What's a T&P valve for?

Why would you install your water heater in hole?

Well....Maybe if your home was in a cave?

There once was a hermit named Dave, Who kept a dead w**re in his cave. He said, "I'll admit, She smells quite a bit, But look at the money I save!"

Jeff

After watching that - you bet! In a very deep hole - far removed from me!

I wonder if your 'average' terrorist is aware of this?

KNOCK KNOCK!! "yes' HOMELAND SECURITY! "and?" ....WATER HEATER............NO FLY LIST............

Ken.

Exactly. I saw the results of a steam explosion in my neighbor's basement. He rehabbed an old cook stove with a water back but plugged the pipes. Must have been a bit of water still in their. Fortunately, no one was in the basement when it went off and no shrapnel penetrated the overhead.

Harry K

Fortunately I'm not a terrorist since I though of the hot water as IED idea a long time ago.

Pete C.

I'm trying to imagine. For the sake of argument, would it really be any greater. The steam is what causes the heater to explode, right? It seems the heater would explode when a certain pressure was achieved, regardless of the amount of water heated to begin with. Wouldn't the 80 gallons of water tend to hold down the remainder of the heater, thus you might get a greater "flight" from the 12 gallon. I don't believe the entire water contents would "instantly" become steam and rupture the tank. Now, as I said, I'm just trying to imagine what would happen...I don't really know and have no intention of experimenting to find out.

Tom G.

I don't know either. I wish I had the info, but I saw where a commercial water heater blew. It was a larger one, 125-250 gallons, but I don't remember, but it about destroyed the building it was in and the heater landed several hundred feet out in the parking lot! Greg

The steam occurs *after* the tank ruptures, not before. The water under greater than atmospheric pressure has a higher boiling temperature than at atmospheric pressure. As the water is heated it expands increasing the pressure in the sealed tank and further raising it's boiling point. At some point when the water is well above it's atmospheric boiling point the pressure is too great for the tank and it ruptures. As soon as the tank ruptures the pressure returns to atmospheric at which point

*all* of the super heated water instantly flashes into steam with the resulting massive increase in volume producing the explosive shock wave similar to chemical explosives. The bigger the tank the more volume of super heated water to flash into steam.

Pete C.

You'd have 80 gallons of water at, say, 290° and 100 psi pressure (I haven't looked this up in a steam table, I just guessed.) The water is still liquid because it's held under pressure*. When the tank blows, it releases the pressure and all that water turns to steam and expands rapidly. [That should absorb a lot of energy due to the latent heat of vaporization, but there must be a huge amount of energy stored in the superheated water.]

*(the critical temperature of water is about 700°F, above this temperature water is always a gas regardless of the pressure.)

Bob

--FWIW the fact that many of these things seem to blow *up* and land a fair distance from where they started out makes me think the rupture is beginning at the bottom of the pressure vessel. This might mean the thing's filled up with crud and has never been drained properly, thus there is a weak band around the bottom, yes?

Try again...

Your physics is right, but your numbers are wrong. The heat of vaporization of water is 965 BTU/lb (British Engineering System) or 540 cal/g (cgs system...). (For sake of completeness, the figure is 2260 kJ/kg).

Jerry

The pressure is highest at the bottom.

In a closed system without pressure relief, the water can get quite a bit / a lot hotter than 212 without boiling. When the thing does burst, there would be a lot of pressure released from throughout the water, I think.

Something sort of like opening a warm 2-liter bottle of soda after shaking it up

>

If you have the water in the tank under pressure and superheated, as long as you can maintain the system pressure you are fine. You can draw off the steam slowly for use. But if the tank ruptures or the safety valve pops off and the pressure drops a bit, all the water will try to flash into steam at once - the ratio is 1600 gallons of steam from one gallon of water.

It WILL turn that tiny rupture into a big one, fast. And if the safety valve opens but is not big enough to vent the volume of steam that is developed and wants out (they are rated) the pressure can quickly spike past the vessel limits and blow the vessel.

You want to see real messes, look back in the history books for boiler explosions on railroad engines and marine boilers. They can launch the boiler hundreds of feet up, and level large structures. This is why boilers and water heaters are not toys.

-->--

I thought pressure was lighter than air and always rose upwards. Which is why airplanes fly and blimps float.

Gunner

The aim of untold millions is to be free to do exactly as they choose and for someone else to pay when things go wrong.

In the past few decades, a peculiar and distinctive psychology has emerged in England. Gone are the civility, sturdy independence, and admirable stoicism that carried the English through the war years . It has been replaced by a constant whine of excuses, complaints, and special pleading. The collapse of the British character has been as swift and complete as the collapse of British power.

Theodore Dalrymple,

I'm surprised that in 14 posts, nobody has yet brought up the explosion vs. BLEVE* distinction. :)

*Boiling Liquid Expanding Vapor Event

At last! A topic I can contribute to on this NG. First, the pressure is higher in the bottom when the contents is all liquid. And yes, it is possible that the crud has corroded a region around or near the bottom. But the real interesting thing is that with a bigger tank you get a bigger explosion. Here's why: The state of a gas is determined by pressure, temperature, quantity of the gas, and the contained volume (remember high school?). For most gasses, if you increase the pressure enough it will liquify. Water, or more precisely water vapor, condenses to a liquid under normal conditions. So the exploding water tank reverses the process. The water is heated throughout so all the water is heated way above the boiling point and most importantly, the pressure is not relieved (it builds up, as a mater of fact). When the tank cracks, even a little, the pressure is suddenly and dramatically reduced within the tank. ALL of the water flashes to steam. Worse, now that the water is a gas it continues to expand and release energy causing more damage (i.e.an explosion). This is why high pressure systems (3000 psi) are tested with liquid and not air. Hope this helps.

The entire contents no. A good portion of it yes. The physics goes something like this. The water boils producing steam until it begins the tank rupture, pressure drops and thus more water turns to steam (boiling point drops as pressure drops). It all happens in an instant but that is the sequence. Thus the total steam is much more than the amount present just before the rupture.

There is story about the Stanleyi Steamer and how the brothers tested a new boiler type in the same way (burying it out in the open). The design failed.

Harry K