MythBuster's Exploding Water Heater...

jpf321

I caught this on the tube last night .. thought it might be interesting to some .. it's quite dramatic to see a water heater launch (through a house) from overpressure condition (350+ psi) .. sorry if this has been posted before .. <a href="http://www.youtube.com/watch?v=pu3FwgIHsQA">http://www.youtube.com/watch?v=pu3FwgIHsQA</a> if you are impatient .. watch from the 7min mark ..



i also found a 2nd clip, presumably another edition of Mythbusters .. where they send a DHW heater through 2 stories rather than just a roof .. <a href="http://www.youtube.com/watch?v=fUkjXGfCLIM">http://www.youtube.com/watch?v=fUkjXGfCLIM</a>

Tim McElwain

Watts made a video many years ago

which by the way is now on disc of water heaters blowing up due to pressure and temperature. It is called Danger Lurks I believe. The danger is when you reach 250° F and 30 pounds pressure and above.



I personally experienced this in a house in Providence, RI many years ago. An old Vulcan heater with attached copper tank blew and went through three floors and out the roof and landed 100 feet up the street. Powerful stuff!

Jean-David Beyer

You can make tanks that withstand higher pressures...

While hot water heaters are not made for very high pressures, there are interesting videos on You Tube that shows what can happen if their design limits are exceeded that demenstrate the power involved.



In ages past, the manufacturers of steam locomotives designed machines to use this power in a more controlled fashion. In the early 1900s, Baldwin delivered a series of locomotives that used 200 to 210 psi (not a record) and exerted 60 to 70 thousand pounds of tractive force. They had over 3000 square feet of heating surface. As time went on, the number of explosions of steam locomotives was greatly reduced, because when one of those blows up, the results can be ghastly.

Jamie Hall

Part of the problem...

is that, with a water heater, let's suppose that something fails and it overheats -- to anything above the boiling point of water at the heater, when a tap is opened somewhere else in the house.  If you do that, and then open the tap, the pressure in the heater drops and the water in it flashes to steam, with no place to go -- and some 1700 times the volume.  Pretty spectacular... which is why you want T&P valves that work (same thing can, potentially, happen to a hot water boiler or a steam boiler).



Locomotive steam engines -- with a couple of rare exceptions -- went to 300 psi or thereabouts (Delaware and Hudson had a triple expansion locomotive running at, as I recall, around 800).  Pressures much higher than that gave lubrication problems.  The catastrophic explosions in steam locomotives usually came about when the water got low and the crown sheet on the fire box became uncovered and got too hot, and got weak -- and blew.  It was a quick way to dissassemble a locomotive, but it was usually pretty tough on the crew.

Tim McElwain

Water heaters today have

an ECO (Energy Cut Off) set to activate at 195° F to break the millivolt circuit to the valve and cause the pilot safety valve to close stopping all flow of gas.



As for pressure I saw in my 9 years in the Navy 600 pound steam systems on the destroyer I was on and the carrier had 1200 pound steam systems. A pin hole steam leak would cut a piece of 1/4" deck plate in half. They used a broom handle to find the steam leak. Passing the handle along the steam line until it cut the handle in half located the pinhole leak..

Jean-David Beyer

Blowing up my indirect-fired water heater...

I think it would be difficult to blow up my indirect-fired water heater. First line of defense is the aquastat in the water heater that is set at 125F and I am at near sea-level.



There is a T&P valve in the water heater that should also prevent it from rocketing around the neighborhood or flattening my house.



Then there is my boiler. If it does not fail, it is currently set to put 165F water into the jacket of the indirect, and that should not boil anything likely to be in there.



Now, depending on just what fails in my boiler, nothing in there should go over 210F. And it it did start boiling, its pressure relief valve should stop pressure build-up.



And if none of that worked, after a while, the low water detector (hot water boiler) should cut everything off. But it might not be soon enough.



Which seems to mean that I should check those relief valves every year as suggested by the boiler manual. I have picked a new contractor and he sems willing to check them. They are easy enough to test, but if they do not shut off completely, they must be replaced and I do not have spares. The contractor should have them in his truck. Maybe I should get one of each and keep them around on general principles. Do they go bad on the shelf?

Jean-David Beyer

Boiler pressures.

The highest boiler pressure I have read about in steam locomotives was 325 psi. These were firetube boilers. At least one watertube boiler was made for a locomotive, but I do not remember the pressure in that. I just looked it up; it ran at 450 psi and was not very successful. If you care, you can read about it here:



http://www.lner.info/locos/W/w1.shtml



Marine steam engines may well have run at higher pressures. I believe some power plant boilers went up to critical pressure (over 3200 psi).