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#1
Old 04-13-2008, 11:23 PM
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Mythbusters and blowing up a hot water heater

I just saw this episode. I recorded it ages ago, but only just watched it. For those who haven't seen it, the Mythbusters bypass the thermostat on a storage electric hot water heater, block the safety valve and keep heating these suckers till they blow up.

My question: why was it that on both occasions the mode of failure was the bottom blowing out, rocketing the boiler sky high?

I would have thought the weak point was the threads on the pipe fittings on the side, or any of the weld seams: top, bottom or side. Yet they did it twice and both times it was the bottom that failed. And they reported that there had been several other accidents in which boilers had exploded, resulting in the boiler going up.

I suppose that they could be designed to fail at the bottom, but I would have thought the boiler going up would be the least safe option, given what results.
#2
Old 04-14-2008, 12:35 AM
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WAG from a non-plumber non-expert: the bottom is where the heating element is. So, when something happens that causes the heating element to overheat, it happens at the bottom.
#3
Old 04-14-2008, 01:36 AM
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Actually, the typical electric water heater has two heating elements, one near the bottom and one near the top. Cold feed and hot discharge ports are usually located on the top, but most of the cold water entering is diverted to the bottom via an anti syphon tube. This is to improve efficiency, i.e: cold water isn't cooling the water at the top of the tank and sending it right back out, leaving the majority of the heated water in the tank. If there is no use for awhile, the top heater tends to shut off before the bottom heater, meaning that over time the bottom of the tank is subjected to much more heating and cooling, which contributes to electrolytic action, the primary cause for internal corrosion. As to welds, they are normally stronger than the surrounding metal and any mechanical joints. In domestic WH's, catastrophic failure is very rare and almost always occurs from human error in defeating required safety devices. Failure of tanks by corrosion is almost always through one, or more, of the mechanical joints' resulting in a noticeable leak, or if left unnoticed, a local flooding.
#4
Old 04-14-2008, 01:46 AM
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The tanks used by the Mythbusters were brand new, so weakening due to corrosion isn't the answer, if that is what you were suggesting.
#5
Old 04-14-2008, 02:15 AM
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If they consistently blew out the top would you wonder why that happened? Products that are mass produced will have the same weaknesses, one part has to be the weakest. I doubt they designed for it to specifically be the bottom.
#6
Old 04-14-2008, 02:22 AM
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Sure I would. And obviously you are right that mass produced products may have the same weakness. My question though is why is it consistently the bottom? I mean I would have thought that if you are producing a cylindrical pressure vessel, you'd use the same process to construct the top and the bottom. So why would the bottom always blow? Or alternatively, maybe the tanks are constructed differently at the top than the bottom. OK, fine. What is that difference?

Maybe it was just a co-incidence that both the tanks tested blew the bottom. Maybe the two "real life" incidents they mentioned on the show were only mentioned because they happened to blow out the bottom and rocket upwards.

I don't know. That's why I'm asking.
#7
Old 04-14-2008, 02:30 AM
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Thanks to Youtube, I have now seen some of it. I'm not sure there's enough data to answer the question, but I have a few ideas.

First, I don't think what you see is what you get. We see a flat-ended cylinder, which is not an ideal container to hold pressure. And from the voice-over, the tanks are rated to hold 150 psi. So I'm guessing that inside is a dome ended cylinder to hold the pressure, surrounded by insulation, and then a thin outer housing, which is what we see.

Secondly, pressure vessels are usually designed to be tough, i.e. they bend before they break. So the pressure vessel inside isn't going to shatter into frag like a glass bottle. It'll pop a seam, or even fail at a thread as you suggested. So it might be only the outer shell that fails at the bottom and gives the rocket effect.

One reason the outer shell might always fail at the bottom is a subtlety of geometry. When the steam blasts blasts out into the shell, it's trying to inflate it like a balloon. The wall is a cylinder, the top is flat but it can dome upwards, the bottom is flat but it's constrained by the ground. So the bottom seam remains at a right-angle for a fraction of a second longer, which makes a nice stress concentrator.

