DOT pressure vessels (cylinders) require periodic inspections to evaluate the integrity and safety of each vessel. After years in use, it is important that authorized holders of a Requalification Identification Number (RIN) conduct a thorough examination of each vessel. Two non-destructive test methods are generally used for requalification: hydrostatic (volumetric expansion) and ultrasonic examination. Ultrasonic Examination (UE) is the most popular and reliable test method for portable cylinders and offers several advantages to equipment operators.
Benefits:
* Reliable and Useful Test Data: The ultrasonic examination equipment locates and quantifies flaw size and accurately measures wall thickness.
* Minimum Down Time: UE testing can be done without draining the gas product out of the cylinders. After the test, the cylinders can immediately be returned to service without the wasted steps required when hydrostatic testing.
* Limited Risk of Contamination: UE eliminates the need for valve removal and internal visual inspection, thus allowing the cylinder to be tested with residual product.
* Inexpensive: Quick testing and zero risk of contamination minimize down time.
* Test Integrity: UE has been used on pressure vessels for over 20 years without incident. UE complies with requalification requirements of the United States Department of Transportation (DOT). UE is also recognized by ISO standards and other international regulatory bodies.
Methodology
Ultrasonic examination uses high frequency sound energy for detection/evaluation of flaws and for dimensional measurements. The UE transducer creates a high frequency sound wave that travels through the test material. When the wave encounters the material’s boundary or a discontinuity (i.e. flaw) the wave is reflected back to the sensor.
Benefits:
* Requires access to only the outer surface of the material
* Deep penetration
* High sensitivity
Two waves directions are used during UE:
1. Longitudinal Waves (L-Wave): The sound waves are directed straight down into the material at a 90° angle. These waves measure wall thickness and locate defects oriented perpendicular to the sound wave.
2. Shear Wave (S-Wave, Angle Beam): The sound waves enter the material at a 45° angle. If no flaw exists the sound wave will not reflect back to the sensor. These waves locate defects in the material in any orientation. Due to the refraction of the sound wave, the transducer does not need to be directly over the defect. This allows the technician to inspect sections of the material that are difficult to access.
The FIBA designed UE cylinder submersion system utilizes a 7-channel system with 2 longitudinal, 2 transverse, 2 oblique and 1 thickness sensor. These channels will identify anomalies such as cracks, folds, wall thinning, pitting and general corrosion.
Cylinder Ultrasonic Examination Test Procedure
1 Visually Inspect the Cylinder Per Applicable CGA Pamphlet ( C-6 for steel cylinders or C-6.1 for aluminum cylinders)
Prior to requalifying the cylinder it must pass a thorough visual inspection.
2 Select the Calibration Cylinder
The proper calibration cylinder is selected based on the cylinder diameter and must be manufactured from an acoustically similar material as the test cylinder.
3 Select the Calibration File
The proper calibration file is selected based on the calibration cylinder.
4 Calibrate In
Load the calibration cylinder on to the test table and run the calibration test.
5 Review the Calibration Results.
The operator must review the on-screen results window and the system print out. All of the machined notches must be found for the calibration results to be accepted. If all of the notches are not found, adjustments will need to be made to transducer sensitivity and C-scan gate placement. Once the test system is calibrated properly, the results are saved and the operator can proceed with the test
6 Cylinder Testing
The first cylinder is loaded onto the test table and the appropriate test file is selected. One or more cylinders can be tested as long as they share the same diameter and material composition. Note: Recalibration is required after 200 cyls, 4 hours of testing, the end of the shift, the completion of tests for a designated cylinder size, an equipment change or if the operators change.
7 Calibrate Out
After testing is completed, the operator must close the test file, re-open the calibration file and re-run the calibration cylinder. If the system is unable to calibrate out successfully the results of the cylinders tested between the calibration in and the failed calibration out are invalid and the entire batch will need to be retested.
8 Cylinder Stamping and Daily Test Reports
The cylinders pass their periodic requalification once the system calibrates out properly. Next the cylinders are stamped with the Month/RIN#/Year/”UE” to mark the test date, the requalification company and the method of the test inspection.
At the conclusion of the shift, the operator must upload the test results onto a Daily Test Log and submit it to a Level III UE tester for data review, cylinder tracking and filing.