Storage tank for a cryogenic fluid with a partitioned cryogen space

1. A method of holding a cryogenic fluid comprising liquefied gas and vapor in a thermally insulated cryogen space defined by a storage tank, wherein said cryogen space is partitioned so that it is divided into a main storage space and an auxiliary space, said method comprising:

(a) establishing a vapor-filled ullage space when said main storage space is being vented and re-filled with said cryogenic fluid, by draining said liquefied gas from said auxiliary space to said main storage space through a first fluid passage provided through said partition, wherein said first fluid passage is sized to allow said liquefied gas to flow there through under the influence of a vapor pressure differential permitted by said partition, said liquefied gas draining at a rate that allows said auxiliary space to be drained of said liquefied gas to establish said vapor-filled ullage space with a liquefied gas volume that is at least as large as a predetermined ullage volume in less than the time needed to vent and re-fill said main storage space with said cryogenic fluid;

(b) preserving said vapor-filled ullage space by restricting a flow of said liquefied gas through said first fluid passage from said main storage space into said auxiliary space when said cryogen space is being re-filled with said cryogenic fluid, wherein said flow of liquefied gas through said first fluid passage is restricted by regulating said flow of liquefied gas with a valve that comprises a valve member that is actuatable between an open and a seated position by fluid forces within said cryogen space, whereby the flow of liquefied gas is restricted by said valve when said valve member is in said seated position;

(c) restricting said cryogenic fluid flow through a second fluid passage provided through said partition, so that said vapor pressure differential can develop across said partition when said main storage space is being vented and re-filled with said cryogenic fluid, and whereby a detectable back pressure occurs when said main storage space is filled with said cryogenic fluid when said cryogenic fluid is being introduced into said main storage space; and

(d) detecting when said storage tank is full and stopping re-filling when said storage tank is full;

further comprising regulating a second fluid flow through said second fluid passage with a second valve comprising a second valve member that is actuatable by other fluid forces within said cryogen space, whereby said second valve opens to allow said cryogenic fluid to flow from said main storage space to said auxiliary space when vapor pressure in said main storage space is higher than vapor pressure within said auxiliary space by a predetermined amount.

2. The method of claim 1 wherein said fluid forces within said cryogen space that act on said valve member comprise vapor pressure and static liquefied gas pressure, and dynamic fluid momentum forces generated by said cryogenic fluid when it is flowing through said valve.

3. The method of claim 1 further comprising orienting said valve within said cryogen space so that gravitational forces act on said valve member to bias it towards said open position.

4. The method of claim 1 wherein said valve member is urged towards said seated position when fluid pressure acting on said valve member from said cryogenic fluid within said main storage space is higher than fluid pressure acting on said valve member from said cryogenic fluid within said auxiliary space by a predetermined margin, and said valve member is otherwise urged to said open position.

5. The method of claim 1 wherein when filling said main storage space with said cryogenic fluid, said method comprises introducing said cryogenic fluid at a flow rate that causes cryogenic fluid level within said main storage to increase at a faster rate than cryogenic fluid level can rise within said auxiliary space, whereby static fluid pressure and fluid momentum pressure acting on said valve member urge it towards said seated position.

6. The method of claim 1 wherein the step of detecting when said storage tank is full comprises processing a signal from a level sensor to determine the level of said liquefied gas within said main storage space.

7. The method of claim 1 wherein flow through said second fluid passage is restricted by making the cross-sectional flow area of said second fluid passage smaller than that of a fill conduit through which said cryogenic fluid is introduced into said main storage space when re-filling said storage tank.

8. The method of claim 3 further comprising venting vapor from said main storage space through said fill conduit prior to re-filling said storage tank with said cryogenic fluid.

9. The method of claim 1 further comprising venting said vapor from said main storage space through a vent pipe prior to re-filling said storage tank with said cryogenic fluid.

10. The method of claim 1 wherein when re-filling said main storage space, said cryogenic fluid is not introduced into said cryogen space until said vapor pressure within said main storage space is below a predetermined value.

11. The method of claim 1 wherein said second valve is a one-way valve, and said method further comprises allowing said cryogenic fluid to flow through said second valve only from said main storage space to said auxiliary storage space.

12. The method of claim 1 further comprising transferring said cryogenic fluid between a lower region of said auxiliary space and an upper region of said main storage space, when said cryogenic fluid is flowing through said second fluid passage.

13. The method of claim 1 further comprising dispensing said cryogenic fluid from said main storage space through an outlet conduit.