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A hydrostatic test is a way in which pressure vessels such as pipelines, plumbinggascylindersboilers and fuel tanks can be tested for strength and leaks. The test involves filling the vessel with a liquid, usually water, and pressurization of the vessel to the specified test pressure. Pressure tightness can be tested by shutting off the supply valve and observing whether there is pressure loss.

Hydrostatic testing is the most common method employed for testing pipes and 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. They are then revalidated at regular intervals according to the relevant standard.

Testing of pressure vessels for transport and storage of gases is very important because such containers can explode if they fail under pressure.

Hydrostatic tests are conducted under the constraints of either the industry's or the customer's specifications, or may be required by law. The vessel is filled with a nearly incompressible liquid – usually water and pressurized to test pressure, and examined for leaks or permanent changes in shape. The test pressure is always considerably higher than the operating pressure to give a factor of safety. This factor of safety is typically166.66%, 143% or 150% of the designed working pressure, depending on the regulations that apply. For example, if a cylinder was rated to DOT-2015 PSI(approximately 139 bar), it would be tested at around 3360 PSI (approximately232 bar).

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, usually by monitoring the water level in a calibrated tube. The vessel is then pressurized 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 pressurized vessel. The vessel is then depressurized, 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 pressurization.

A leak will give a similar result to permanent set, but will be detectable by holding the volume in the pressurized vessel by closing the inlet valve for a period before depressurizing, as the pressure will drop steadily during this period if there is a leak. In most cases a permanent set that exceeds the specified maximum will indicate failure. A leak may also be a failure criterion, but it may be that the leak is due to poor sealing of the test equipment. If the vessel fails, it will normally go through a condemning process marking the cylinder as unsafe.

The information needed to specify the test is stamped onto the cylinder. This includes the design standard, serial number, manufacturer, and manufacture date. After testing, the vessel or its nameplate will usually be stamp marked with the date of the successful test, and the test facility's identification mark.

Testing frequency

Most countries have legislation or pressure vessel codes which requires vessels to be regularly tested, for example every two years (with a visual inspection annually) for high pressure gas cylinders and every five or ten years for lower pressure ones such as used in fire extinguishers. Gas cylinders which fail are normally destroyed as part of the testing protocol to avoid the dangers inherent in them being subsequently used.

These common US standard gas cylinders have the following requirements:

·        DOT-3A gas cylinders must be tested every 5 years and have an unlimited life.

·        DOT-3AA gas cylinders must be tested every 5 years and have an unlimited life. (Unless stamped with a star (*) in which case the cylinder meets certain specifications and can have a 10-year hydrostatic test life).

·        DOT-3AL gas cylinders must be tested every 5 years and have an unlimited life.

·        4B

·        4B240ET

·        4BA

·        4BW

·        UN ISO 7866

·        UN ISO9809-1

Typically organizations such as DOT PHMSA, ISO, ASTM and ASME specify the guidelines for the different types of pressure vessels.