Since I’ve never seen one trip, is it steam or water that comes out? They vent at 210 degrees, just under boiling, correct? Would it likely vent for just a couple of seconds? Repeated if somehow the overheating continues? Like maybe it would fire for 3 seconds every five minutes?
Water should come out. There should not be air or steam in your water tank.
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Under normal conditions it will release water in form of liquid each and every time water reaches 210F or 150psi, whichever is greater. However, if something goes very wrong and the water inside gets superheated and the TPRV finally gives way, it will end up releasing superheated water that instantly flashes into steam. Don’t try it at home 
Look up superheated water. It’s the reason why we have the TPRV to begin with.
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It would probably not be superheated. With the sensor working correctly it will release because of temperature long before it reaches 150 psi. It may flash boil as the pressure is reduced (temperature of the water would then instantly reach 212 F). As long as the TPR remains open pressure will continue to drop. (Note also that as the tank volume is reduced cold water will enter) Superheating requires maintaining high pressure to achieve temperatures past the boiling point. Water temperature will never exceed 212 F under normal (atmospheric ) pressures.
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Robert and Bert have you covered. Unless there is a catastrophic failure of the T&P valve, steam will never be produced. Steam can be produced in the sediment bed of a water heater but it causes no harm. Even then one is to assume all of a sudden the T&P valve operates after allowing the contents of the water heater to turn into steam, really.
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Not sure what I stated is inaccurate. If the burner gets stuck and TPRV does not release and or something else does not fail in the connected plumbing system to release the pressure, the water will superheat after it goes above 212F. TPRV is there to prevent superheating in case all other safety controls fail.
I would imagine, that whether water or steam, it would be at no less than the house water pressure, say 40 psi, and that in code legal installations with the pipe terminating 6" above a drip pan, boiling water would instantly splash all over the area and scald anyone who might happen to be standing nearby.
No, if properly maintained, the TPRV will crack open to release water slowly as the pressure or temperature builds up to release point. It does not go from fully closed to fully open unless it was stuck in a very unusual way and then finally gave way.
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Did I say you were inaccurate? Just referenced the term, you brought it up.
Exploding water heater 2 min. 55 sec.:
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Yup I just knew this thread was going to end up with an exploding water heater video. I thought the video from Arizona was going to show up, disappointed 
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wow. You could probably hear that a mile away. massive.
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Whiner! LOL! 
But, I did forget about that one It would have been good and still can be, Martin. 
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Alright I’ll post it😃
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I love that one. Enjoy the music!
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I remember that episode. 
Those guys blew up a lot of stuff. Kinda miss that show.
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That valve will never open🤨
One has to take into account Jr High chemistry though. “Boiling” is a relative term. At Sea level, water “boils” at 212 F, however in a near vacuum ice will vaporize or “boil” away. Where I live, 5000+ ft above Sea level, water boils at 185 F. So if a TPRV were to discharge here, the water would be “Boiling Hot” at 200 F.
Just some food for thought.
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