Hydrostatic and Ultrasonic Cylinder Tests

Jerrold D. Sameth, chief technologist, director of gas packaging, Matheson Tri-Gas
Edited by Kimberley Gilles, associate editor

The U.S. Department of Transportation requires periodic testing of most gas cylinders to ensure that they do not rupture or fail when the gas inside is compressed. The compressed gas industry uses two types of tests to re-qualify tanks: hydrostatic and ultrasonic.

Standard hydrostatic test
A hydrostatic test measures the elastic expansion of a cylinder after it has been pressurized in excess of the maximum pressure level that is stamped on the tank or that is stated in the Code of Federation Regulations. The test pressure for high-pressure, seamless cylinders usually is ⁵⁄₃rds the stamped service pressure, and the test pressure for welded cylinders typically is twice the stamped level. For example, cylinders specified as Department of Transportation 4A have a minimum retest pressure of ⁵⁄₃rds, but cylinders specified as 4AA480 have a minimum retest pressure of 2 times the service pressure. Cylinders specified as 4 have a minimum retest pressure of 700 psi.

To perform a hydrostatic test, the cylinder is drained of its residual gas, "de-valved" and cleaned if necessary. The cylinder then is filled with a specified amount of water, allowed to reach the ambient temperature, and lowered into a test jacket that is filled with a measured amount of water. While the cylinder is in the test jacket, the pressure inside the cylinder is raised to the required level and held there for 30 seconds before it is released.

As the pressure is released, the walls of the cylinder "relax," and the cylinder returns to its pre-stressed condition. The amount of water that entered the cylinder during the pressurization is measured and compared to the amount of water that exited as the tank depressurized.

A cylinder passes this part of the test if it returns to 90% of the prestressed condition, as indicated by measuring the volume of water that entered the cylinder during the test, and comparing that with the amount of water that was released upon depressurization. If the test equipment malfunctions during a test, a re-test must be conducted using pressure that is that is the lesser of 10% higher or 100 psig higher than the original test pressure.

The final part of testing is a rigorous external and internal inspection.

Generally, cylinders that pass both parts of the test can be charged and offered for transportation for five years. In many cases, this time period can be extended to 10 years. The Federal Code of Regulations should be consulted to determine any exemption to the 5-year rule.

Hydrostatic test equipment
Regulations require that test equipment be calibrated with a cylinder that has expansion characteristics that are defined in the Code of Federal Regulations. Also, the person performing the calibration must be trained and certified to 49CFR180.25 of the Code of Federal Regulations (see sidebar). Proper calibration for the test equipment requires that the calibration cylinder does not have permanent expansion after the "pre test." This calibration is conducted each day that testing takes place, and within 500 psi of any test pressure at or above 3,000 psi. Test equipment must be verified to within ±1 percent of the calibrated cylinder's pressure

and corresponding expansion values.

Additionally, the calibration must take into account the testing of a variety of cylinders on any single day. For example, retesting a Department of Transportation-specification DOT 3AA 1800 cylinder along with DOT 3AA 2400 cylinders requires test pressures that range from 3,000 psig to 4,000 psig. Because those test pressures are above the 3,000 psi daily maximum, multiple calibrations are required for the day's testing.

Record-keeping
Records of calibration tests and observations taken during the tests, including external and internal visual inspections, must be accurate and must be signed by the operator and the supervisor. These records must be kept on file and must be readily available for inspection by officials of the Department of Transportation and others during site audits. The Transportation Department requires that all calibration tests be repeatable: Inspectors can demand that a test be repeated, and that the new test be started at any point in the testing process and with any of the equipment that was used during the documented test.

Exceptions to the test
There are instances when regulations allow for the complete external inspection of a cylinder in lieu of a hydrostatic test. The guidelines are strict, and can be found under 49CFR180.209(g) of the Code of Federal Regulations.

Charged and shipped
Cylinders cannot be charged and shipped after their re-qualification due dates, but they can be shipped if they were filled (charged) before the due dates, according to Department of Transportation regulations. Cylinders that are at customer sites when their expirations dates come due are not required to be returned immediately to the filling plant. However, once the cylinders are returned, they must be retested before they are recharged and shipped again.

Ultrasonic testing
Ultrasonic inspection eliminates problems created by hydrostatic tests in the case of cylinders that are used in high-end specialty gas and electronic services. These cylinders receive special surface treatments and, after hydrostatic tests, those treatments must be re-applied, followed by a thorough drying of the cylinder interior. Water is considered a contaminant in gases used in the manufacture of electronic components.

Ultrasonic examination tests measure the thickness of cylinder walls over their full lengths. Cylinders do not pass the test if, at any point, the thickness of their walls falls below the established minimum.

One advantage of ultrasonic testing is that it identifies small faults, such as cracks, corrosion, pits and gouges, that are not always found during the internal visual inspection required as part of a standard hydrostatic test. Because ultrasonic testing produces highly accurate measurements, the Department of Transportation waived the required internal visual examination for cylinders used in certain services.

Other advantages include:

• Residual contents do not have to be drained.
• Cylinders are not de-valved.
• Ultrasonic testing provides a more detailed analysis than the standard hydrostatic test.

Rejection rates Rejection rates for cylinders tested with the ultrasonic method are slightly higher than those for the standard hydrostatic test. In Matheson Tri-Gas's experience, the rejection rate for the standard hydrostatic test is approximately 0.5 percent, and approximately 1.5 percent for the ultrasonic test. Cylinder regulations Regulations for cylinders are found under Title 49 of the Code of Federal Regulations, part 180, sections 201-215. A citation such as 49CFR180.205 directs the reader to Title 49, section 205 of part 180 of the Code. The proper value for a test pressure can be found in Table 1 of 49CFR180.209 or the applicable Special Permit stamped into the cylinder. The Code of Federal Regulations — 49CFR180.205(g)(4) — mandates that hydrostatic test equipment be verified within ±1 percent of the calibrated cylinder's pressure and corresponding expansion values. Multiple testing of cylinders is covered in 49CFR180.205(g)(3)(i). Department of Transportationapproved testing facilities must display the Letter of Authorization in the area where the testing is taking place, and all required Transportation Department and referenced Compressed Gas Association publications must be on hand at the testing site. Illustrations of typical equipment layout for hydrostatic testing can be found in the Compressed Gas Association's Publication C-1 and the Compressed Gas Association Handbook. Compressed Gas Association Pamphlet C-1.1 addresses personnel training and certification guidelines for hydrotest facilities.