OK, a chap here at work posted something on our company bulletin board about having recently changed his water heater–and finding out that a water heater can explode with the aforementioned explosive force.
This was “verified” by the service folks at the store where he bought his replacement, and he posted a link that “supported” their positions.
I did a quick Google search, and–sure enough–the first link that bubbled to the top was the the one he cited.
However, I was not able to find either corroborating information or a solid debunking–but I didn’t buy it.
IANAP but the key may be bubbles. If a vessel is full of water under extremely high pressure and it ruptures the results are mostly inconsequential. Since water doesn’t compress there is the same volume as when it’s not under pressure. The only difference is the walls of the vessel are displaced a bit outward. When the vessle cracks the water runs out, the vessel returns to shape apart from the break and that it. That’s why gas cylinders are pressure checked with water.
A compressed gas cylinder, or boiler full or steam or faulty hot water heater, has many times it’s unncompressed volume of gas squeezed inside. IIRC a SCUBA tank 200 times the normal volume of air in it. When the vessel lets go there is plenty of force available to fragment the remains and toss them about at high speed. The force isn’t dissipated until the compressed gas has returned to atmosphere pressure. Very dangerous to be around.
If you have a sealed tank of water heated to boiling point, the vapor pressure will be enormous. Boiler explosions were fairly common in the early days of steam locomotives and ships, until reliable safety valves (pressure relief valves) were developed.
According to the How stuff works article, water heater tanks are usually tested to 300 psi. Does someone want to calculate the internal energy of 40 gallons of 300 psi steam? (Sorry to leave the excercise to the reader, it’s late…)
No but I can add this. 1 cubic foot of water (about 8 gal) will create roughly 1,800 cubic feet of steam at 1 atmosphere. So you have about 5 cubic feet of water that if brought to 212+ degrees farenheit will try to create about 9,000 cubic feet of steam. Thats enough to fill a typical 2 bedroom house floor to ceiling.
Firefighters have been killed by exploding water heaters in house fires on numerous occasions.
In my neck of the woods they are required to have check valves to vent the pressure over X point to avoid this problem. The water will literally push back down the supply line if pressure greater than line water pressure is developed. It takes a couple unlikely failures to blow up a present day water heater, but if it does…you dont want to be in the house. I have 2 brick walls between my bed and my water heater so I don’t sweat it much.
I’m skeptical of the “200 sticks” figure. Wouldn’t 200 sticks of dynamite completely level a house? Whenever a house explodes, they always seem to suspect gas, not hot water heaters.
At the top of this page is a report of a hot water heater exploding and shooting through the roof, but apart from the holes, the house was OK. It landed about 150 feet away.
Like I said a few unlikely failures( stuck thermostat and bad check valves)…or extreme circumstances (house fires). We were taught in fire academy to open drain valves on water heaters if we encountered them in involved areas to make sure they didn’t burst.
My fire investigation book IIRC made a statement to the effect that it only takes 5-10 PSI immediate pressure change to blow walls out of a house. A bursting water heater cauld easily generate that much of a pressure spike in a room.
Most likely split a lower seam and effectively made a crude little steam powered rocket. Not quite a steam explosion but I’m sure it was quite impressive nonetheless.
A house can be leveled with a lot less than 200 sticks of dynamite.
This table of Saturated steam pressure suggests that water needs to be heated to over 400 °F to achieve a water heater’s bursting pressure of 300 psi. By the time that happens, nearby objects will likely be heated to ignition temperatures and the house will burn down. (According to R. Bradbury, paper will ignite at ~451 °F) Even in the unlikely even of the internal pressure passing 300 psi, there still isn’t enough heat stored in the water to make it all vaporize when the tank ruptures. There’s enough energy to wreck a house and propel the tank through concrete walls and such, but that’s still a lot less than what 200 sticks of dynamite can do.
Those T&P relief valves can get old, corroded and not work. That’s why it is a good idea to check them every so often, about every 6 months or so, or more if you live in an area with very hard or corrosive water. Just pull on the little tab enough to get some water out. If it is hard to pull, or if it won’t shut off again, you need a new one. (Know where your shutoff valve is, just in case you have trouble getting it shut off.) What comes out will be very hot water, so don’t let shoot on your bare feet or your cat. Ideally, this line should be plumbed outside onto the ground. Testing the T&P valve should also increase its serviceable lifespan, because like any valve, it gets stiff if it is not used, and using it busts up the corrosion.
When you install a new water heater, always get a new T&P valve. Don’t try to salvage the old one off the old water heater. They aren’t designed to last the lifetimes of multiple water heaters.
I agree that 200 sticks of dynamite is absurd, but that doesn’t mean a stuck T&P valve, coupled with a bad thermostat, won’t kill you. I’d wager that more than 10% of T&P valves on water heaters in places like Corpus Christi, where the water is hard and corrosive, are totally frozen, and thus worthless.