Conversion of pressure into energy equation

It has very little energy. The approximate formula is

E = 0.5λV_oP²

where λ is the compressibility of water (≈4.3 x 10^-10 m²/N), V_o is the volume before pressurization (we aren't told that, but can just use 1 m³, and p is the pressure. I'll probably now screw up the units, so check with somebody who has at least a two digit IQ (). Anyway, I get 0.02 J. That seems too low - I would have guessed at least 10J - so I probably screwed up the compressibility. HELP!

"pressurized water in a cylinder has a pressure of 10 kn/m^2"

Are you sure?

http://www.engineeringtoolbox.com/pressure-units-converter-d_569.html

10,000 pascals = 1.450377 psi

The energy that might be contained in your cubic metre hardly seems bothering with!

I'm still not sure about my units, but I worked it another way - how much work to compress it and got 0.05J, so maybe the 0.02J was OK.

The whole idea of doing hydraulic testing for pressure vessels is that the stored energy is very low and, should the vessel rupture during tests, the damage will be negligible.

Since water is very "stiff" it should be better to consider as well the energy in the vessel (cylinder) deformation under pressure, it can be higher!