If high pressure gas were used, then the gas would expand to V=(nRT)/p with its compressed volume resulting in an explosion, with the attendant risk of damage or injury. These fluids are nearly incompressible, therefore requiring relatively little work to develop a high pressure, and is therefore also only able to release a small amount of energy in case of a failure - only a small volume will escape under high pressure if the container fails. Hydraulic fluids and oils may be specified where contamination with water could cause problems. Water is commonly used because it is cheap and easily available, and is usually harmless to the system to be tested. For example, if a cylinder was rated to DOT-2015 PSI (approximately 139 bar), it would be tested at around 3360 PSI (approximately 232 bar). This factor of safety is typically 166.66%, 143% or 150% of the designed working pressure, depending on the regulations that apply. The test pressure is always considerably higher than the operating pressure to give a factor of safety. Red or fluorescent dyes may be added to the water to make leaks easier to see. The vessel is filled with a nearly incompressible liquid – usually water or oil – pressurised to test pressure, and examined for leaks or permanent changes in shape. Hydrostatic tests are conducted under the constraints of either the industry's or the customer's specifications, or may be required by law. Testing of pressure vessels for transport and storage of gases is very important because such containers can explode if they fail under pressure. In some cases where a hydrostatic test is not practicable a pneumatic pressure test may be an acceptable alternative. They are then revalidated at regular intervals according to the relevant standard. Newly manufactured pieces are initially qualified using the hydrostatic test. Using this test helps maintain safety standards and durability of a vessel over time. Hydrostatic testing is the most common method employed for testing pipes and pressure vessels. Strength is usually tested by measuring permanent deformation of the container. The location of a leak can be visually identified more easily if the water contains a colorant.
Pressure tightness can be tested by shutting off the supply valve and observing whether there is a pressure loss. The test involves filling the vessel or pipe system with a liquid, usually water, which may be dyed to aid in visual leak detection, and pressurization of the vessel to the specified test pressure.
A hydrostatic test is a way in which pressure vessels such as pipelines, plumbing, gas cylinders, boilers and fuel tanks can be tested for strength and leaks.
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