Compressed gas tanks must be hydrostatically tested periodically
Compressed gas tanks are required periodically to be hydrostatically tested to ensure that they can safely hold their designated pressure. During the process the tanks are filled with water, then sealed and immersed in a water-filled hydrochamber. This chamber is sealed and the tank is then pressurized to much greater than its working pressure.
The tank expands, pushing water from the hydrochamber into a measuring device called a burette. It is then depressurized and a further reading is taken from the burette to see what water has remained due to the tank expansion. The difference between the two volume readings is noted and the expansion calculated. To pass the test, the tank must meet certain requirements.
Pressurization Levels During Testing
A tank’s working pressure is the pressure to which a tank is normally filled. In a hydrostatic test it must be filled to 1 2/3 of its working pressure. This is to establish that the tank is capable of holding pressures much greater than it is normally required to. For example, a tank which has a working pressure of 300 bar would be pressurized to 500 bar. This pressure must be held for at least 30 seconds before depressurization to allow for full expansion of the tank walls.
The acceptable expansion of the tank following depressurization is to within 10 percent of the expansion of the tank, when pressurized. An example of this is a 10 liter tank that expands by one liter during hydrostatic testing. To pass a test its expansion following depressurization cannot be more than 0.1 liters. Any more than this and the metal is deemed to be not resilient enough and fails the test.
The dates of the hydrostatic test must be stamped on the neck of the air tank. This is so users of the tank can see if the cylinder is currently acceptable to use and when the next test is due.
Tanks must be hydrostatically tested every five years. They can be required to be tested sooner if dropped from a height, exposed to heat damage, painted with inappropriate substances, or anything that could affect the structural integrity of the tank.