Cryogenic fluids comprise liquefied gases that generally have boiling points below −100° C. (about −150° F.) at atmospheric pressure. Examples of cryogenic fluids include liquefied natural gas (LNG), and other gases, such as nitrogen, oxygen, carbon dioxide, methane and hydrogen, that are storable in liquefied form at cryogenic temperatures.
To prevent cryogenic fluids from boiling off and to increase the time that they can be stored in liquefied form, cryogenic fluids can be stored in thermally insulated storage tanks that consist of an inner storage vessel mounted within an outer shell, with thermal insulation provided by insulating materials and a vacuum disposed in the space between the inner vessel and the outer shell. The inner vessel defines a cryogen space in which a liquefied gas can be stored at cryogenic temperatures. Such an arrangement reduces the transfer of heat from the ambient environment to the cryogenic fluid stored within the cryogen space, but some heat transfer into the cryogen space, which can also be referred to as “heat leak” is inevitable. Heat leak warms the cryogenic fluid, which lowers the density of the liquefied gas and increases the bulk temperature and pressure of the cryogenic fluid. In a fixed volume, the lowered density of the liquefied gas causes an increase in the density of the cryogenic vapor and some vapor will be condensed back into the expanding liquid. Overall, the volume of liquefied gas in the cryogen space will increase steadily as the bulk temperature and vapor pressure increase due to heat leak. If the overall pressure in the cryogen space rises above the set point of the pressure relief valve, vapor is vented from the cryogen space to atmosphere, or to a recovery system or directly to an end user. For example, with what is known in the industry as an economizer system, vented vapor can be delivered directly to an engine. However, it is still preferable to reduce venting cryogenic fluid from the cryogen space, and so it is desirable to design storage tanks to reduce heat leak so that cryogenic fluids can be stored for longer periods of time without venting. Each pipe that penetrates through the insulating space and into the cryogen space provides a thermal conduction path that can contribute to heat leak. Reducing the number of pipes that extend between the inner storage vessel and the outer shell can reduce heat leak. Heat leak can also be reduced by selecting materials with lower thermal conductivity for the structural supports for the inner storage vessel.
Another method of increasing holding times and reducing the possibility of venting vapor from the cryogen space is to reserve a portion of the cryogen space for vapor when the storage tank is filled. This vapor space is known as the ullage space, and the ullage space provides a volume for cryogenic fluid to expand into so that the tank does not become liquid full before reaching the relief valve set point pressure. If a storage tank is filled completely with a liquefied gas, without reserving a vapor-filled ullage space, even a very small amount of heat leak can result in a rapid increase in storage pressure, because there is little space into which the liquefied gas can expand. Accordingly, it is common practice when filling a storage tank to reserve an ullage space that is not filled with liquefied gas. To assist with preventing an ullage space from being filled with cryogenic liquid while filling a storage tank, U.S. Pat. No. 5,404,918, entitled, “Cryogenic Liquid Storage Tank” (the '918 patent), discloses a storage tank with a partitioned cryogen space with only one passage means between the main tank and the ullage space. The cross-sectional flow area of the passage means is smaller than the cross-sectional flow area of the fill line so that the liquefied gas is restricted from flowing into the partitioned ullage space when the cryogen space is being filled. A problem with this arrangement is that although the flow area of the passage means is smaller than the cross-sectional flow area of the fill line, it can still allow a quantity of liquefied gas to flow into the ullage space during filling, especially when the passage means is located near the bottom of the partition to assist with draining liquefied gas back into the main tank. Perhaps more importantly, another problem with this arrangement is that the flow restriction caused by the passage means also acts to restrict flow of the cryogenic fluid from the ullage space back into the main tank. When a storage tank is being filled, if the storage tank is not initially empty there can be some liquefied gas already inside the ullage space. This is common for storage tanks used to carry fuel for a vehicle engine because completely emptying the storage tank will leave the vehicle out of fuel and stranded. When re-filling a partially empty storage tank, the newly introduced liquefied gas flowing into the main tank during the re-filling process can condense the vapor inside the main tank thereby lowering the pressure in the main tank. Meanwhile, the flow restriction provided by the passage means