Consider a 5,000 gallon hot water tank which has a working pressure of 0.5MPa.?

Question

As a result of some malfunction, the pressure in the tank rises to 2 MPa at which point the tank explodes. Taking the atmospheric pressure to be 100 kPa and assuming the liquid in the tank to be saturated at the time of the explosion, determine the total explosive energy of the tank in terms of TNT equivalence. (The explosive energy of TNT is about 3250kJ/kg, and 5kg of TNT can cause total destruction of unreinforced structures within about a 7-m radius

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Answer 1

The previous answerer has a point, conceivably the pressure increase could be because of thermal expansion and the temperature could be below the boiling point, however, I'll assume there is space filled with vapor, and to get to the 2MPa pressure the temperature is about 212° Celsius.
At this point we are getting beyond my field of expertise, but I fooled around a while and came up with (don't ask me how) a number that is probably wrong, but sounds sort of plausible; perhaps around 5,000kg, however since the blast would affect a 3D area, I'd guess the total destruction radius would be the cube root of 1000, multiplied by that of your 5kg example, or 70m



I told you not to ask, but here is what I did, I figure you have about 19,000kg of water, entering 2000kPa into the calculator at the link below, I got an enthalpy of about 908kJ/kg for saturated liquid, and not having a clue what I was doing just multiplied that by the mass of the water, getting about 17,252,000kJ which I divided by 3250 to get the (probably incorrect) TNT equivalent.

Answer 2

If the tank is full of water chances are it will not explode bcause water is incompressible and will not expand upon a loss in pressure.

We used to hydrostatic pressure test all of our pressure vessels to 1 -1/2 x the maximum allowable working or design pressure, and sometimes we would test them to destruction in a concrete lined pit with a heavy steel/concrete cover.

With water in the tank it doesn't take much of a leak to drop the pressure way down to the point where the tank won't explode, and usually the high pressure would first cause a flange to leak, or would blow a threaded plug out of a coupling. Those that were able to be tested to destruction always split longitudinally and never really exploded.

Now, with steam or gas in the tank, I think that you could probably get a pretty big explosion.