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On factory sites, tanks are often insulated to ground level. For tanks with convex bottoms supported by legs or a skirt, this is the simplest and cheapest option. However, this limits ventilation, which promotes corrosion under the tank. Welds at legs or skirts are especially vulnerable to dynamic loads. With skirts, inspection is often impossible due to insulation.
An example is an overturned stainless steel condensate tank (180m³) with a carbon steel skirt welded to the tank bottom (316L) via a stainless steel transition ring. Periodic filling and emptying creates dynamic bending loads on the weld, leading to fatigue failure. In addition, the weld often contains a gap on the inside, which accelerates this process.
This site contained 28 tanks, half of which were insulated to ground level. We recommended that tanks with large fluid fluctuations be inspected directly by opening the insulation above the lap weld. This revealed cracks in five tanks. In addition to level fluctuations, construction work and trucks passing by also caused vibrations.
MCI performed laboratory testing after inspections and sampling, including fracture surface examination with SEM (Scanning Electron Microscopy). This showed that only fluid fluctuations were the cause of fatigue failure.
Many manufacturers, engineers and inspectors are unaware of this risk, so full insulation is often preferred. In addition, the dynamic loading of these tanks is underestimated and the design is wrongly based on static loading.
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