Problems with the maintenance and renovation of vertical cylindrical storage tanks.
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This post was last edited by The sun is about to return on 2022-4-21 at 15:09. A client needs to modify a 3,000-cubic-meter vertical storage tank (with an inner diameter of 18.9 m and a tank wall height of 11.725 m; it has 7 layers of tank wall panels, the first 6 layers of which each have a width of 1.8 m), and they provided the design institute with the inspection report for this tank. During the analysis of the inspection report, the following issues were identified: 1. The minimum average thicknesses of the sixth and seventh layers of the tank wall were 3.9 mm and 2.6 mm, respectively. According to the formula for calculating tank wall thickness specified in GB50341-2014, the calculated thicknesses for the sixth and seventh layers are 2.81 mm and 1.53 mm, respectively. According to the wall evaluation in 5.4.4.2 of SY/T5921-2017, the minimum average thickness of each ring of wall plates shall not be less than the calculated thickness of that ring of wall plates plus the corrosion allowance. Here, the corrosion allowance is taken as 1 mm. Thus, 3.9 > 2.81 + 1, and 2.6 < 1.53 + 1. It can be concluded that only the seventh layer of plate fails to meet the strength requirements; if the storage tank is to be modified, it is sufficient to replace only that seventh layer of plate. So the question is: Table 6.3.4 in GB50341-2014 specifies the minimum nominal thickness for tank wall plates. According to this table, the minimum nominal thickness for a 3000-cubic-meter tank’s wall plates is 6 mm; thus, the value obtained by subtracting the negative deviation of the steel plate thickness from this nominal thickness and then adding the corrosion allowance is 4.45 mm. 3.9mm < 4.45mm, which also does not meet the specification requirements. Moreover, if only the seventh layer of tank wall panels is to be replaced, and given that the thickness of the sixth layer is only 3.9 mm, what should be the appropriate thickness for the replacement seventh layer? According to the standards, the minimum thickness is 6 mm; however, this would result in the upper layer of tank walls having a greater thickness than the lower layer, which also violates clause 6.1.2 of GB50341-2014. 2. The minimum average thickness for inspection of the tank top is 3.8 mm; the evaluation is to be carried out in accordance with item a) of 5.4.4.3 in SY/T5921-2017, but the instructions given are quite vague. As a supplementary note: the minimum average thicknesses for the inspection of layers 1 to 5 of the tank wall are 11.4 mm, 9.1 mm, 7.6 mm, 5.6 mm, and 5.4 mm respectively. After verification, everything is fine.
I added some further explanations: the inspection reports showed that there were no issues with layers 1 to 5, and all strength checks passed. The owner preferred to carry out repairs rather than having the tank demolished and rebuilt, as the cost of constructing a tank with a capacity of 3,000 cubic meters is very high.
1. Design evaluation； 2. Renovation plan ； 3. Request for approval!
This post was last edited by AtlantisH on 2022-4-21 at 15:52. According to the internal regulations of a certain state-owned design firm, if the minimum average thickness of upper wall panels is less than 80% of the nominal thickness, they must be replaced regardless of the results of strength calculations； Complete replacement of the ribbed roof panels where the minimum average thickness is less than 4.45 mm.
This is quite similar to the corrosion detection analysis I do here, but the basis for it is still insufficient; it’s all based on internal regulations. It’s still very difficult for current property owners, who only follow the regulations. I really appreciate your advice.
The wall thickness mentioned above might be fine for use, but it’s still best to be cautious; it seems better to replace both of the upper strips. If the medium is not extremely hazardous, Q235B should be sufficient.
If updating is truly not desired, it is recommended to use 5mm thick plates for the sixth and seventh sections, along with wind-resistant rings, for modification.
As long as the calculated thickness is less than the measured value, it’s fine. Corrosion margin is expected, and it holds little significance for retrofitting or reusing existing equipment. The key is to assess the remaining useful life of the existing conditions. Shorten the periodic inspection cycle. Meanwhile, to improve the stiffness of the equipment tanks, stiffening rings are added to the equipment at intervals of no more than 3 meters.