(57ax) Dynamic Modeling of Inbreathing Requirements for Low-Pressure Storage Tanks

Dynamic Modeling of Inbreathing Requirements for Low-Pressure Storage
Tanks Dona Abou-Chakra, Georges A. Melhem, PhD, Ronald J. Willey
Fixed roof storage tanks are known to have a weak resistance to slight vacuum or slight pressure. Typically, the minimum design vacuum is -0.036 psig and the maximum design pressure is 15 psig according to API 620 (12th Edition, 2013). Because these storage tanks have very thin shelled walls, a slight vacuum can cause tank distortion and failure. Upon a sudden change in weather conditions such as a rainstorm occurring suddenly, atmospheric storage tanks experience thermal inbreathing of ambient air into the tank. If air does not enter rapidly, a pressure drop occurs inside the tank that can lead to tank wall failure by implosion due to negative pressure. Therefore, relief devices must be sized properly based on the maximum inbreathing rate to provide safe venting of the tank.
This study aims at calculating the maximum thermal inbreathing rate by performing dynamic simulations for different tanks using ioMosaic’s 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.
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.
References:
ï‚·API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks, 7th Edition,March 2014.
ï‚·API Standard 620, Design and Construction of Large, Welded, Low-Pressure StorageTanks, 12th Edition, October 2013.
ï‚·ioMosaic Corporation, SuperChemsâ„¢ component of Process Safety Officeâ„¢.