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A hydrostatic test is the common way in which leaks can be found in pressure vessels such as Pipeline transport|pipelines and plumbing. This method of testing should not be confused with the Hydrostatic Body Fat Test that uses the principles of buoyancy to calculate a person's body fat. Hydrostatic tests, on the other hand, performance verify fluid 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 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.

Testing procedures[]

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. Red or fluorescent dyes are usually added to the water to make leaks easier to see. 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 (compressible only by weight, not air pressure), so will only expand by a very small amount should the vessel split. If high pressure gas were used, then the gas would expand to perhaps several hundred times its compressed volume in an explosion, with the attendant risk of damage or injury. This is the risk which the testing is intended to mitigate.

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. This measures the overall leakage of a system instead of locating the leaks and additives can be added to the water to reduce resistivity and increase the sensitivity of the test. Hydrostatic test fluid can also clog small holes (1x10^-6 std cm^3/s or smaller) as a result of the increase in pressure. Which is another reason why water is commonly used.

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 hydrotested.

A simpler test, that is still considered a hydrostatic test but has probably been performed by anyone who has a garden hose, is to pressurize the vessel by filling it with water and to physically examine the outside for leaks.

Examples[]

Portable fire extinguishers are safety tools that are required to be on hand in almost every public building. Fire extinguishers are also highly recommended in every home. Over time the conditions in which they are housed and the manner in which they are handled have an impact on the structural integrity of the extinguisher. A structurally weakened fire extinguisher can malfunction or even burst when it is needed the most. To maintain the quality and safety of this product, hydrostatic testing must be utilized. All critical components of the fire extinguisher must be tested to ensure proper function. The cylinder would be tested by using the water jacket test.

As previously mentioned, the water pressure inside the tank will usually be 150% of the normal operating pressure. The change in volume of the cylinder is calculated by measuring the change in the water levels outside the cylinder. This can be done with a digital scale as well to detect the slightest changes most always in grams. The cylinder can also be visually checked for leaks or the pressure drop method can be utilized to measure the overall efficiency of the cylinder.

Pipeline testing[]

Buried high pressure oil and gas pipelines are tested for strength by pressurising them to at least 125% of their maximum operating pressure (MAOP). Since many long distance transmission pipelines are designed to have a steel hoop stress of 80% of specified minimum yield (SMYS) at MAOP, this means that the steel is stressed to SMYS and above during the testing, and test sections must be selected to ensure that excessive plastic deformation does not occur.

Leak testing is performed by balancing changes in the measured pressure in the test section against the theoretical pressure changes calculated from changes in the measured temperature of the test section.

Australian standard AS2885.5 "Pipelines—Gas and liquid petroleum: Part 5: Field pressure testing" gives an excellent explanation of the factors involved.

Testing frequency[]

Most countries have legislation or building code that requires pressure 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 gas cylinders have the following requirements:

In the U S and Canada, organizations such as ASTM, and ASME specify the guidelines for the different types of pressure vessels.

External links[]

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