A hydrostatic test is a way in which leaks can be found in pressure vessels such as pipelines and plumbing. The test involves placing water, which is often dyed for visibility, in the pipe or vessel at the required pressure to ensure that it will not leak or be damaged. It is the most common method employed for testing pipes and 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. They are then continually re-qualified at regular intervals using the proof pressure test which is also called the modified hydrostatic test. Hydrostatic testing is also a way in which a gas pressure vessel, such as a gas cylinder or a boiler, is checked for leaks or flaws. Testing is very important because such containers can explode if they fail when containing compressed gas. Organizations such as ASTM, API, CEN and ASME specify the guidelines for the different types of pressure vessels.
Australian standard AS2885.5 "Pipelines—Gas and liquid petroleum: Part 5: Field pressure testing" gives an excellent explanation of the hydrostatic testing
Eurotech provides third party testing and inspection services for pressure equipments