We had a case in Kingston that took out the back wall of a council house. A combination of factors:
Feed to cylinder was a screwdown stopcock (which acted as a non return valve), not a gate valve
Cylinder vent pipe was 15mm which had furred up solid because
The immersion stat wasn't cutting out and so the water got really hot. Tenant turned off the immersion when he heard the water starting to boil. The tenant couldn't call in maintenance to fix it because
He'd got the meter illicitly bridged which would have been spotted.
If you heat water in a sealed vessel the temperature can rise to above 100C. If the cylinder is then ruptured the superheated water turns to steam: IIRC 1 cubic inch of water = 1 cubic foot of steam.
Interesting but how can that be relevant since there should be a NRV in the feed anyway?
Indeed that's the real risk.
But where is the reference to the council's installation sign-off and regular testing? Presumably not done? The system should have had both a PRV and a temperature relief valve. On mine its a separate discharge to the tundish so no single pipe to fur.
If you pressurised a cylinder full of water to 10 bar and it went pop, what do you suppose would happen? The answer is not much, and certainly not "some kabboooom" as some self proclaimed usenet expert has been noted as stating.
There would be lots of water about, but not much in the way of an explosion. This is because water is inelastic and unable to store much energy in compression. (Hence why steam boilers and pressure vessels are always pressure tested with water and not air or steam - it is a relatively safe way to do it).
The danger with a water heater, and it is a danger that is only really of particular relevance to an electrically heated one[1], is that of superheated water. Under pressure the boiling point of water rises[2]. Taking the extreme example of 10 bar storage pressure, the boiling point would be in the region of 450 degrees C. Beyond this you finally start boiling and manufacturing steam. Once you start making steam the pressure will rapidly rise far in excess of the static mains pressure. Now when the cylinder finally fails you have a new and very serious problem, that of the de-pressurised superheated water. The moment you release the pressure it will spontaneously convert to steam and undergo a massive (i.e. explosive) expansion in volume.
[1] With other indirect heating methods the maximum temperature rise of the cylinder is limited by the maximum primary temperature that can be reached by the boiler etc which is obviously far lower, given the much lower pressure of the primary water and the presence of yet further safety interlocks in the boiler.
[2] The relationship between boiling point and temperature is a complex non linear regression. google Clapeyron's Equation for more information.
It sounds like Tony is describing a conventional vented hot water cylinder here (i.e. no inspection required, and no additional safety features like PRVs etc), where both the normal expansion routes have been occluded, at the same time as the heater stat has failed.
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