Simple. In order to obtain certification and burst damage liability cover from the Inspector's company, follow that individual's instructions.

However, it is a wise precaution to test on an incompressible fluid to a pressure above that specified for the test on the compressible fluid. Ruptures in pressurised systems cause rather less damage when an incompressible fluid is used.

I have seen instructions for using air to test a feedwater heater in a power plant. The test pressure was much lower than the liquid hydrostatic test pressure and seemed intended to find leaks in the weld or valves instead of testing the integrity of the vessel.

I could see some specialized containers being tested without using liquids but safety precautions should be taken to prevent explosive failure causing harm.

Drew K

I just went back to the home page and see this (and the other seemingly related post) has been removed...guess the overlords don't think talking about potentially fatal testing procedures is not good enough?!?!

It all depends on what you are testing.

If you are testing water piping, hydro-testing is appropriate.

If you are testing nitrogen piping, nitrogen pressure is appropriate.

What are you testing ?