Or maybe I'm over-complicating things. Maybe the bottom of the shell is simply crimped on, while the top is more substantially attached, for obscure manufacturing reasons.

Another alternative is the effect of orientation and headspace. Presumably the pressure vessel isn't entirely full of water, so you have a headspace of pressurised steam at the top. Boiling is a vigourous process so there is agitation, steam bubbles travelling to the top and bursting. Maybe a shockwave effect, or something else to do with this asymmetry, means the bottom dome always blows off first.

All WAGs. More investigation is needed! We need before-and-after dissection of a tank. More burst tests with a tank upside down, a tank on its side, a tank suspended off the ground by a collar around the cylinder...
#8
Old 04-14-2008, 02:34 AM
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Hot water heater?

What other kind of water heater is there?!?!

I hate how redundant that word is.
#9
Old 04-14-2008, 02:43 AM
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Quote:
Originally Posted by matt
So I'm guessing that inside is a dome ended cylinder to hold the pressure, surrounded by insulation, and then a thin outer housing, which is what we see.
There is no doubt a thin outer shell. If you watch the high speed footage you can see that the outer shell gets blown away like tissue near instantly while the heavy pressure cylinder goes skyward. At one point Adam (picking over the remains) finds the base of the pressure vessel and comments that it had gone from concave to convex in the explosion.

Quote:
So it might be only the outer shell that fails at the bottom and gives the rocket effect.
No, they showed the inner and it had blown out at the bottom.

Quote:
One reason the outer shell might always fail at the bottom is a subtlety of geometry. When the steam blasts blasts out into the shell, it's trying to inflate it like a balloon. The wall is a cylinder, the top is flat but it can dome upwards, the bottom is flat but it's constrained by the ground. So the bottom seam remains at a right-angle for a fraction of a second longer, which makes a nice stress concentrator.
Interesting, and the best idea yet.

Quote:
All WAGs. More investigation is needed! We need before-and-after dissection of a tank. More burst tests with a tank upside down, a tank on its side, a tank suspended off the ground by a collar around the cylinder...
Agreed.

Quote:
Originally Posted by Argent Towers
What other kind of water heater is there?!?!
The kind that doesn't provide domestic "hot water". Like in industrial applications or swimming pools or whatever.

Last edited by Princhester; 04-14-2008 at 02:44 AM.
#10
Old 04-14-2008, 02:54 AM
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I've watched a longer feature now.

http://youtube.com/watch?v=pu3Fw...eature=related

Confirmations and refutations:

At 5:22 you can see insulation fragments rolling around on the ground. That's a guess right.

At 5.40 you can see the internal pressure vessel popping out through the top of the casing. That's another guess right, but also ruins my theory that it's all to do with the casing. The internal pressure vessel itself goes up like a rocket.

At 7.55 we get a BIG clue. The voiceover says "the bottom of the tank went from concave to convex". If he's talking about the pressure vessel inside, rather than the casing, then it's simply the geometry of the pressure vessel. A concave bottom lets you stand the thing up easily while you're welding and drilling and manufacturing. But it means the weakest point is the bottom seam, because of that concave end.

On edit: Hey, we think alike!

Last edited by matt; 04-14-2008 at 02:55 AM. Reason: Simulpost!
#11
Old 04-14-2008, 03:03 AM
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So are you assuming that the top is convex and the bottom concave. Could be right in the instant case.

The only thing is, I once pulled one apart and I'm pretty sure it was concave top and bottom. That's why I like your ground constraint theory.
#12
Old 04-14-2008, 03:16 AM
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Quote:
Originally Posted by Princhester
So are you assuming that the top is convex and the bottom concave. Could be right in the instant case.
Yes, I was.

Quote:
Originally Posted by Princhester
The only thing is, I once pulled one apart and I'm pretty sure it was concave top and bottom. That's why I like your ground constraint theory.
Bugger. I don't like the ground constraint theory any more. The casing and insulation isolate the pressure vessel from the ground, to my mind.
#13
Old 04-14-2008, 03:35 AM
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No, you had it right and we have our answer. I just found this cite:

Quote:
Today, the most common of domestic water heaters comprise a pressure vessel having a cylindrical wall, a hemispherical top and a concave, hemispherical bottom which is directly exposed to a gas or oil burner.
and also from here:

http://maib.gov.uk/cms_resources/fleur-de-lys.pdf (I can't cut and paste from this .pdf but if you search on "concave" you'll find a description of a hot water system which is described as typical British domestic and which has a convex top and concave bottom.)

