Cylinder Requalification


Hydrostatic Testing of Compressed Gas Cylinders

Hydrostatic testing is the most common method employed for testing cylinders. A hydrostatic test is a way in which pressure vessels such as gas cylinders can be tested for strength and leaks. The test involves filling the cylinder with water and pressurization of the cylinder to the specified test pressure.

Newly manufactured cylinders are initially qualified using the hydrostatic test. Cylinders are then requalified at regular intervals according to Canadian standards. Testing of cylinders is very important because such containers can explode if they fail under pressure.

The test system is calibrated daily prior to running production. Prior to being tested hydrostatically a cylinder must pass internal and external visual inspections. The cylinder is then tested in a water jacket in which it is pressurized to its specified test pressure, and held for 30 stable seconds, and the volumetric expansion (VE) is measured by an expansion indication device (EID) that measures the amount of liquid that has been forced through the expansion line into the EID by the volume increase of the pressurized cylinder in the sealed water jacket. The cylinder is then depressurized and the permanent volumetric expansion (PVE, i.e., the permanent volume increase of the cylinder due to deformation while under pressure) is measured by comparing the final volume in the EID with the volume before pressurization.

The information needed to specify the test requirements is stamped onto the cylinder’s shoulder. This includes regulating authority, a design standard/specification, serial number, manufacturer, manufacture date, and sometimes other relevant information. If the cylinder fails, it will go through a condemning process which shall prevent any further use. Cylinders that pass the hydrostatic test are stamp marked on their shoulder with the date of the successful test and the test facility’s registered mark.

The required testing frequency is as follows:

  • TC-3ALM/DOT-3AL gas cylinders must be tested every 5 years
  • TC-3AAM/DOT-3AA gas cylinders must be tested every 5 years, unless stamped with a star (*) following it’s most recent test date in which the cylinder met certain specifications allowing it to be tested every 10 years
  • TC-3FCM, TC-3HWM, TC-3CCM gas cylinder must be tested every 5 years and have a limited life of 15 years from the date of manufacture
  • ISO9809-1 gas cylinders must be tested every 10 years

Our facility performs hydrostatic tests using the Galiso Rec4 Open.


Ultrasonic Testing

Ultrasonic testing (UT) is a non-destructive testing (NDT) method used to analyze and evaluate the properties of a cylinder, based on the propagation of ultrasonic waves in the cylinder being tested, without causing damage, as NDT methods do not permanently alter the cylinder being inspected.

Using 5 different ultrasound transducers, ultrasonic pulse-waves are transmitted into the cylinder, using water as a coupler, to detect external & internal flaws as well as thickness measurements.

The advantages of ultrasonic testing are numerous. It has high penetrating power which allows the detection of flaws deep in the cylinder as well as high sensitivity which permits the detection of extremely small flaws. Also, ultrasonic testing has greater accuracy than other NDT methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces, such as the cylinder wall. It is more precise than hydrostatic testing.

Additional advantages, in comparison to hydrostatic testing, are:

  • In most instances, it eliminates the need to remove the cylinder valve as the equipment only needs access to one surface of the cylinder being inspected. This is especially useful for cylinders with toxic or flammable gas services.
  • Since the valve is not removed and the cylinder is not filled with water, as necessary in a hydrostatic test, water consumption is limited, making it an environmentally conscious method for testing cylinders. Additionally, it also assists in avoiding off-gassing the cylinder contents which is required before removing the valve for a hydrostatic test.
  • Some cylinders are equipped with residual pressure valves (RPV), in which the valve will always maintain positive pressure. This is done to prevent contamination and is often used in gas services that require a high level of purity. Since the valve does not need to be removed for the cylinder to be tested there is no concern for contamination.
  • The results are immediate, and an on-the-spot decision can be made providing quicker turnaround for customers.


Eddy Current Testing

Eddy-current testing is used to detect sustained-load cracking (SLC) in aluminum alloy cylinders. The majority of aluminum cylinders in service today are made from aluminum alloy 6061 (AA6061), which is not susceptible to SLC. However, there are still many aluminum cylinders that are made from aluminum alloy 6351 (AA6351), which is susceptible to SLC and would require an eddy-current inspection at the time of requalification, whether by hydrostatic test or ultrasonic examination, if in Oxygen, Scuba or SCBA applications.

At CCS we use the Visual Plus 3 produced by Advanced Inspection Technology. This is the best inspection technology you can buy today, and it is the equipment of choice for inspecting all aluminum alloys used in cylinder manufacturing.

Even the best trained and most experienced technician can miss or incorrectly diagnose small imperfections, such as hairline cracks and folds in the neck/shoulder area of aluminum cylinders using hydrostatic testing alone. Visual Plus, a non-destructive testing (NDT) device, identifies 100% of these defective cylinders using the latest in eddy-current technology.

An electromagnetic wave is introduced into the metal of the neck-threads of the cylinder. The wave causes currents to flow in a circular manner in the metal. An imperfection can increase the path that the currents have to flow and make the material appear higher in resistance in the region where the imperfection is present. This generates a spike on the computer-generated report which alerts the technician to a problem that requires further investigation.

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