What's a T&P valve for?

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Tom G wrote:

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.
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All of what Pete said except not ALL the water flashes to steam, just a lot of it. Water holds 1 BTU per pound per degree F, heat of vaporization is 540 BTU's per pound. So if the unit gets to say 100 psi, it will be at 338 degrees, and you have 126 BTU's to flash into steam, or merely 20% of the water goes to steam. But that is more than enough for serious havoc.
Pete C. wrote:

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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

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Tom G wrote:

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 700F, above this temperature water is always a gas regardless of the pressure.)
Bob
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wrote:

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.
--<< Bruce >>--
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
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Tom G wrote:

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
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Now, I have learned something. Saw a story on TV the other day about the Mississippi Riverboat explosion at the end of the Civil War that killed about 2500 returning Civil War soldiers and former P.O.Ws. It was supposedly the worst maritime accident ever but we've forgotten it because it's importance was lost to the story of the assassination of President Lincoln. Only a small paragraph or two was reported in the newspapers of the time.
Tom G.
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Yeah! And it was completely blank out on the TV.
Bob Swinney

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Tom G wrote:

It would be much greater.
80 gallons of super-heated water (say 400 degrees) turns to much more steam (at 212 degrees) than 12 gallons of water when the pressure is relieved.
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Tom G wrote:

Similar to the dynamics of a locomotive boiler going. The initial rupture is of enough violence to releive the pressure on the remainder of the superheated water in the boiler. When it all goes to steam at the same time, it becomes something that one would rather read about than experience. Part of the design of safety valves looks at limiting the rate that pressure is releived for that purpose.
Cheers Trevor Jones
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Jeff Wisnia wrote:

Isn't it wonderful that I live in a place where they don't put all sorts of safety stuff in the lines. If my water heater overheats, the pressure just equalizes with the incoming cold water line since there are no restriction between the water tank to the street water supply.
First the water tank would boil and the pressure would push the water back in the cold water line. When the boiling water level dropped to the upper electrode, the electrode would burn out, possibly turning the power off, but if not, the water would continue to boil until the lower electrode was uncovered which at that time the electrode would burn out and coldwater would fill the tank.
Of course if I had a gas heater (which I do now), it would just boil dry and then the bottom would burn out and cold water would poor in.
I wouldn't notice anything until the water flooded.
I'll bet way more people are killed by lightening each year than by an exploding water tank. Probably about 10,000 to 1 more by lightening.
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George E. Cawthon wrote:

I've wondered the same thing. You'd have to have a closed valve (or a check valve) before the water heater tank and all the hot water faucets closed to get in trouble. But if it ever did BLEVE, there's a *lot* of energy released.
Bob
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So you could trigger the explosion opening any faucet, hot or cold?
I do not know about check valves but most utility have a antysiphoning valve, should not that vent as the pressure rise above the main?
Not trying to imply the T&P is not necessary, just want to understand all the physic involved.
Mauro
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MG wrote:

<snip>
-------------------------------- Not likely IMO as the explosion is caused by a sudden rupture of the tank, If the rupture hasn't happened before you open the faucet, opening it only reduces the pressure. Yes, more steam will be released as the pressure drops but the pressure is coming down. ---------------------------------

Any house sytem with a PRV (pressure reducing valve) or on a well system will be a closed system with nowhere for excess pressure to bleed off.
Anti siphoning vavles open when a pressure -drop- occurs. They wouldn't open on a rise in pressure.
Harry K
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MG wrote:

I don't know that "most" utilities have an antisiphon valve. I don't know that "most" utilities have any restriction. In fact, I don't know what "most" utilities do. But I know that there are utilities in various places that have no restriction in the incoming line. And if there are no restriction then then a T&P valve is just another possible failure point.
But obviously if there are backflow preventers of any kind, you would be foolish to not have a T&P valve.
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Many municipal water suppliers run over 100 psi, thus requiring each home to have a pressure reducer where the water line enters the home. The pressure reducer would act as a closed valve to a higher downstream pressure.
Shawn
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zxcvbob wrote:

I would bet that in most cases a hot water line would fail before the tank, especially in the supply line to faucets which are rubber hose, skinny metal, or plastic.
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You assume electric. Ok - does the stove top or oven element burn out ? It is the same material - cal rod.
Many places have anti-back flow devices at the street so some dufus doesn't set up a sprayer of some agent orange class of material to spray bugs or such - and siphon it into the water system. If the dufus doesn't have his own on the hose line, he will do himself/themselves in.
Which then means another valve in the house must be open to take the expansion.
How about gas ?
Martin Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member http://lufkinced.com /
George E. Cawthon wrote:

-
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Martin H. Eastburn wrote:

I don't know how many "many" is but I would say that many do not. I say this because heater installation instructions call for a vacuum breaker on the supply line. This prevent tank collapse if water is sucked out the supply line, as when a fire truck is pulling water from a hydrant. In this case water is flowing "backwards" to the street.
My $.02, Bob
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Bob Engelhardt wrote:

I have never seen a vacuum break on a water supply line to a hot water tank, and it sounds completely useless.
First if your fire truck scenario even worked, it would be the responsibility of the water system to install backflow preventers at the turnouts to the houses. Second, collapse of a tank is virtually impossible because I don't know how you would be able to suck the water out through the supply line. Water has a relatively low adhesion rate, so a strong enough pull (vacuum) would break the water column.
Second, if hot water tank lines did have vacuum breaks and there were no back flow preventers and the firetruck made the water flow "backwards," then the truck would soon be pumping water and air.
What would happen to the water if one end of a tangled system of pipes filled with water is closed and a strong vacuum is put on the other end? Nothing significant. Yes if you got a low enough pressure the water would boil and vapor would be slowly drawn off.
You are not going to crush a water tank (or any part of your water system) by putting a vacuum on the system.
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