Also there is this manual from Rheem, a large Australian manufacturer, which is confusing because the diagrams don't agree with the text (I think the text confuses convex and concave). It says it used to produce tanks that were double minus (ie concave top and bottom) pre 1981 (which would explain what I saw) but post that a plus minus (convex top, concave bottom) design has been used.

So we've cracked it.

Last edited by Princhester; 04-14-2008 at 03:39 AM.
#14
Old 04-14-2008, 03:41 AM
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Although welds on pressure vessels and piping are designed to be under tensile stress as opposed to shear, I have seen some water heaters where the bottom head is placed with the concavity towards the outside of the tank and its flange joined to the skelp (cylinder) with lap weld, which would be the weakest point.
But a simpler explanation for the bottom blowing first is not only taking the strain of thermal expansion but the considerable weight of water contained.
#15
Old 04-14-2008, 03:46 AM
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The pressure created by head of a few feet of water is of the order of 2 or 3 psi. The Mythbusters' cylinders blew at about 350 psi. Not likely to make the difference.

Last edited by Princhester; 04-14-2008 at 03:47 AM.
#16
Old 04-14-2008, 03:57 AM
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Quote:
Originally Posted by Princhester
The kind that doesn't provide domestic "hot water". Like in industrial applications or swimming pools or whatever.
I have just a water heater in my house. It "provides domestic 'hot water.' "
#17
Old 04-14-2008, 04:58 AM
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Quote:
Originally Posted by Carson O'Genic
I have seen some water heaters where the bottom head is placed with the concavity towards the outside of the tank and its flange joined to the skelp (cylinder) with lap weld, which would be the weakest point.
That's where the things seem to fail, for sure! I think the failure mode is more complicated than a simple shear of the weld though.

The Mythbusters experiments show that the concave bottom head inverts during the failure. This makes sense, in that the rest of the vessel wall sees purely tensile loads whereas the bottom head sees compressive loads and can fail by buckling. Buckling and catastrophic inversion (or should that be EXversion?) of the bottom head subjects the weld to high bending/tearing loads, and a kinetic shock as well.
#18
Old 04-14-2008, 06:04 AM
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Quote:
Originally Posted by Contrapuntal
I have just a water heater in my house. It "provides domestic 'hot water.' "
So you have a domestic hot water heater?
#19
Old 04-14-2008, 08:19 AM
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Quote:
Originally Posted by Princhester
So you have a domestic hot water heater?
No, just a water heater. Like the kinds you can purchase here. If I had hot water I wouldn't need a heater. Mine heats water that comes into the house from pipes laid underground, so my best guess is the temperature ranges from the low thirties to the low fifties Farenheit, depending upon the season, which is at the low end of the range where water remains a liquid. I understand that "hot" and "cold" are relative terms, but after my water heater heats the water it is around 120 degrees, so I feel confident that most folks would understand that it is not heating hot water.

Last edited by Contrapuntal; 04-14-2008 at 08:20 AM.
#20
Old 04-14-2008, 09:34 AM
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Quote:
Originally Posted by Argent Towers
Hot water heater?

What other kind of water heater is there?!?!

I hate how redundant that word is.
Quit complaining and go eat your tuna fish sandwich.
#21
Old 04-14-2008, 10:17 AM
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Quote:
Originally Posted by matt
The Mythbusters experiments show that the concave bottom head inverts during the failure. This makes sense, in that the rest of the vessel wall sees purely tensile loads whereas the bottom head sees compressive loads and can fail by buckling. Buckling and catastrophic inversion (or should that be EXversion?) of the bottom head subjects the weld to high bending/tearing loads, and a kinetic shock as well.
This is what I was concluding also. When the concave base "pops," there's enough pressure that it just basically keeps going.

