Chlorine Dioxide Storage Tank

Data-dense robotic inspection technology discovered pinhole-sized damage, avoiding potentially fatal outcomes at a kraft paper mill.

80%

through-wall pit found

201

million UT readings

corrosion monitoring locations established

Challenges

Chlorine dioxide (ClO₂) is used for the paper bleaching process, but it is toxic to humans at ambient pressures and temperatures
This vessel was located directly outside of a regularly manned control room
ClO₂ is highly acidic and will degrade carbon steel very quickly if exposed
The storage tank was tile lined on the interior and insulated on the exterior
Semi-annual visual inspection of the tile liner revealed no signs of liner failure or degradation
Process Safety Management (PSM) audit determined the vessel needed to be stripped of insulation and inspected for corrosion under insulation (CUI)
Previous inspection methods only covered corrosion monitoring locations (CML) or drop scans

Full-coverage external visual and ultrasonic testing (UT) inspection for CUI
Robotic Rapid Automated Ultrasonic Testing (R-AUT) phased array corrosion mapping tool
High definition S-scan data visualization and reporting to identify, locate, and size damage

Rapid AUT’s millimeter resolution found a small, isolated (1.75” or 44 mm diameter) corrosion cell on the inner diameter (ID) of the fourth shell course with a nearly 80% through-wall pit
At the most severe location, Gecko Experts calculated a short term corrosion rate of 0.152” (4mm)/year, driving the removal of a 2’x2’ (61cm x 61cm) patch of shell and installed a flush patch and subsequent liner repairs
Gecko Experts identified an additional 20 CMLs of ID corrosion with 20% wall loss or greater, signifying failure or damage to the tile liner
Robot localization came within 1” (25mm) of the corrosion cells for all 21 CMLs
Traditional A-scan, B-scan, or C-scan inspection methods and data visualization would have missed the insidious corrosion cell, potentially resulting in fatal outcomes
The inspection found no CUI on the ClO₂ storage tank but found significant manufacturing flaws