The code requires all vessels to be pressure tested before they can be stamped and certified. The standard pressure test is a hydrostatic test, in which the vessel is filled with water and pressurized to at least 30% over the rated pressure. Since water is incompressible, if a failure were to occur in the vessel during the test, the pressure would quickly drop to zero.
The code also allows the pressure test to be performed with air – a pneumatic test. This test might be needed if water cannot be introduced into the vessel for process reasons. Since air is compressible, a large amount of energy is stored as pressure is increased, making this test inherently dangerous. A failure during a pneumatic test would release a shock wave of energy like a bomb. If pneumatic testing must be performed, certain safety measures should be put into place such as additional NDE, testing in a pit or behind reinforced walls, and visually inspecting the unit only after reducing the test pressure. Pneumatic testing can be a useful option, but only when performed under controlled conditions to ensure safety.