You know what this means, of course: The MythBusters need to explore this "why does the bottom fail and the tank go up like a rocket?" question, and validate the analysis in this thread, by testing a variety of water heaters and other high-pressure vessels to see how the design of the tank influences the likely failure points. We get concrete data, and lots more things blowing up. Win-win.
#22
Old 04-14-2008, 06:11 PM
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Quote:
Originally Posted by Contrapuntal
No, just a water heater.
So it provides hot water?
#23
Old 04-14-2008, 06:22 PM
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Quote:
Originally Posted by Princhester
So it provides hot water?
I apologize for being unclear. The city provides cold water to my house, via a system of pipes that are pressurized by water towers. Some of this cold water is diverted to my water heater, which heats the water and returns it back into my house system, where it can be accessed by opening one of two valves (the one labeled "hot") at various sinks, a washing machine, and a shower.

It doesn't provide the water. It heats the water. Hence the name -- water heater.
#24
Old 04-14-2008, 06:23 PM
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And at what temperature is the water when it comes out of the heater? Hot, is it?
#25
Old 04-14-2008, 06:24 PM
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I think the weight of the water does add to the stress and when the whole container is near failure the added weight would make the bottom pass the fail point first, This is if the bottom and top have the same strength in construction.
#26
Old 04-14-2008, 06:31 PM
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Quote:
Originally Posted by Princhester
And at what temperature is the water when it comes out of the heater? Hot, is it?
You know, I now believe that your confusion is not my fault. Can you really have read my posts and wonder if my water heater actually heats water? I said it was around 120 degrees F, just upthread. I suppose you could argue that that isn't hot, but it would seem to me to be quibbling.

You're having a go at me, aren't you?
#27
Old 04-14-2008, 06:33 PM
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I just want to see you confirm, in terms, that the water is "hot".

When you've done that, we can go on to the next step.
#28
Old 04-14-2008, 06:35 PM
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Quote:
Originally Posted by Princhester
I just want to see you confirm, in terms, that the water is "hot".

When you've done that, we can go on to the next step.
Correct. After the water heater heats the cold water, the water become hot.
#29
Old 04-14-2008, 06:47 PM
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It would be a hot water thermos if it didn't heat the water, and it does heat the hot water that is starting to cool down, but is still hot. Technically it is a hot water heater.
#30
Old 04-14-2008, 06:52 PM
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Thing is that English as used isn't logical. Or, it is but at a contorted level. We refer to the stuff that comes out of the tap with the red "H" on it as "hot water" not merely because it is water and it is hot, but because that is the usual term for domestic water that comes out of the tap with the red "H" on it.

The thing that produces said "hot water" is a water heater. So just as we call the thing that produces bread a bread maker, we call the thing that produces the hot water a hot water heater. Yes, there is redundancy, but there you go.
#31
Old 04-14-2008, 06:55 PM
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I watched the vid via provided link and paid attention to the footage during the comment "went from concave to convex". There are remnants of weld visible with shell metal surrounding, not surprising with a mild steel at ~45k PSI and weld at 70k PSI.
The inverted bottom head is forced outward and rips the weld seam loose.Note the shell maintains cylindricity until return to tarmac.
Had they maintained a true hydrostatic test ( Let's not fill it all the way!) it would have made for boring TV. The ullage allowed the formation of steam and corresponding enormous expansion.
#32
Old 04-14-2008, 07:04 PM
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Quote:
Originally Posted by Princhester
My question: why was it that on both occasions the mode of failure was the bottom blowing out, rocketing the boiler sky high?

I would have thought the weak point was the threads on the pipe fittings on the side, or any of the weld seams: top, bottom or side. Yet they did it twice and both times it was the bottom that failed. And they reported that there had been several other accidents in which boilers had exploded, resulting in the boiler going up.
It's because the bottom was the weakest point. If you reinforced the bottom, you would then find the second weakest part.
#33
Old 04-14-2008, 07:09 PM
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Quote:
Originally Posted by Rick
It's because the bottom was the weakest point. If you reinforced the bottom, you would then find the second weakest part.
Thanks, Einstein

My question amounts to: why is the bottom consistently the weakest point. And between Matt pointing me in the right direction and my own research (see #13) the answer would seem to be: it is constructed completely differently, as it is concave while the top is convex.

