Testing Cryogenic Tank Walls with AE

 

Large cryogenic tanks can contain high risk products such as ammonia, butane and propane.

20,000-ton ammonia tank with 95 AE sensors mounted for in-service testing

High frequency AE sensors are attached at typically a 6-meter spacing over the entire test area.

 

Where the vessel is insulated with very thick insulation, the sensors are often mounted permanently underneath the insulation and cabled back to a connection point, so that repeat installation costs are not incurred.

 

The fluid level in the vessel is then increased while monitoring for acoustic emission. The fluid level has to be preferably below 95% of maximum for the six months prior to the test. It then has to be increased slowly to 100%.

 

Following the test, the AE data is processed to remove any extraneous noise, this is known as interpretation. The severity of emission is then assessed against a database known as evaluation. This method gives clearly-defined grading system with interpretation and recommendation:

 

A Very minor source None

B Minor source Visual external inspection

C Source further Evaluation/possibly NDT

D Active source Immediate follow-up NDT

E Intense source Immediate action

 

In some cases, it is possible to obtain a more precise indication of the emission source by analyzing arrival times for the stress waves at each sensor in the same way that earthquakes are located. For this to be possible, the stress waves must be energetic enough to reach three sensors. This is most likely in the case where cracking is the source.

 

Acoustic methods of tank inspection can offer considerable financial, safety and environmental benefits by providing information on tank integrity without draining or incurring extensive down time. The tests are non-invasive and pose no threat to the integrity of the tank. When used as part of a predictive maintenance program for tanks, they allow maintenance resources to be targeted to the areas with problems, thereby minimizing costs while focusing on problem tanks.

When ammonia tanks are taken out of service for inspection and repair, oxygen enters the tank and this tends to restart the corrosion process once again.

 

Benefits of AE Testing

 

Global Monitoring - Sensors detect AE signals from considerable distances, making this method ideal for global monitoring of large vessels and systems. Identified problem areas can then be inspected using other NDT methods.

Minor Disturbance of Insulation - Only small holes in insulation are required for sensor mounting. On high-temperature applications, waveguides are used to contact the surface. Sensors are then mounted outside the insulation.

On-Line Testing. Opening a tank introduces oxygen into the tank. When the tank is put back into service the corrosion process starts all over again. For most storage vessels, on-line testing is possible either by filling it with product, introducing gas into the vapor space, controlling the temperature or other process parameters.

 

Cost Reduction. In ammonia applications, it can cost up to $1,000,000 just for a nitrogen purge. The cost increases further when you add thousands of dollars for scaffolding and the cost of traditional ultrasonic or other traditional inspection, not to mention lost production time. The use of AE can reduce plant maintenance costs considerably, while increasing the information available about plant integrity. Plant downtime for inspection is also minimized.

 

Rapid Inspection - The actual AE test takes a matter of hours, and in some cases, considerably less. No comparable method can provide 100% volumetric inspection in the same amount of time.

 

Permanent Record of Test - Data is digitized and stored on disk, providing a permanent record of the test that can be reanalyzed at any time.