Posted: Posted on December 21, 2021 Hydrostatic Requalification

One of the most common methods of testing high pressure gas cylinders in the United States and Canada is a water jacket hydrostatic test. This test consists of filling and pressurizing a tube with water inside of a water filled jacket.

Tube requalifiers must have a Requalification Identification Number (RIN) and are required to comply with 49 CFR Subpart C, Qualification, Maintenance and Use of Cylinders, which includes UN tubes. FIBA holds 8 different RIN numbers which are renewed every 5 years, for all of our locations across the globe.

This blog post will describe a typical water jacket requalification of a jumbo tube, which is a seamless steel pressure vessel manufactured to various standards such as DOT 3AAX, DOT 3T, TC 3AAXM, TC 3TM, or ISO standard 11120.

Water jacket testing allows for volumetric measurements of several key criteria such as: Total Expansion, Permanent Expansion, Elastic Expansion, and the Expansion Ratio.

The tube specification and conditions such as gas service, age, and packaging determine whether the tube needs to be hydrostatically requalified either every 5 or 10 years.

3-Series tubes are hydrostatically tested at 5/3rds of the tube service pressure and ISO 11120 tubes are hydrostatically retested at 1.5 times the tube service pressure.

Before the test begins, FIBA ensures the system is calibrated in accordance with CGA C-1 or ISO 18119 and preforms visual internal and external inspections according to CGA C-6 or ISO 18119 to identify any defects, excessive deterioration or corrosion of the tube.

The necessary parts of a hydrostatic test system are:

• A water jacket of sufficient size to allow the tube to expand freely in all directions and prevent water leakage or air entrapment. FIBA’s water jacket is over 43 feet long and over 30” in diameter.

• An Expansion Indication Device which can be burettes, weigh bowls or microprocessors. FIBA uses burettes.

• A Pressure Indicating Device such as a calibrated pressure Gauge. FIBA uses both analogue and digital pressure gauges

• Plumbing that is sized and installed correctly in order to prevent leakage, air entrapment, and exposure to any external forces that could affect test results.

• A timing device to measure the appropriate hold time. FIBA uses a simple digital timer to ensure the tube holds pressure for the required amount of time

After the test system is calibrated, the tube is placed into the water jacket. As the tube is filling with water, the jacket is raised to a slight incline to ensure there is no trapped air inside of the tube dome. Once the tube is full of water the jacket is sealed and filled with water. The burette is then set to zero.

The test may now begin with a gradual increase in pressure of the tube.  FIBA opts to stop pressurizing the tube at 90% of minimum test pressure to test for leaks, before continuing to full test pressure.

If a tube is pressurized below 90% of minimum test pressure at time of requalification and a system failure or operator error occurs, this does not count as a test.  Retests cannot be repeated more than twice and test pressure shall be increased on repeat tests.  If a successful result cannot be achieved after two repeat tests, the tube shall be condemned.

The tube is then pressurized and held at 100% of test pressure to ensure there are no leaks.

Once pressure and expansion readings have stabilized (as exhibited by the Pressure Indicating Device and burette) the actual test pressure and total expansion readings (cubic centimeters of water) shall be recorded.

Next, the pressure is drained from the tube and any permanent expansion after the tube has fully contracted is recorded.

The technician can then review the test results and calculate both the percent permanent and elastic expansions to confirm the tube is within acceptable limits and passes the test. In some cases the elastic expansion may also be compared to standard or marked rejection elastic expansion limits (REE) for suitability for 10% overfill or 10-year requalification.  Standard REE are published in CGA C-5. Percent permanent expansion is calculated by dividing the permanent expansion by the total expansions and multiplying by 100. The tube passes the hydrostatic test if the permanent expansion is less than or equal to 10%. Elastic expansion is calculated by subtracting permanent expansion from total expansion.

The hydrostatic jacket is then drained and opened, the tube is drained, dried and visually inspected again for any flaws as described in CGA C-6 or ISO 18119.

Finally, the tube is externally blasted and stamped with requalification marks per 49 CFR 180.213 to include the date of requalification, the facility RIN number, and other symbols such as a plus symbol for 10% overfill allowance or a star for a 10-year requalification cycle.

The tube is now ready for painting, and reassembly into a tube trailer or multiple element gas container (MEGC)

A video showing a typical hydrostatic requalification test can be found at the top of this post.