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At industrial sites, storage tanks are often insulated up to ground level. For tanks with dished bottoms supported by legs or skirts, this is the simplest and most cost-effective option. However, it limits ventilation, which increases the risk of corrosion under the tank. Welds at the skirt legs are particularly vulnerable to dynamic loads. With skirts, inspection is often impossible due to the insulation.
An example involves a fallen stainless steel condensate tank (180 m³) with a carbon steel skirt, welded to the tank bottom (316L) via a stainless steel transition ring. Periodic filling and emptying created a dynamic bending load on the weld, leading to fatigue failure. Additionally, the weld had a crevice on the inside, accelerating the process.
At the site, 28 tanks were present, half of which were insulated down to ground level. We recommended that tanks with significant fluid fluctuations be inspected directly by opening the insulation above the tank bottom. This revealed cracks in five tanks. In addition to fluid level changes, nearby construction work and passing trucks contributed to vibrations.
MCI conducted inspections and took samples for laboratory testing, including fracture surface analysis using SEM (Scanning Electron Microscopy). This confirmed that fluid fluctuations alone were the cause of fatigue failures.
Many manufacturers, engineers and inspectors are unaware of this risk, which is why full insulation is often preferred. Moreover, the dynamic loads acting on these tanks are often underestimated, and the design is wrongly based on static loading conditions.
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