Unvented Cylinder explosion...

Discussion in 'Plumbers' Talk' started by farmerboy, Sep 23, 2006.

  1. farmerboy

    farmerboy New Member

    (Sorry for all those diving in here hoping its an example of one blowing up!).

    I've been reading alot on here about unvented cylinders and their risk of explosion (since I had to fix a faulty thermostat in situe on a Megaflo and had serious paranoia - needless to say, although working fine it'll be replaced in days).

    I totally agree all round on the safety features that UV has, and to be frank I think those harping on about explosions are being dramatic in the extreme. For example in this case if the thermostat overheats, then the safety will cut in, if that fails then P&T, then Expansion valve etc, bla bla).

    One thing I can't get out of my head with all those saying explosions will occur if <u>ALL</u> safety systems fail; how about the strength of the HW pipes leading from the top of the system, or those of the feed?!

    Would they not burst before the cylinder does?!, surely a 22mm copper pipe or indeed any soldered joints/compression fittings by taps etc would give in long before an unvented cylinder.

  2. sinewave

    sinewave Screwfix Select

    Depends if it's 'Farmers Welding' or not! :^O
  3. farmerboy

    farmerboy New Member

    Ha, weld?

    We use baler twine on our pipes, works a treat.
  4. tgs

    tgs New Member

    The fact that something is theoretically possible does mean a slight probability of it happening. The question is: what that probability means. If you say the probability is 1 in 1 million per year (for example) and there are 2 million devices then you can expect 2 accidents per year.

    The difficulty arises from working out the probability in the first case. If there are 2 million units and 2 accidents per year then the calculation is easy. I don't believe there have been any accidents so how do you calculate your probability? You could do on a World wide basis but that assumes the products and conditions of manufacture and use are the same everyhwere as here. Other than that it becomes a guess or a lower limit.

    The fact is that, as you say, several items have to fail first, then if all the pipework stays intact, kaboom. It might be possible in thoery but until one does explode catastrophically (and hopefully this won't happen) then it is difficult to say what will fail. It will be the weakest point but where that will be is anyone's guess.
  5. doitall

    doitall New Member

    assuming the cylinder is heated by a boiler, then all the boiler safety bits and bobs have to fail as well, so after the boiler stat has failed, and the overheat stat has failed, and the pressure relief valve has failed, and the cylinder stat has failed, and the thermal cutout has failed, and the temperature/pressure relief valve has failed on the cylinder, together with the pressure relief valve, then the cylinder would be the first to go being the larger area, of course the boiler may already have taken the wall down as it headed for the neighbours garden.

    But are we not forgetting all the plastic pipe connecting A to B etc which would have melted a long time ago, sending steaming water through the house, and draining the boiler at the same time. We now have a boiler that cannot deliver boiling water to the cylinder because there is no water left. Oh dear anyone know what happens to a boiler on full burner without any water, yep it gets very hot.

    As for any copper pipe, they are safe and sound and the joints are the strongest part, mind you they will melt if the fire gets hot enough, but by then you will probably be running down the road and hiding.

  6. Onetap

    Onetap New Member

    Would they not burst before the cylinder does?!,
    surely a 22mm copper pipe or indeed any soldered
    joints/compression fittings by taps etc would give in
    long before an unvented cylinder.

    The cylinder would usually fail before the pipe, assuming they are properly joined. The pipes are narrow cylinders, so a failure would happen in the same way. The stress in the pipe/cylinder wall is proportional to the force x surface area. The surface area is proportional to the circumference, so a wider pipe/cylinder will fail first.

    I totally agree all round on the safety features that
    UV has, and to be frank I think those harping on
    about explosions are being dramatic in the extreme.
    For example in this case if the thermostat overheats,
    then the safety will cut in, if that fails then P&T,
    then Expansion valve etc, bla bla).

    Yes, possible but very improbable. I'd think it would be most likely if someone has tampered with the installation, e.g, capped off leaking or discharging safety valves.

    Something like that happened in the case of the Fleur-de-Lys, a UK fishing boat sunk by an exploding cylinder, in that case a domestic vented cylinder installed without a vent. There are ocasional explosions, most of those I've heard of were in the US where unvented systems are common.
  7. Garfield

    Garfield New Member

    A few years ago i saw a video of a specially staged demonstration of what happens when a cylinder explodes.As we all know,the boiling point of water rises when water is under pressure,so imagine a cylinder with 40ish gallons of water,under pressure,at something over boiling point,now if that pressure is suddenly released,possibly either by someone opening a tap,or,a pipe or the cylinder itself rupturing,the water inside instantly turns to steam.
    So we have 40 gallons of water multiplying in volume many thousands of times,in the blink of an eye.
    The timber house they had specially built to stage the experiment was totally blown to pieces,anyone in it would have been killed instantly,and ,about 20 seconds later,the cylinder landed in front of the camera.
  8. doitall

    doitall New Member

    Although my previous post was light hearted.

    On a serious note, All the heat source safety features/devices would have to fail first, before the UV cylinder could even get near to exploding.

    As I sort of said, with a complete failure of the boiler, it would probable fail, slit heat exchanger, water all over the place etc, a long time before the cylinder was in danger of joining the mear space station.
  9. r2d2

    r2d2 New Member

    What happens if an immersion heater fails and the safety valves are seized up solid. ?.
  10. What happens if an immersion heater fails and the
    safety valves are seized up solid. ?.

    seized up solid.... why... due to lack of maintenance!

