Why should air be vented prior to hydrostatic testing?

Venting air before hydrostatic testing is crucial because it helps to prevent violent and dangerous failure of the cylinder. During hydrostatic testing, the cylinder is filled with water and subjected to high-pressure conditions intended to assess its integrity and ability to hold pressure. If there is air trapped inside the cylinder, it can compress and create pressure conditions that are not uniform. In the event of a failure, the presence of compressed air may turn a standard failure into a catastrophic event, leading to potential fragmentation or a violent rupture.

By venting the air, you ensure that only the water is present in the cylinder during testing, which allows for a controlled and safe environment when the cylinder is pressurized. This not only safeguards personnel conducting the test but also prevents damage to equipment and surroundings that could occur from an explosive failure.