Mythbusters and blowing up a hot water heater

Factual Questions
1

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

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

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

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

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

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

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

Hot water heater?

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

I hate how redundant that word is.

9

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.

No, they showed the inner and it had blown out at the bottom.

Interesting, and the best idea yet.

Agreed.

The kind that doesn’t provide domestic “hot water”. Like in industrial applications or swimming pools or whatever.

10

I’ve watched a longer feature now.

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!

11

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

Yes, I was.

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

No, you had it right and we have our answer. I just found this cite:

and also from here:

http://www.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.

14

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

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.

16

I have just a water heater in my house. It "provides domestic ‘hot water.’ "

17

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

So you have a domestic hot water heater?

19

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.

20

Quit complaining and go eat your tuna fish sandwich.