Hydrostatic test
A hydrostatic test (also called hydrotest for short) 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.
Contents
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 higher than the operating pressure to give a margin for safety. This margin of safety is typically 166.66% or 150% (in the case of many exemptions) of the designed 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 (approximately 232 bar). 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 pV=nRT 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. In other words using water is safer and takes less energy to do. Just one drop of water in every 5 seconds can cause a pressure change even up to 25 psi, while it would take a huge amount of gas to produce the same pressure change. Water is used mainly because the density of water is very close to 1.001 so using scale 1 g of water in the bowl would be 1 ml of cylinder expansion.
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. A gauge is used to measure the amount of change. The vessel is then pressurized for a specified period, usually 30 or more seconds, and then 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 percentage of permanent expansion is more than 10%, or not up to DOT or customer standards, the cylinder fails, and then goes through a condemning process marking the cylinder as 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. The hydrostatic test fluid can also clog small holes (1x10^-6 std cm^3/s or smaller) as a result of the increase in pressure. This is another reason why water is commonly used.
All the information the tester needs is stamped onto the cylinder. This includes the DOT information, serial number, manufacturer, and manufacture date. Other information is stamped as needed such as the REE 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 can be 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. The pressure level achieved in this sort of test does not come close to the pressure level that would be used in a professional testing facility.
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, almost 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
Hydrotesting of pipe, pipelines and vessels is performed to expose defective materials that have missed prior detection, ensure that any remaining defects are insignificant enough to allow operation at design pressures, expose possible leaks and serve as a final validation of the integrity of the constructed system. ASME B31.3 requires this testing to ensure tightness and strength. ASME B31.3 section 345
Buried high pressure oil and gas pipelines are tested for strength by pressurizing them to at least 125% of their maximum operating pressure (MAOP) at any point along their length. 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.
Test pressures need not exceed a value that would produce a stress higher than yield stress at test temperature. ASME B31.3 section 345.4.2 (c)
Other codes require a more onerous approach. BS PD 8010-2 requires testing to 150% of the design pressure - which should not be less than the MAOP plus surge and other incidental effects that will occur during normal operation.
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:
- DOT-3AL gas cylinders must be tested every 5 years and have an unlimited life.
- DOT-3HT gas cylinders must be tested every 3 years and have a 24 year life.
- SP-Cylinders Special Permit
- 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)
In the U S and Canada, organizations such as ASTM, and ASME specify the guidelines for the different types of pressure vessels.
Precautions
Controlling the hydrotest water quality is critical. Otherwise, it could introduce oxygen and bacteria, both of which pose threat to system integrity. The water typically needs to be filtered, deoxygenated, and biocided. Iron content analysis of the hydrotest water is recommended after the test is finished to evaluate any potential corrosion that has occurred.
Safety barrier must be set around the equipments that are being hydrotested because any leaked high pressure water jet can pose great safety risks.
External links
"OSHA". http://www.osha.gov/SLTC/etools/evacuation/portable_hydro.html. Retrieved 2006-10-05.
"Bancroft: Equipment and further information". http://www.bancroft.co.uk. Retrieved 2009-12-27. "Water Jacket Hydrostatic Test Equipment". http://www.galiso.com. Retrieved 2011-08-16.
