How to inspectors test compressed air tanks and steam boilers? They fill them with water and hydro test them. If the tanks fail, they just split and water squirts out and falls to the ground. When a steam boiler ruptures with steam, bad things happen.
http://lubbockonline.com/stories/073...l#.VeNOZvlViko
Now when an air tank goes, it isn't pretty:
https://www.youtube.com/watch?v=KVP_A7eGYxw
When using piping or valves, check for the "WOGS" rating. W= Water, O=oil, G= Gas, and S=Steam. Some valves might have 100W 60GO 40S, meaning 100psi for water, and 60psi for both oil or gas, and 40psi for steam.
So in short, 100psw water is not the same as 100psi Steam, nor 100psi Oil, and not 100psi Gas. You can have a really bad day if you mix them.
I had a neighbor use plastic water pipe to plumb his garage shop, it was left over from replacing his water feed lines in the house. The house pressure was 65psi and rated for 100psi. He ran 80psi air in the same pipe. One day there was a large bang as he was working in the garage. They took a 3" long peice of the plastic pipe out of his chest. Needless to say, when he recovered out went the plastic pipe and in went black pipe.
Be safe. Go the distance make sure you stay safe.