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Coutesy of Jeff Lawrence - BOOM (PAH)

hot_rod Member Posts: 21,790

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Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream


  • Eric Taylor_35
    Eric Taylor_35 Member Posts: 29

    Where is the energy storage for a release like that in a 20 gallon tank? A large air pocket at the top? If the tank were full of water right to the top there is no place for the steam to accumulate and therefore no energy storage. The tank would burst for sure, but not like a rocket. That thing must have been mostly empty.

    Heres a thought-- was it plumbed in backwards? Draw the hot water out through the dip tube and air gets trapped at the top of the tank with no place to go.

    I wonder if they were taking REALLY short showers for a while before this happened.


  • Glenn Harrison
    Glenn Harrison Member Posts: 405
    That's why...

    I believe in testing relief valves, and if they leak after the test, OH WELL, time for new. And yes, I have found relief valves that were stuck closed or required a pair of channel locks on the handle to open them.

  • hot_rod
    hot_rod Member Posts: 21,790
    cause of boom


    Water under pressure can't boil at 212 F. It becomes superheated instead and can easily hit temperatures in excess of 300 F !

    All that's needed for that to flash over into steam is a sudden change in pressure or when it reaches the saturation point. The saturation point is where the temperature finally exceeds the pressure's ability to keep the water in a liquid state.

    That 20 gallons of water increases to 45,000 gallons (in volume) of steam & it does so in a nano-second = BOOM!

    Enough energy is realeased to equal several sticks of dynomite.

    This used to happen with great regularity in our country until one day a poor engineer walked into the Watts Regulator Co and presented Burchard E. Horne with his design for a T&P valve. Horne was smart enough to realise the engineer had hit on why water heaters were exploding! They'd been using pressure only relief valves and popping off with superheated water behind them = BOOM. With a probe that openes at 210 F, no more superheated water & no more BOOM. Horne signed him to an exclusive contract.

    That design was the basis for codes governing all T&P relief valves manufactured to this day! But the contractors of his day were skeptical about spending the extra money for a T&P when they'd been utilizing P valves. Horne's son went on a cross-country trip to convince contractors their T&P valve was the answer. He did this by exploding water heaters at each sales demonstration & the results were captured on film in a video available from Watts called "Danger - Explosion Lurks". Get a copy, you won't regret spending the few dollars.

    We're working on a video for Watts that is the sequel and deals with the new dangers - Legionella & scalding.

    To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jeff Lawrence_16
    Jeff Lawrence_16 Member Posts: 8

    Many thanks to Dave Yates for posting this for me.

    There are many times I work (actually play) with computers that I remember the little duck with the large hammer about "Press Any Key To Proceed."

    Some people aren't aware how dangerous water heaters really are.

    Show this to your customers.

  • Mark Hunt
    Mark Hunt Member Posts: 4,909
    But she said

    they turned the breaker off.


    They are both VERY lucky.

    Mark H

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  • Dave Yates (PAH)
    Dave Yates (PAH) Member Posts: 2,162

    I thought about that too, but given that the water was superheated, I suspect someone flushed a toilet or drew water, which would have triggered the reaction.

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  • Gary Fereday
    Gary Fereday Member Posts: 427

    Years ago, I worked for a explosives engineer,(Hercules Powder co) I asked him how much power a 40 gallon w/h had with 125 psi and 300*F water in it. He Calculated it and said that it was easily worth the heat release of 97 sticks of 90% Jelligunite, the best dynamite Hercules made at the time! Install those Temperature & Pressuer relief valves and check them frequently. Also Watts years ago had a movie of a 40 gallon W/h that was under the same problem. They busted a hole in the bottom, it landed 5 miles away,like a rocket!
  • ScottMP
    ScottMP Member Posts: 5,884

    In regards to the breaker being turned off, what about a fly wheel effect with the rising temperature. She claims they both were in bed ? You may be correct in that some one flushed a toilet.

    It made me think back to a customer of mine who had an old Sears electric HWH that had straps around the tank that where heating the tank. She called about 12:00 on a Sat. night. She was sorry to bother me but the tank was glowing red and the room was smokey " Sorry to bother you, but should I call the fire dept ?". I screamed yes and told her to get out of the house. How lucky she was.


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  • Dave Yates (PAH)
    Dave Yates (PAH) Member Posts: 2,162

    Could be the relief valve finally cut loose, which would have had the same effect.

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  • Art Pittaway
    Art Pittaway Member Posts: 230

    Dave, I have been looking for something powerful, interesting, and credible to show in my class at Rock Valley College on water heater danger and proper installation. The Watts vidio looks perfect. A picture is worth a thousand words and there is not enough time to pack everything into the class as it is. I've already ordered it. :) Thanks again! Art
  • Irene
    Irene Member Posts: 22
    time for a change

    You guys just sold me on a tankless....
  • Eric Taylor_35
    Eric Taylor_35 Member Posts: 29
    Thanks Dave

    I get it now. It's like opening a really big beer!


    Boiling Liquid Expanding Vapor Explosion. NFPA has a good film of this. It is what happens to LP tanks in a fire. With LP, the liquid explosion looks like water bursting out, and where and when it mixes 5%-15% Gas (in air)at the edges, the flame starts. It's a big flame, because LP expands 300 times when it turns from liquid to vapor, and only needs to be 5% gas in 95% air to explode.

    BLEVE -- boiling liquid expanding vapour explosion -- a BLEVE is the explosive release of expanding vapour and boiling liquid following the catastrophic failure of a pressure vessel holding a pressure liquefied gas such as propane or LPG. A BLEVE is the worst possible outcome when a propane or LPG tank is exposed to fire. BLEVE hazards include fireballs, blast, projectiles and possible toxic clouds or vapour cloud explosions. If you are a firefighter, emergency responder, plant safety professional, or a plant manager, you need to know what a BLEVE is and what it can do.

    From: http://www.thermdyne.com/

    Steam is expanded 1700 times from the water it came from. The math makes me realize how much steam formed instantly when the bottom of that tank let go.

    This topic is meant to illustrate how important MAINTENANCE is, not how dangerous any water heater is. Tankless Coils in Boilers are just as susceptable, as are direct fired and indirect fired water heaters.

    Gas and Oil boilers also need their safety devices tested every year.

This discussion has been closed.