Gippeto wrote:Brettel is absolutely correct.
With hydrotesting, when/if a failure occurs, there is a very small "pfft" and that's it. It's not dramatic at all, even at over 6000psi. Been there, done that.
If you fill a vessel with water, and then pressurize it with air(or co2, etc.), you are defeating the whole reason for hydrotesting.
The air used to pressurize, is STORED energy. It WILL force the water out through any failure,..at pressure.
Ever heard of a water jet cutter? A lot more pressure, but if you think that you're being "safe" by "only" pressurizing with a gas, you are dead wrong.
Failures with stored energy, will be dramatic, and dangerous.
Please don't fool yourself into believing otherwise.
Gippeto, you are correct but I believe you have overstated things a bit.
Yes, at very high pressures a jet of water can cut through steel. At 300 PSI it won't.
In hydraulic systems there is a significant hazard anytime the pressure is high enough to inject hydraulic fluid through the skin. You don't have to slice a limb off with the jet. If you inject a small amount of hydraulic fluid under the skin that is a big health hazard. If you inject a small amount of water under the skin there are minimal health issues.
The TOTAL energy in the container is what matters if a catastrophic failure occurs. Like you said, if the only thing in the bottle is compressed water then even a catastrophic failure is pretty unimpressive. You only have to expand the fluid by a couple percent before you've dropped the pressure back to ambient. But even if the bottle was filled with water then pressurized with air you've still significantly decreased the energy in the bottle. If the bottle is 5% air then it only contains 5% as much energy as it would if the bottle was completely filled with air.
So, bottom line, pressurizing with water would be best but not many people have access to something that'll pump water to the needed pressures (like a pressure washer). The next best thing is to fill fully with water then pressurize with compressed gas. No matter how it is done it should be done remotely and with adequate shielding (like dropping everything into a large hole).