Is the choice of steel plate based on the medium temperature or the ambient temperature?
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During recent discussions regarding a horizontal storage tank in Xinjiang (an underground tank at a gas station used for storing diesel and automotive gasoline), it was mentioned that the lowest temperatures in Xinjiang during winter can reach -30 degrees; in such cases, using Q235B steel might not be suitable, and Q345R should be chosen instead. Some people say that the oil temperature cannot be this low; the design should select materials based on the temperature of the medium, not on the ambient temperature. Which of these two statements is correct?
The design temperature should be selected based on the most severe operating conditions
It should be determined based on the equipment pressure. If low temperature and low stress conditions are met, Q235B is suitable. The diesel and gasoline tanks should be pressure-free.
The determination of the design temperature for equipment is clearly specified in clause 4.3.4 of GB/T150. Q235B can only be used at temperatures above 20 degrees; for pressure vessels, corresponding material requirements must be met. It is also necessary to conduct detailed calculations to determine whether the application involves low-temperature and low-stress conditions. For reference, we design that the edge plates and bottom ring plates of storage tank bottoms in Xinjiang are sometimes made of 16MnDR.
Burial and insulation have little to do with the ambient temperature. However, Q235B can only be used at temperatures above 0°C; there’s no need to worry about this – use Q345R instead. The cost increase is minimal, while safety is **improved**.
The lowest temperature that metal may reach during service.
The interpretation of the GB/T150-2011 standard states that Q235 steel is not a steel grade designated for use in containers, and there was consideration to remove it from the standard. Finally, considering some low-parameter vessels such as glass-lined containers and vacuum-insulated containers, as well as their support pads, which still use this steel grade, it was ultimately included in Appendix D of the materials section; however, the conditions for its use are detailed, especially with a temperature limit above 20°C. This reflects the official stance: the production of containers using Q235 will eventually be restricted, so in this case it is appropriate to use Q345R for manufacturing the containers. If a Q235 container has already been manufactured, it can only be verified under low temperature and low stress conditions; however, it should be noted that the impact toughness of actual Q235 material is extremely poor, posing a risk of brittle fracture at low temperatures!
From the most important perspective, namely safety, the design should be based on parameters corresponding to the most severe operating conditions as the key criteria. If there’s a problem with the design and something goes wrong, one will be sentenced.
Designed for the most demanding conditions; 16MnDR is chosen for reliability
Take a look at Appendix E of GB150.3 – the ambient temperature refers to the lowest average monthly temperature over the years. HG20580 also has relevant regulations on this. In addition to the ambient temperature, factors such as insulation are taken into consideration as well. If low temperature and low stress conditions are met, it might not be necessary to use materials designed for low temperatures
Diesel and gasoline tanks are both atmospheric pressure storage tanks and are not governed by GB/T150. Put simply, it’s like a water tank – it can’t be damaged by even the lowest temperatures. Moreover, since it is a buried tank, the temperature underground should be above 0 degrees.