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eyager
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| Joined: | Sun Jul 8th, 2007 |
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Posted: Sun Nov 11th, 2007 02:02 pm |
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For those who are looking for some new videos of water heater explosions, the show Mythbusters on the Discovery Channel recently did a segment about water heater explosions. They tested the "myth" that a water heater with a shorted thermostat/high limit switch, and a plugged T&P valve can explode and rocket itself though the roof of a house. After blowing up 2 different brand new heaters, they conclude the myth is true. Both heaters failed after the pressure in the tank exceeded 300 psi, and rocketed themselves high into the air.
However, there was a weakness in their testing procedure. They filled the tank by hand and plugged up both the inlets and outlets of the tank. In a typical hot water heater installation without a check valve at the water meter, wouldn't the extremely high pressure in the tank push steam into the city water supply at a rate sufficient to avert explosion?
Both heaters lauched themselves into the air instead of violently exploding down the middle. It still destroyed the small shed they built to simulate the house, but it could have been much worse. Do tank manufacturers deliberately produce a weak section on bottom of the tank to cause it rip open and rocket upwards rather than explode and produce large amounts of shrapnel?
The hosts stress the importance of testing the T&P valve, but they don't show anyone how to do it! Not much help as a public safety message.
The show will be rerun Nov 12 @ 7:00p EST on the Discovery Channel.
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elenano
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| Joined: | Sat Sep 11th, 2004 |
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Posted: Sun Nov 11th, 2007 03:04 pm |
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Watts, a company that makes T&Ps, has an old film showing all that stuff, too. With the right water heater, you can almost get to the moon!
Water heaters really so explode several times a year (we have news articles).
You test by pulling up on the handle and see if water will flow and see if it will stop when you let go of the handle.
Randy Schuyler
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mauvaisgenre
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| Joined: | Sat Nov 3rd, 2007 |
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Posted: Mon Nov 12th, 2007 05:44 pm |
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eyager wrote: In a typical hot water heater installation without a check valve at the water meter, wouldn't the extremely high pressure in the tank push steam into the city water supply at a rate sufficient to avert explosion?
Ideally in an open water system (one without a check valve), expanding water that exceeded the capacity of the water heater would flow back to the city main where the pressure would be dissipated. If it were a closed system though, thermal expansion could cause a rapid and dangerous pressure increase in the heater and piping, and if the relief valve didn't open, the system would blow. Usually (hopefully) a closed system would have a thermal expansion tank though.
I would imagine that if you had enough heat/pressure build rapidly on an open system, the pressure might not dissipate quickly enough (too slow of a rate), and you still might get an explosion. I haven't researched this though, so I don't know for sure. I'd like to know though, so if I find out, I'll post it. Or if anyone else on the forum knows, I'd also like to know the answer to this question.
Cheers,
Todd
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eleent
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Posted: Mon Nov 12th, 2007 10:12 pm |
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Hello: Although it is a matter that still raises opinions, basically, there is a direct relationship between steam, temperature and pressure. Steam will only form if it's hot enough and the pressure is low enough. Water boils at less than 212* at altitude. Water boils at 298* at fifty psi. If your heater won't shut off the heat source and the water is 298* and the tank breaks, the expansive forces of steam will not allow pressure to fall in the tank until a lot of the water has turned to steam. Basically once a tear starts in the steel tank there is a good chance the tear will grow until water/steam in the tank is gone. The tank may be 400 feet in the air by then  The same tear in the tank would stay small if the tank had cold water in it, as pressure would fall off with the leak.
It's interesting to note that a pound of steam is 1700 times bigger than a pound of water and under normal water pressure can contain more energy than a pound of black powder.
Yours, Larry
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