What operating conditions need to be calculated during the analytical design of spherical tanks?

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This post was last edited by Freestyle-sky on 2024-5-7 at 18:33. Search for and follow the WeChat official account “ANSYS Analysis and Design Professionals” – a platform dedicated to discussions on the analysis and design of pressure vessels! In learning, it is important to have a teacher; even more so, it is important to have friends! Learn together*, progress together! Appendix D of GB/T12337 specifies that the load combination conditions for the analytical design of spherical tanks as stated in D.4.3 of Appendix D of GB/T12337 are as follows: Under operational conditions, the following three conditions need to be considered: (1) internal pressure + self-weight ； (2) Internal pressure + self-weight + wind load ； (3) Internal pressure + self-weight + 25% wind load + seismic load ； The following conditions need to be considered under the voltage withstand test: (4) internal pressure + self-weight + wind load. It should be noted that the aforementioned self-weight refers not only to the weight of the spherical tank shell itself, but also includes the static pressure of the operating medium or the liquid column used for hydraulic testing, the weight of supports and tie rods, the weight of the insulation layer, the weight of snow accumulation, as well as the weight of accessories such as manholes, connections, level gauges, internal components, and ladder platforms. It is clear from the above provisions that the effects of wind load need to be taken into account in all cases (2), (3), and (4). In case (3), a wind load of 25% is specified, reflecting the extremely low probability of wind loads being at their maximum level simultaneously during an earthquake. In cases (2) and (4), only wind load is mentioned without specifying the percentage of wind load that needs to be considered. For condition (2), which is an operating condition and a long-term condition throughout the service life, it is undeniable that the impact of the maximum possible wind load must be taken into account; that is, calculations should be carried out assuming a 100% wind load. As for condition (4), it is the condition under pressure testing, a short-term condition measured in hours. It is almost impossible for the maximum wind load to occur precisely during the pressure test – it is a event with an extremely low probability, even lower than the chance of winning the lottery. Therefore, it is certainly not possible to consider a 100% wind load; doing so would be too conservative. However, Appendix D does not specify a clear percentage of wind load that needs to be taken into account, as it does for condition (3). Question 1: What wind load needs to be considered under the withstand voltage test conditions? Although Appendix D does not provide a clear answer, it can be found in the formulas from the regular calculation sections earlier in the standards. https://pic4.zhimg.com/80/v2-d659d46002f5a14520cc49b80b1bfc93_720w.webp As can be seen from point 6.7.1.4, in the formula for calculating the maximum vertical load on the pillar under hydraulic testing conditions, Fw/Fmax represents the horizontal wind force acting on the spherical tank divided by its maximum horizontal force; therefore, under hydraulic testing conditions, only the effect of wind loads needs to be considered, while seismic loads need not be taken into account ； The coefficient of 0.3 earlier indicates that only the effect of 30% of the maximum wind load needs to be considered, and this is the answer we are looking for. In addition to the formula in 6.7.1.4, further verification can be found in the formula for the shear stress hydrostatic test condition at point a in 6.11.1, as shown in the figure below: https://pic4.zhimg.com/80/v2-2b82283fe4b1884f41de250dc9fa203f_720w.webp. It can therefore be concluded that, for conditions under pressure testing, the calculations during design should take into account internal pressure + self-weight + 30% wind load. Question 2: What operating conditions need to be calculated for the analysis and design of spherical tanks? As mentioned earlier, it is necessary to consider 3 operating conditions and 1 condition under pressure testing, for a total of 4 conditions. So, does that mean we only need to take these 4 conditions into account? No! Due to the direction of wind loads and seismic loads, it is also necessary to consider the relative direction between the maximum horizontal force resulting from the combination of horizontal wind loads and horizontal seismic loads and the pillars. As shown in the figure below, this directly affects which pillar will experience the maximum stress at point a, as well as the magnitude of that maximum stress. https://pic2.zhimg.com/80/v2-007be8ff777c278b6295dd796b885611_720w.webp In summary, by taking into account the direction of the maximum horizontal force, the 4 original conditions become the following 7 conditions: https://pic3.zhimg.com/80/v2-58da9d9e41b5e9ea95a35fa02865d2a6_720w.webp The content of this article was compiled and published by the author; it represents only the author’s personal views and is intended for academic purposes, as well as for discussion and exchange.