    Also they have at least 4 safety devices on electric heated cylinders.

    normal thermostat
    O'heat thermostat
    Prv valve
    Temp/PRV valve.

    On gas heated (indirect) there is no way the water can boil within the unvented.
  11. doitall

    doitall New Member

    Immersion heater stat.
    immersion heater cut-out.
    Thermal cutout on the UV cylinder.
    Temperature and pressure relief valve.
    Pressure relief valve.
    Cylinder stat.

    If that lot fails at the same time you are very unlucky.
  12. r2d2

    r2d2 New Member

    seized up solid.... why... due to lack of maintenance!

    Yep. :(
  13. johnr

    johnr New Member

    it the basic physics that you all need to understand. water boils at 100 degrees right? WRONG!! water under normal atmospheric pressure boils at 100 degrees at sea level. water in a sealed hot water system will actually boil at over 100 degrees, in fact, water held at 3 bar will boil at 150 degrees. now theres a huge danger here that i will try to explain. if you dont get it, then dont touch unvented ***! when water boils, it expands by 1600 times its volume, therefore 100 litres of water will become 160,000 litres of steam. now if water is at 3 bar, the pipework in the system will be able to contain it, cos, well, its 3 bar, about average domestic water pressure in many places near me. HOWEVER, if its at 3 bar and all the safety devices have failed, unlikely i know but possible, then you have 100 litres of water at, lets say for example 150 degrees c, now its still at 3 bar. BUT, if it gets just a degree hotter, or if the pressure in the system drops by even a fraction, for example by someone opening a hot tap. then physics takes over, that 100 litres of water will instantly, and i mean instantly flash off in a high pressure steam explosion, not a  hollywood slo mo type explosion either , but a full on, house destroying explosion of superheated steam and water. the person who opens the tap wont even know whats hit them. theres plenty of cases of buildings being utterly destroyed by mains pressure hot water explosions. like they say, if you dont understand it, leave it alone.
  14. G Brown

    G Brown New Member

    Oh yeah?

    Give us some examples.....
  15. Onetap

    Onetap New Member

    "Oh yeah?

    Give us some examples...."

    For example, Fleur-de-lys above or

    Or Japanese nuclear reactors, it is the same principle. An uncontrolled/uncontrollable heat source putting heat into a sealed system, without any  means of removing the heat or releasing the pressure will eventually result in a bang-type pressure release.

    Domestic cylinder explosions in the UK have mostly been vented types (frozen, blocked OV, incorrectly installed). Unvented cylinders, installed properly and maintained, are as safe as can be practically achieved. Although they said that about those reactors as well.
  16. Farmacist

    Farmacist Guest

    Opening a tap thats connected to an overheated pressure vessel will not cause the water to instantly boil and explode. Steam will come out of the tap at high pressure, which could be nasty, depending on were you are standing, but thats about it. Just think, if what you said was true then very time a pressure relief vale opened the cylinder it was protecting would blow up! If water did boil in the  tank a small amount would expand to form steam, increasing in volume, but it has nowhere to go, so the pressure is increased and the boiling point goes up, water stops boiling. In other words its in equilibrium.
  17. joinerjohn1

    joinerjohn1 Screwfix Select

    On a completely unrelated subject (but about safety devices). Years ago I worked in a plastics factory. As senior shop steward, H&S was one of my areas of interest. One day a co worker asked me to come and have a look at an injection moulding machine he thought was unsafe. I had a look at it and on the machine there were 5 safety interlocks to stop the machine working if the main guard was opened. Each interlock ( 2 mechanical, 2 electrical and one hydraulic) had been "tampered" with, to ensure the machine would continue to run.
    I turned the machine off and fetched the manager to have a look. He told me that "This machine has to run."  and he knew the interlocks had been bypassed. He had allocated one person per shift to ensure that this machine ran 24/7. I asked him if he realised that a worker could lose their hand/arm if they opened the guard and reached in to remove anything caught inside the machine, to which he replied that the operators had been told to "Be Careful.". Hmmmmm, HSE certainly had something to say, once informed.;)

    I know it's nowt about unvented boilers, but I thought I'd use it as an example whereby safety devices had been bypassed.

    On the original subject, I did see the one on, Mythbusters where they took an unvented cylinder and applied heat until it did explode (quite literally too) But as pointed out on the program, they had made sure that no safety devices would operate at all. As I remember, the cylinder took off  like a rocket, reaching some height before falling back to earth.
  18. sinewave

    sinewave Screwfix Select

    Cue Walter!
  19. honestbob

    honestbob New Member

    When I did my unvented at a local college they said that they had one go several years earlier taking out a whole computer suite, but then everyone who teaches a course like that has to have known of at least one incident.  Don't know what type of system it was.  You would have to have a very poorly maintained cylinder to go that wrong, but at the same time how many people have their uv cylinders serviced?  And how many actually explode?  Have to agree that I don't think opening a tap will cause an explsion, only if tank itself splits and instantly unpressurises the super heated (?) water.
  20. Onetap

    Onetap New Member

    Walter seems to be absent from his post, so allow me;


    It's always good for another watch.

    All exploded cylinders were unvented, if they'd been vented, they wouldn't burst.
    It's the cylinder's way of letting you know the pressure relief didn't work. 

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