Dynamic Modeling of Inbreathing Requirements for Low Pressure Storage Tanks

This study aims at calculating the maximum thermal inbreathing rate by performing dynamic simulations for different tanks using ioMosaic’s Process Safety Office® SuperChems™ software. The first objective of this research was comparing the detailed SuperChems™ single-phase and two-phase wall dynamics model to existing large scale test data and models. The results were successfully reproduced using this software with error margins between ± 5%. Previous to this work, the software had not been evaluated for this important modeling.

The second objective was to compare results from the SuperChems-based model against API 2000 (7th Edition, 2014), which is the current standard used for venting atmospheric and low-pressure storage tanks. This work found under a number of scenarios that API 2000 relief equations are considered conservative for non-condensable gas services where the relief device may be overdesigned by up to 60%. However, API 2000 modes fail to predict appropriate relief sizing for tanks storing condensable vapors, such as methanol, and wide-boiling-point mixtures, such as gasoline-ethanol. The relief device capacity can be underestimated by as much as 270% using API 2000. This work recommends adjusting the free-convection heat transfer coefficients according to the vapor type to ensure adequate relief sizing for safe venting.


Heat transfer across the wall is modeled in detail for each wall segment

Heat transfer across the wall is modeled in detail for each wall segment in SuperChems™.

The third and final objective of this research was to assess the impact of the solar radiation. Solar radiation varies with the geographical location of the tank and impacts the thermal inbreathing and out-breathing. The two locations chosen for this study were Montreal, Canada and Jubail City, Saudi Arabia. Examined were three types of colors for external wall covering with different values of emissivity. Colors examined were: white, aluminum bronze, and black. Rainstorms were simulated at the time of maximum solar flux (i.e. highest tank wall temperature) to create the worst-case scenario and thus the maximum inbreathing rate. Preliminary results for dry air showed that a 600 m3 tank in Saudi Arabia experiences 10% higher inbreathing and 8% higher out-breathing as compared to a tank located in Canada. API 2000 relief calculations were adequate in this case. However, it should be noted that the comparison is for tanks filled with non-condensable dry air only. Future work in this objective is recommended for tanks containing condensable vapors and verification of the maximum inbreathing rates determined at the two locations.

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