This includes the design standard, serial number, manufacturer, and manufacture date. The information needed to specify the test is stamped onto the cylinder. If the vessel fails, it will normally go through a condemning process marking the cylinder as unsafe. A leak may also be a failure criterion, but it may be that the leak is due to poor sealing of the test equipment. In most cases a permanent set that exceeds the specified maximum will indicate failure. A leak will give a similar result to permanent set, but will be detectable by holding the volume in the pressurised vessel by closing the inlet valve for a period before depressurising, as the pressure will drop steadily during this period if there is a leak. The vessel is then depressurised, and the permanent volume increase due to plastic deformation while under pressure is measured by comparing the final volume in the measuring tube with the volume before pressurisation. The vessel is then pressurised for a specified period, usually 30 or more seconds, and if specified, the expansion will be measured by reading off the amount of liquid that has been forced into the measuring tube by the volume increase of the pressurised vessel.
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, usually by monitoring the water level in a calibrated tube.
Small pressure vessels are normally tested using a water jacket test. 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.