The Important Structural Safety Question
“Is our plant free of corrosion?” – A question every operator should ask themselves. These questions are often followed by a pat on the back, knowing that regular visual inspections are made and all legislation is adhered to. The unfortunate truth of the matter is that corrosion can rear its ugly head from many unseen directions.
Storage tanks – Large, high surface area structures, that are difficult and time consuming to regularly inspect in great detail. So visual inspections should do the job, right? There’s no visible corrosion, so everything is ok? Unfortunately life is not this simple.
Hidden Corrosion on Structures
On many structures, especially storage tanks, it is the side that is not seen in a visual inspection that is at the most risk of concealing corrosion. Storage tanks have huge internal surface areas and the slightest breach of the surface coating or lining can create a corrosive environment between the parent material and the surface coating. In the picture shown here you can see the corrosion that has been revealed by blasting the surface coating from the parent material.
The example here is the external, easily visible surface. But what if it was the internal surface of an acidic process fluid storage tank? Those small surface indications of a larger sub-surface coating corrosion issue just got a lot more difficult to see. This can have disastrous effects on your structural assets if an effective maintenance program is not carried out.
“… Failure to carry out an effective maintenance program led to the catastrophic failure of this tank” – DMP Safety Director, Andrew Chaplyn. Source – DMP News Article
Safety Learning – Visual Examination vs Thickness Testing
While visual examination is an effective tool for analysing the exterior health of a structure, it provides no indication of structural health beyond the visual.
In the case of storage tanks, especially those storing acidic substances, corrosive environments can form between the internal lining and the tank shell. Namely, between the metallic surfaces and the paint or rubber lining layers.
An effective way to mitigate against these hidden dangers is to perform ultrasonic thickness (UT) testing of the tank walls.
Ultrasonic Thickness Testing
UT testing can be used to determine the thickness of the tank shell, even with a surface coating, revealing the internal corrosion. These kind of inspections can heavily reduce the risk of vessel breaches by identifying areas of concern and implementing a solution into the maintenance program like closer, more frequent, monitoring and shell patches. When tanks are designed they have a corrosion allowance specified in the design. This is because although in a perfect world there would be none, in the real world corrosion is always a risk that must be mitigated.
UT Thickness testing goes far beyond just Storage tanks. UT can be used on pipes, structures, aircraft frames and numerous other structures and components. Rapallo employ a wide array of manual UT thickness, weld testing and automated scanning robots such as the Scanmap VS RMS2 Crawler.
Condition Monitoring of Tanks and Vessels
Acoustic Emissions (AE) technology has also been used successfully on vessels and tanks, among other structures, as a condition monitoring tool. AE works by listening for energy being released by active corrosion and crack propagation. The AE software then uses the time difference of the signals picked up by different sensors to triangulate the location of the corrosion or cracking.
Rapallo have been using AE systems to closely monitor critical process vessels and tanks that are not easily accessible for many years. The data from AE systems has also been used to assist Plant and Machinery Inspectors to make informed decisions on future inspections including using AE reports in lieu of internal inspections and extending inspection intervals.
An Unexpected Lesson
The famous American writer, Mark Twain, often referred humorously to his lack of success at mining before his career in literature took off. However, while his physical services never contributed much to the industry his words still can.
“It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.” – Mark Twain
Need a Structural Engineering Audit or Non-Destructive Testing? Contact Rapallo’s team at firstname.lastname@example.org and utilise our broad knowledge and experience in asset management and engineering. Want to know more about any of Rapallo’s services? Click the respective links below.