restricts the flow rate of liquefied gas that is flowing from the ullage space back into the main tank, driven under such circumstances by the pressure in the ullage space being higher than the pressure in the main tank. At the end of the filling process, the restricted flow of liquefied gas from the ullage space can result in a significant amount of liquefied gas being trapped inside the ullage space, resulting in a reduced volume reserved for vapor. This outcome is disadvantageous because a reduced volume of vapor at the end of the re-filling process means that there is less volume for cryogenic fluid to expand into, which in turn means shorter hold times before vapor is vented from the cryogen space. Also, during use after re-filling, if the liquefied gas is being delivered from the storage tank at a high rate, another disadvantage of storage tanks with one restricted fluid passage through the partition is that the flow restriction provided by the single fluid passage can slow down the rate at which the liquefied gas can be delivered. Some, of the problems associated with the design taught by the '918 patent are overcome by the design taught by U.S. Pat. No. 6,128,908 entitled “Cryogenic Liquid Storage Tank with Integral Ullage Tank” (the '908 patent). The '908 patent teaches an arrangement similar to that of the '918 patent except that the passage between the ullage tank and the main tank connects the lower part of the ullage tank with the upper part of the main tank. This prevents liquefied gas from flowing into the ullage tank during the first part of the filling process, but the small diameter of the provided passage still restricts the rate at which liquefied gas can flow back into the main tank. However, this design introduces new problems because it relies upon a pressure differential between the ullage tank and the main tank to remove liquefied gas from the ullage tank and this can also result in liquefied gas being trapped inside the ullage tank. The effectiveness of the storage tank taught by the '908 patent can be compromised if heat transfer through the partition wall between the main tank and the ullage tank cools the vapor in the ullage tank to such a degree that it condenses vapor in the ullage space thereby reducing vapor pressure therein. If the pressure differential is not high enough to effectively remove the liquefied gas from the ullage space through the vertical passage, when the tank is re-filled this can result in the reserved vapor space being smaller than desired. In addition, another disadvantage of the design taught by the '908 patent is that, because of the small diameter of the vertical passage between the ullage space and main tank, it appears that blockage of the passage is possible, leading the developers of this design to introduce into preferred embodiments a pressure relief device operative to prevent over-pressurization of the ullage space, adding to the cost of manufacture, as well as adding to the possible failure modes should the pressure relief valve fail.
U.S. Pat. No. 5,685,159 entitled, “Method and System for Storing Cold Liquid” (the '159 patent), discloses an arrangement that comprises a main tank and an auxiliary tank, which can act as an ullage space for the main tank. In a disclosed preferred embodiment, when the auxiliary tank is external to the main storage tank, the system can comprise a solenoid valve that controls flow between the main storage tank and the auxiliary tank. An electronic controller can be programmed to automatically actuate the solenoid to close the valve when the tank is being filled and to otherwise keep the valve open to allow the auxiliary tank to communicate with the main storage tank and to act as the ullage space. A disadvantage of this arrangement is that the conduit between the main tank and the auxiliary tank introduces another heat transfer path into the cryogen space. However for maintenance purposes the solenoid valve should be located outside of the storage vessel where it can be accessed for servicing and where the solenoid will not itself introduce heat into the cryogen space.
The '159 patent also discloses other preferred embodiments in which the auxiliary tank is located inside the main storage tank. In these embodiments, instead of a valve, capillary tubes are employed to restrict flow of cryogenic fluid into the auxiliary tank during filling and to allow communication between the auxiliary tank and the main storage tank so that the internal auxiliary tank can act as the ullage space and so that cryogenic fluid (liquid and/or vapor) can drain back into the main storage tank. One problem with this arrangement is that at least one capillary tube is preferably associated with the low point of the ullage space so that liquefied gas that enters or condenses inside the ullage space can be drained there from through the capillary tube that is associated with the low point of the ullage space. Since the capillary tubes remain open, they can allow flow of the cryogenic fluid into the auxiliary tank during re-filling, which can result in a reduced volume of vapor space.