Testing Cryogenic Tank Walls
Large cryogenic tanks can contain high risk
products such as ammonia, butane and propane.
20,000-ton ammonia tank with 95 AE
mounted for in-service testing
High frequency AE
sensors are attached at typically a 6-meter spacing over the entire test
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%.
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:
Very minor source None
Minor source Visual external inspection
further Evaluation/possibly NDT
source Immediate follow-up NDT
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.
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.
ammonia tanks are taken out of service for inspection and repair, oxygen
enters the tank and this tends to restart the corrosion process once
Benefits of AE
- 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
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
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
- 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.
of Test - Data is digitized and stored on disk, providing a permanent
record of the test that can be reanalyzed at any time.