Large Liquid Hydrogen (LH2) storage tanks are vital infrastructure for NASA. Eventually, air may leak into the evacuated and perlite filled annular region of these tanks. Although the vacuum level is monitored in this region, the extremely cold temperature causes all but the helium and neon constituents of air to freeze. A small, often unnoticeable pressure rise is the result. As the leak persists, the quantity of frozen air increases, as does the thermal conductivity of the insulation system. Consequently, a notable increase in commodity boiloff is often the first indicator of an air leak. Severe damage can then result from normal draining of the tank. The warming air will sublimate which will cause a pressure rise in the annulus. When the pressure increases above the triple point, the frozen air will begin to melt and migrate downward. Collection of liquid air on the carbon steel outer shell may chill it below its ductility range, resulting in fracture. In order to avoid a structural failure, as described above, a method for the safe removal of frozen air is needed. Two potential methods for air removal are evaluated here. The first method discussed is the connection of a vacuum pump to the annulus which provides pumping in parallel with drainage of LH2. The goal is to keep the annular pressure below the triple point so that the air continues to sublimate, thus eliminating the threat that liquefaction poses. The second method discussed is the application of heat to the bottom of the outer tank during tank drain. Though liquefaction in the annular space will occur, the goal of the heater design is to keep the outer shell above the embrittlement temperature, so that cracking will not occur. In order to evaluate these methods, it is first necessary to characterize some the physical properties and changes that take place in the system. A thermal model of the storage tank was created in SINDA/FLUINT (C&R Technologies, 2014) to identify locations where air can freeze. This model shows the volume that is capable of freezing air under varying conditions. It is also necessary to characterize the changes in thermal conductivity of perlite which has nitrogen frozen into its interstitial spaces. The details and results of an experiment designed for that purpose is outlined. All data, including operational data from existing LH2 tanks, is compiled and a physics-based evaluation of the two proposed air removal techniques is performed. Due to small pumping capacities at low pressure and the large quantity of air inside the annulus, the pumping option is not deemed feasible. It would take many years to remove a significant amount of air by pumping while maintaining the annular pressure below the necessary triple point. Application of heating devices is a feasible option. For a specific case, it is shown that approximately 105 kilowatts of power would be required to vaporize the air in the annulus and keep the temperature of the outer tank wall above the freezing point of water. Several engineering solutions to accomplish this are also discussed. There are many unknowns and complexities in addressing the problem of safely removing frozen air from the annulus of an LH2 storage sphere. The work that follows utilized: research, modeling, experimentation, analysis, and data from existing tanks to arrive at possible solutions to the problem. Heating solutions may be implemented immediately and could result in significant savings to the user.
| Identifer | oai:union.ndltd.org:ucf.edu/oai:stars.library.ucf.edu:etd-2381 |
| Date | 01 January 2015 |
| Creators | Krenn, Angela |
| Publisher | STARS |
| Source Sets | University of Central Florida |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Electronic Theses and Dissertations |
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