What really held me up was that (as I mentioned above) I once pulled one apart and the top and bottom were constructed the same. So I couldn't see any obvious reason why they would always blow out the bottom. But it seems, according to one cite I found, that at least in Australia the common design changed from "top and bottom both concave" to "top convex, bottom concave" in about 1981.

Last edited by Princhester; 04-14-2008 at 07:12 PM.
#34
Old 04-14-2008, 07:24 PM
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Quote:
Originally Posted by Argent Towers
Hot water heater?

What other kind of water heater is there?!?!

I hate how redundant that word is.
Surely it ought to be a cold water heater.
#35
Old 04-14-2008, 07:43 PM
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Quote:
Originally Posted by Mangetout
Surely it ought to be a cold water heater.
Apparently in Australia, they like to heat their hot water.
#36
Old 04-14-2008, 08:57 PM
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Correct. In mid summer, the heater just makes it hotter than it already is.
#37
Old 04-15-2008, 02:35 AM
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Quote:
Originally Posted by Princhester
The thing that produces said "hot water" is a water heater.
Correct.

Quote:
So just as we call the thing that produces bread a bread maker, we call the thing that produces the hot water a hot water heater.
And the thing that produces sliced meat is a sliced meat slicer. The thing that sews button holes is a sewn button hole sewer. The thing that paints lines on the road is a painted line painter. Right?

The thing that produces hot water is a hot water producer perhaps, but not a hot water heater. My cite is your quote, above. As well as my link upthread to where they sell water heaters.
#38
Old 04-15-2008, 02:42 AM
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Quote:
Originally Posted by Contrapuntal
And the thing that produces sliced meat is a sliced meat slicer. The thing that sews button holes is a sewn button hole sewer. The thing that paints lines on the road is a painted line painter. Right?
There's a hill in the UK the name of which, if you translate it, means "hill hill hill", each "hill" being in a different original language. Nonetheless, the name of the hill, is the name of the hill

"hot water heater" gets 1.7m hits on google including hits on manufacturer websites, wikipedia, howitworks etc.

It's a recognised term. It may be illogical, but that's English for you. Get over it.
#39
Old 04-15-2008, 02:51 AM
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Why not just call it a boiler?
#40
Old 04-15-2008, 03:01 AM
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Quote:
Originally Posted by Princhester
There's a hill in the UK the name of which, if you translate it, means "hill hill hill", each "hill" being in a different original language. Nonetheless, the name of the hill, is the name of the hill.
The proper name, perhaps, but the thing named is simply a hill, as you point out yourself.
Quote:
"hot water heater" gets 1.7m hits on google including hits on manufacturer websites, wikipedia, howitworks etc.
And "water heater" gets 2.44m hits.

Quote:
Get over it.
Brilliantly played. I was a little foggy on where you were going with your pseudo Socratic musings, but now I see that it was all a clever trap. So clever you could put a tail on it and call it a weasel. My hat is off to you, sir!
#41
Old 04-15-2008, 03:03 AM
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Furthermore, if you google on "hot water heater" AND "water heater" you get about 2.4m hits, showing that the terms are used interchangeably. Plus if you google "hot water heater" and limit the search just to homedepot.com (ie your chosen cite) you get 137 hits, and not all of them to reviews etc written other than by Home Depot itself.

Mangetout, because it doesn't boil.

Last edited by Princhester; 04-15-2008 at 03:05 AM.
#42
Old 04-15-2008, 03:04 AM
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Quote:
Originally Posted by Contrapuntal
And "water heater" gets 2.44m hits.
Showing that both terms are in common usage. Yours rather more than mine, signifying little. I must say I get nearly 9m hits for "water heater".

