Hydrostatic testing is the common way in which leaks can be found in pressure vessels such as pipelines and plumbing. Hydrostatic tests are also used to verify pressure vessels. Using this test helps maintain safety standards and durability of a vessel over time. Newly manufactured pieces are initially qualified using the hydrostatic test and continually re-qualified at regular intervals using the proof pressure test which is also called the modified hydrostatic test.
Hydrostatic tests are conducted under the constraints of either the industry's or the customer's specifications. The vessel is filled with a nearly incompressible liquid - usually water or oil - and examined for leaks or permanent changes in shape. The test pressure is always considerably more than the operating pressure to give a margin for safety, typically 150% of the design pressure. An example would be if a cylinder was rated to DOT-2015 PSI it would be tested at around 3360 PSI to ensure maximum usage and to provide more safety. Water is commonly used because it is almost incompressible thus it is much safer than testing with a gas.
Small pressure vessels are normally tested using a water jacket test. The vessel is visually examined for defects and then placed in a container filled with water, and in which the change in volume of the vessel can be measured by monitoring the water level. For best accuracy a digital scale is used to measure the smallest amounts of change. The vessel is then pressurized for a specified period usually 30 or more seconds and depressurized again. The water level in the jacket is then examined. The level will be greater if the vessel being tested has been distorted by the pressure change and did not return to its original volume or some of the pressurized water inside has leaked out. In both cases, this will normally signify that the vessel has failed the test. If the REE (Rejection Elastic Expansion) is more than 10% or not up to DOT or customer standards the cylinder fails, and then goes through a condemning process dubbing the cylinder unsafe.
All the information the tester needs is stamped onto the cylinder such as: The DOT information, Serial number, manufacturer, and manufacturer date. Other information is stamped as needed such as the REE (Rejection Elastic Expansion) or how much the manufacturer specifies the cylinder should expand before it is considered unsafe. All this information is usually taken down and stored on a computer prior to the testing process. All this information is necessary for keeping track of when the cylinder has been or needs to be hydro-tested. Gayston currently hydro-tests over 1500 parts a week with two pieces of hydro-test equipment.
