cylinder where the piston valves are enclosed is a much stronger part of the casting.

    So if there was nothing to increase the "bendage" after the failure, how did the piston rod bend so much. It seems to me that it must have occurred at the point of catastrophic failure. And here's the thing I find difficult.....

    The displacement at the bottom of the piston, as a result of a bent piston rod, is much larger than the size of the "core plug" that is supposed to have caused the failure. The displacement seems about six inches, where the plug is only around a couple of inches. I fully accept that the piston bears witness marks around it's edges that indicate that it chewed on something, perhaps the "core plug". Could these have also been caused by the piston biting on the still intact parts of the cylinder as the train slowed to a halt. None of these marks seem large enough to be involved in a collision that could have bent the piston rod.

    I propose that the cylinder was around one quarter full of water. It seems to me to be the only reasonable explanation at the moment. Being non compressible, the cylinder pressure reached in excess of 2000psi, which the fairly old and rusty iron casting simply could not stand. It is not clear that the cylinders are fitted with automatic drain cocks, or if they were working, but even then they would not have had the capacity to flow such a quantity of water. In addition the relatively poor quality iron, far from being smooth and with many stress crack propagation sites just gave up.

    It is already seen that the cylinder for the piston valve cylinder adds considerable strength to the casting at the top, simply by virtue of mass of material.

    I propose that the energy stored in the pressurised water could not escape upwards, and thus the bottom of the cylinder "burst" forwards and downwards, with the piston rod bending at the instant of the burst. Imagine a slow motion picture of the exit of a bullet from an apple, where the exit causes the apple to splay out from the bullet.

    As I say I don't know, but that's my guess looking at the pictures.

    As a partial answer to the original poster on this thread, priming is a byproduct, of what makes steam engines wonderful. They're alive and they're unpredictable, despite still being highly controllable. It is my belief, that priming is brought about by virtue of the surface tension in the boiler water. Over time the boiler makes steam, and the salts in the water cannot escape with the steam. Also over time much water goes into the boiler, and leaves as steam, so the salts in the boiler water build up. The surface tension of the water decreases as the salts build up. The lower the surface tension, the easier it is for water to "glop" up from the surface. This is why it is important to regularly blow down and wash out regularly used boilers.

    Proximity of the regulator valve opening to the top of the water(height of the dome), rate of steam draw, and size of regulator orifice all contribute to the flow of steam within the boiler. If the boiler is moving or vibrating, this too can affect where and how the water moves in relation to the steam.

    It may not sound natural, but it is not uncommon for a big slug of water to leap off the surface of the water in the boiler, up into the steam space, into the regulator and on to the cylinders.

    It can be avoided by removing the salts before boiling the water, regular blowdowns and washouts, having less water in the boiler, using smaller regulator openings and generally working the engine less hard.

    All of the damage is fixable. Someone mentioned the idea of fabricating cylinders and no-one responded. It's not a bad idea in my opinion. Modern fabricated steel cylinders would certainly be better than ageing cast ones, and probably better than new cast ones too. Not historically correct though.

    This is pain I'm sure for those who have worked the engine, but preservation is about recreating the past, failures included. It would have happened then, and it still happens now. In some crazy way that's a good thing. No-one was hurt, and since this is all done for fun, I hope things are not too harsh for those actually involved, especially where they may be seen as "responsible".

    I might be wrong, in part, or in full. That's what I think.
     
  10. Dan Hamblin

    Dan Hamblin Part of the furniture

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    A very good post streuth, but there is something that I am a bit confused with concerning the missing core plug. The MHR website close-up of the piston shows that the hole where it had come from has had the thread stripped from it. Now that suggests to me that the core plug has been violently ejected from the piston head.

    Any thoughts on that?

    EDIT: I will answer my own question! The thread was only stripped on the back half of the piston head, and, if you look closely, you can see a circular imprint on the inside face of the back piston cover. This could well be the imprint of the loose core plug, which would account for the thread being stripped on one side only.

    Regards,

    Dan
     
  11. 73129

    73129 Part of the furniture

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    I’ve one or two things I don’t understand. When the piston is at its full travel what is the distant between the piston and the cylinder cover and if the distant is smaller than the length of the core plug how can the core plug drop out of the piston when the loco is motion.
     
  12. ghost

    ghost Part of the furniture

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    Another couple of points for 'Streuth' to consider:

    - You mention 'the fairly old and rusty iron casting' and 'the relatively poor quality iron, far from being smooth and with many stress crack propagation sites' Have you any evidence to support these allegations? Just because something is old does not mean it isn't up to the job. Any rust that I have seen on the cylinder casting looks to be light surface rust which is very unlikely to cause any significant problems in the locos operation. I would imagine that any stress crack propagation sites would be thoroughly checked/repaired by the MHR as part of their overhaul/maintenance routines.