Last edited by Princhester; 04-15-2008 at 03:06 AM.
#43
Old 04-15-2008, 03:21 AM
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Quote:
Originally Posted by Princhester
Mangetout, because it doesn't boil.
http://dictionary.reference.com/browse/boiler (4)

Last edited by Mangetout; 04-15-2008 at 03:21 AM.
#44
Old 04-15-2008, 03:31 AM
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Hey look I don't doubt that the term "boiler" is used to mean a domestic water heater. And by my own arguments above, if the term is in common usage that way then feel free.

But you asked why not, and I answered.

Last edited by Princhester; 04-15-2008 at 03:31 AM.
#45
Old 04-15-2008, 09:02 AM
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Quote:
Originally Posted by Mangetout
Why not just call it a boiler?
A boiler is for providing heating for buildings.
#46
Old 04-15-2008, 09:15 AM
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Quote:
Originally Posted by Harmonious Discord
A boiler is for providing heating for buildings.
I much regret interjecting up there in the thread. But no. A boiler (here, at least) is for heating water, for any purpose.
#47
Old 04-15-2008, 02:40 PM
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On the Googlefight, note that every page that uses the string "hot water heater" will necessarily also use the string "water heater", not because the terms are interchangeable, but because the one is a substring of the other. By the numbers above, "hot water heater" is about 2.5 times more common than the unmodified "water heater".
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#48
Old 04-15-2008, 04:28 PM
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Quote:
Originally Posted by Carson O'Genic
Had they maintained a true hydrostatic test ( Let's not fill it all the way!) it would have made for boring TV. The hullage allowed the formation of steam and corresponding enormous expansion.
Don't think that's the case. It wouldn't be the same as a COLD hydrostatic test where you keep pumping water in till the thing pops. That would be pretty boring, as you say!

Thing is, the water was at WAY above the boiling point for ambient pressure, so even without hullage space, when the tank let go a lot of the water would instantly flash to steam from the reduced pressure.


Back of the envelope calcs:

temperature of water-steam equilbrium at 350psi is 432 Farenheit, or 222o C. That is 122o C above boiling point at 1 bar, meaning that there was 122 calories per gram available for turning the water into 100o C, 1 bar steam when the tank let go.

Heat of vaporisation of water is 539 calories/gram, so there was enough heat energy stored in the water for 23% of it to flash to steam when the tank let go. that's 23% of the water volume expanding ~1600 times.

By comparison, a tank filled only with steam at 350psi and no water is at 24 bar pressure. At most, the steam will expand to 24 times the volume when the tank goes. (I know it won't be isothermal. It's an upper bound. Sue me!)
#49
Old 04-15-2008, 04:44 PM
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Hey, putting the semantic dancing aside and trying to get back to the actual question, here's a WAG that is almost certainly not true and is giving the manufacturers of [hot] water heaters more credit than they deserve, but it might be fun to contemplate:

What if the heater was actually designed to fail at the bottom? It could be argued that that would be a last ditch safety measure. Look at the amount of power that an exploding heater wields. If the vessel could fail at any random point, chances are that the vessel would be driven wildly around through the building housing it, increasing the chances of causing injury to a person. But when the vessel fails at the bottom, as we saw, it's driven straight up. Sure, anyone close by or directly above it is toast, but I'd maintain that from a strictly statistical standpoint, blowing up will cause fewer injuries than blowing sideways.
#50
Old 04-15-2008, 05:07 PM
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Quote:
Originally Posted by Roadfood
What if the heater was actually designed to fail at the bottom? It could be argued that that would be a last ditch safety measure
That's just about possible. This kind of thinking can be seen in liquified flammable gas cylinders, where a "burst disc" is backed by a low melting point alloy. In a fire, the alloy melts, the disc bursts and you get a nice tornado of flame as the gas comes out. Not great, but preferable to letting the thing store up energy as heat in the liquid and then rupture in an uncontrolled way.

However, I consider it unlikely. The truth is, there's absolutely no reason why these tanks should be able to hold more than a few bar of pressure, just enough to run a decent shower! Constructing the tanks with a weak spot that pops at 10 bar would be way safer than designing the things to hold 150 bar and let the bottom blow out first. Hell, a rupture disc at the top, backed with Newton's metal (melts at 97 Celcius, 207 F) would solve all the problems by venting the tank before it even gets to boiling.
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