Hydrostatic testing of air tanks must occur every five years by a certified technician. The idea is to determine whether the tank is structurally sound and will not burst under pressure. During this procedure the tank is filled with water and pressurised to check the effects of higher pressures on the tank. It is an industry requirement in most countries to have tanks hydrostatically tested periodically.
The tank is first filled with water from a high pressure water pump. It is fitted with a special valve to allow it to be pressurised and then submerged in a sealed chamber, which is also filled with water. O-rings around this chamber ensure that no leakage will occur when it is closed for pressurisation.
The tank is then pressurised to 5/3 of its working pressure–the pressure to which the tank is normally filled. The idea of the hydrostatic test is to test the integrity of the cylinder at greater pressures, in this case 5/3. For example, in the case of a tank whose working pressure is 300 Bar, then it would be pressurised to 500 Bar. Under pressure the tank expands, displacing the water surrounding it from the chamber and forcing it into a small hose attached to the chamber and into a measuring device, called a burette. Once a stable reading (normally over a period of 30 seconds) of the displacement is measured on the burette, then the tank is depressurised. Another reading will then be taken from the burette to note any change in tank volume.
If the tank does not return close to its original volume reading–within at least 10%–but remains stretched further, then the metal is considered not resilient enough to be used working under pressure again. Once the volume reading has been taken, then the tank is drained and visually inspected once again. Tanks that pass the hydrostatic test have the test date stamped on the neck of the tank for verification of the next due test date. The valve is then reinserted and the tank refilled.
Federal law requires a hydrostatic inspection every five years, unless something happens to the tank that would warrant testing sooner. This would be something that can affect the structural integrity of the tank, such as being dropped from a height, exposure to heat, or painting with inappropriate substances.
The reason for using water over gas is that in the event of a rupture, the results would be minor compared to a rupture with pressurised gas, which expanding to several hundred times its compressed volume, would cause an explosion.
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