    - Your theory on water carryover due to the salt content is interesting, but the MHR uses water treatment so I would contend that the problem as you describe it would not be possible due to the salts not being present.


    Keith
     
  13. Martin Perry

    Martin Perry Nat Pres stalwart Staff Member Moderator

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    Could the dislodged plug have caught in one of the ports for the cylinder drain cocks?
     
  14. streuth

    streuth New Member

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    I was really hoping to post a non-partisan representation of what I think.
    I don't really want to get into defending anything. Answering questions makes an implicit statement of authority, which I just don't have.

    The plug in the piston does seem odd, but I like most have only seen the pictures on the web. To me the hole I assume to be the "plug" seemed more like a drilling in a solid piston. I can't refute the idea of a hollow piston, because I just don't know. Obviously I've not seen the plug it's self, but by logic, it's maximum dimensions could only be the thickness of the piston and as it would appear from the hole, an inch or two in diameter.

    If the piston was hollow, why did the plug not break the piston, rather than the cylinder?

    The "bendage" on the piston seems much more than that of it's thickness, the logical maximum dimension of the plug.

    As to the quality and state of the iron of the cylinder. I think crack analysis an metallurgy for that would be the preserve of academics and the seriously wealthy. Obviously it makes clear sense for boilers and the like. What's the difference between this and standing at the side of the road, when a lorry engine explodes and fires a piston out the side, or even a motorbike shedding it's chain. Stuff happens, and by nature a railway is a pretty safe place.

    The way I see this, is that cast iron is rough old stuff anyhow. Even today people still make far more critical things than this out of it.

    I'm not trying to lay any blame anywhere. I am interested in the reason why the failure occurred, and that's just down to time.

    Boiler explosions are the bad ones. As long as we never have one of those it's all rosy. If it comes to it then not being able to make a brand new boiler should prevent the whole preservation movement turning a wheel. If an engine falls apart because it was old, it's not the end of the world.

    Edit;
    As for where the dislodged plug might have gone, I just couldn't say.
     
  15. Swan Age

    Swan Age Member

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    I believe that in an earlier post it was stated that the missing core plug had been found in the debris field where the incident occurred, however as to what part of the cylinder it may have dropped to after working loose i have no idea.

    Regarding how near the piston travels forward on its stroke towards the front cover, it must be pretty close as the front cover has a recess for the piston rod securing nut to travel into and looking at the photos it would seem from the intact top part of the cylinder near the front steam port that the bore marks would suggest it could be as little as an inch.
     
  16. martin butler

    martin butler Part of the furniture

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    if the core plug had came loose on the forward stroke and jamed the piston head, would that have bent the rod ? which then would have rendered the piston unable to withdraw, and the increase in steam presure against a by now siezed piston on both sides would that have been to much for the cylinder to withstand ? and coupled with the age of the casting made it disintergrate? , i am only offering what i think might have caused the damage, but to be honest i dont expect that we will ever know what the cause was mostly because i expect the parties involved wont want it made public
     
  17. Steve

    Steve Resident of Nat Pres Friend

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    With respect, Dave, I believe that it is you who has not fully understood. The forces in question are nothng to do with steam and the energy contained within that steam. The forces in question are those created by the piston hitting a solid object (the cylinder cover) and that object breaking. They have got to be significantly higher than the forces normally found in a cylinder, otherwise the thing would be breaking up with monotonous regularity. Cast Iron may not be the best of materials when it coms to tensile forces but it still takes a significant force to do the kind of damage we have seen here and that force has got to have a reaction. This will go through the con rod to the crankpin, through the wheel seat to the axlebox and to the frames. It will be dissipated by elasticity or movement in all of these.
     
  18. std tank

    std tank Part of the furniture

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    Well, 73129 you have hit the nail on the head with that question. The answer is 3/8".
     
  19. dhic001

    dhic001 Member

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    Its easy really, the stroke is considerably more than the length of the core plug, therefore the plug could fall out anwhere in the stroke, apart from the fore most inch or two. The stroke of the Standard 5 is 28 inches, that leaves at least 26 inches of travel during which the plug could fall out.
    Daniel
     
  20. chessie

    chessie Member

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    Let's throw something else into the mix - I had a good look at this thing on Sat. Whilst it's impossible to say exactly what happened, here's another one for the conspiracy theorists. Back cover goes first due to displaced core plug - we're all forgetting that this was ripped away from the cylinder too. This causes piston to run out of true, causing damage to piston rod. Bent piston (rod) then 'bursts' cylinder due to not running in correct axis.

    As I said, just another theory.
     
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