Hydrostatic testing is required for cylinders or vessels that contain gases under pressure. Common examples of these cylinders include self-contained breathing apparatus tanks used by firefighters and compressed air tanks used for scuba diving. Regular testing helps to reduce the chances of failure or explosion due to weak points in the tank structure. U.S. hydrostatic testing requirements are outlined in Title 49 of the Code of Federal Regulations.
What Is a Hydrostatic Test?
A pressurized cylinder has a rating that is referred to as the working pressure. A scuba cylinder, for example, has a normal rating of 3,000 pounds per square inch. A hydrostatic test is performed by filling the cylinder with a liquid, usually water, and putting it under 1 2/3 times the rated working pressure. Water doesn't compress like gases, and if the cylinder fails, the sudden release of water won't cause an explosion like a sudden release of gas.
The increased pressure during the test causes expansion. A technician observes instruments that identify leaks and measure expansion to be sure the cylinder does not exceed allowable limits. After the test, the water is drained out and the cylinder is dried. The technician then performs a visual inspection to check for cracks or pitting in the cylinder structure. If the cylinder fails any part of the testing, it must be removed from service.
When Is Testing Required?
Both steel and aluminum cylinders must undergo hydrostatic testing every five years. Once the testing is completed, the neck of the cylinder is stenciled with the date of the test and the identification number of the facility that performed the test. A hydrostatic test is also required any time the cylinder has been dropped, damaged or exposed to excessive heat. Any of these occurrences may compromise the integrity of the tank.
Between hydrostatic tests, cylinders must be visually inspected every year. A label is affixed to the tank showing the date and the facility that performed the inspection.
Who Can Perform the Test?
Qualified facilities have registered with the U.S. Department of Transportation and have received requalifier identification numbers, or RINs. In DOT terminology, testing the cylinder is referred to as requalifying it for service. The RIN is part of the stenciled or stamped information that is marked on the cylinder after testing. Qualified technicians have received thorough training in proper testing procedures and inspection parameters. There is always an element of risk because of the potential for cylinder failures, so untrained people should never attempt this testing.
Warnings and Cautions
The consequences of the rupture of a cylinder filled with compressed gas can be fatal. Never try to have a cylinder filled that does not have a current hydrostatic test marking. A reputable servicing facility should always check the cylinder before filling it to verify its status. If you notice any cracks or anything unusual in the structure of a tank, take it for a visual inspection. The technician who does the visual inspection will recommend a hydrostatic test if he thinks it is warranted.
- diving tank image by Marcin Wasilewski from Fotolia.com