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DOT pressures vessels (tubes) require periodic inspections to evaluate the integrity and safety of each vessel. After years of use over the land and sea, it is important that authorized holders of Requalification Identification Numbers (RIN) conduct a thorough examination of each vessel. There are several non-destructive test methods used for requalification, including hydrostatic (volumetric expansion), ultrasonic and acoustic emission. Acoustic emission testing (AET) is a unique method that offers several advantages to equipment operators.
Benefits
| Portability | The equipment required for acoustic emission testing is relatively small and it can be shipped directly to the location of a trailer or skid container (MEGC). Technicians can travel with additional equipment. No other method allows for such portable testing of tubes. Shipping a unit to a test facility can be expensive. |
| Minimum Down Time | When disassembly is not required to examine neck threads, the test procedure allows for testing to occur with the unit assembled. A unit can come directly from service to a test center and be tested within a few hours. For trailers in certain gas services, the unit can be tested with the same gas it is in service for. |
| Inexpensive | When a unit only requires an acoustic emission test, the scope of work and time required for inspection is minimal, thus keeping the cost down. |
| Test Integrity | Acoustic emission testing has been used on pressure vessels for over 30 years without incident. AET complies with requalification requirements of the United States Department of Transportation (DOT). AET is also recognized by ISO standards and other national regulatory bodies. |
| Limited Risk of Contamination | Unlike hydrostatic testing, acoustic emission testing does not introduce water or other contaminates into the vessel during a test. |
Methodology
Acoustic emission testing involves recording and interpreting sound waves as they travel through a material. The test involves introducing an external stimulus to the material and listening for emissions that may be generated by a flaw in the material. The right equipment and test setup can determine whether or not an emission is normal or requires further investigation.
In the case of pressure vessels, the stimulus applied is pressure. As the vessel is pressurized, the equipment records acoustic emissions in a specific frequency range. Each emission is recorded and if it is outside of certain threshold ranges, it is possible that the emission may come from a crack or other flaw in the material. Generally, localized ultrasonic testing is used for follow-up inspection of an area that exhibits acoustic emissions of concern. If the vessel shows no indication of a flaw, the test results are documented and the vessel is requalified.
Tube Trailer Acoustic Emission Test Procedure
Acoustic emission testing on tube trailers and skids is a practical and convenient process and can be completed quickly. To allow for the test, the proper environment and equipment must be available. The acoustic emission testing equipment is portable and can be transported to almost any test location around the world, but the equipment is sensitive and fragile, so it is important that the environment is clean and, ideally, the space is covered. To avoid damaging the equipment and/or interfering with the test, rain, wind, dust and other environmental variables need to be mitigated. In addition to addressing the environment, the test requires pressure, so a high pressure compressor and related equipment must be available to fill the vessels. Nitrogen is the preferred gas to be used for the test because of both its cost and its properties. A source of electrical power is also required. The better the environment and available equipment, the faster the test will be completed and the unit will be back in service.
Below is an overview of the test process:
1. Equipment is Placed in Inspection Area
The inspection area should be large enough to freely work around the unit and be free of debris or other matter that could disrupt the test.
2. Unit is Reviewed
The unit is reviewed for safety and to ensure that an accurate test can take place. Burst discs and valves are replaced as necessary.
3. Equipment is Setup
The acoustic emission equipment is setup. Cables and other wiring is put into place. The supply of high pressure gas is arranged and connected to the trailer.
4. Trailer is Pressurized to Start Pressure
Each vessel is pressurized to 50% of the AE maximum test pressure of 1.1 times the highest fill pressure experienced by any individual vessel in the unit for the last 12 months prior to requalification.
5. Acoustic Emission Sensors Are Setup
The acoustic emission sensors are placed on both ends of each vessel. The sensors are required to be located within one inch of the bell end of the vessel.
6. Equipment is Calibrated
The acoustic emission equipment is tested and calibrated using a series of lead break tests on the vessel.
7. Test Begins by Applying Pressure
The test pressure must equal 1.1 times the highest fill pressure experienced by any individual vessel in the unit for the last 12 months prior to requalification.
8. Acoustic Emission Data is Evaluated
Recordable events are sized and characterized and will require follow-up shear-wave inspection using angle beam sensors (ultrasonic testing).
9. Follow-up Ultrasonic Testing is Performed
The extent of the examination shall be 18 inches on either side of the axial location (on the cylindrical portion) as determined through AE.
10. Ultrasonic Data is Evaluated
Ultrasonic data is reviewed to confirm or reject AE data that is indicative of a flaw. Any rejected tubes shall be further evaluated before being condemned.
11. Report is Prepared and Test is Complete
The test is completed and a final report is prepared for the customer.
