Method Of And Apparatus For Transporting Cryogenic Liquids

Abstract

A method of and apparatus for reducing the rate at which the heat content of a cryogenic liquid such as liquefied oxygen or nitrogen increases as a consequence of its being necessarily shipped in a partially filled container. The method includes dividing a predetermined volume of the liquid which ordinarily would be loaded into a shipping container therefor into major and minor fractions, the first of which is significantly larger than the second. The major fraction is confined within a container compartment having substantially the same volume as that of the major fraction, and the minor fraction is confined within a container compartment having a substantially larger volume than that of the minor fraction so as to accommodate any enlargement in the volume of the major fraction as a consequence of increases in the heat content thereof. Any such increases in the volume of the major fraction are withdrawn from the container compartment confining the same and are delivered to the container compartment confining the minor fraction. The apparatus includes a tank car having a large container provided with inner and outer wall structures separated from each other to define a heat-insulated space therebetween. The container is subdivided by a bulkhead into major and minor compartments, and means are provided for filling the container with a cryogenic liquid and for withdrawing such liquid therefrom. The minor and major compartments are flow interconnected by valve-equipped conduits that enable any overflow of liquid from the major compartment resulting from temperature-induced volumetric increases in the liquid confined therein to pass into the minor compartment.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my copending application, Ser. No. 756,554 filed Aug. 30, 1968, now abandoned for "Method of an Apparatus for Transporting Cryogenic Liquids."

Description

DISCLOSURE

This invention relates to a method of and apparatus for reducing the rate at which the heat content of a liquid increases as a consequence of its being shipped in a partially filled container, and it relates more particularly to a method of and apparatus for transporting relatively large quantities of a cryogenic liquid, as for example, a railway tank car of liquid oxygen or nitrogen.

Whenever a liquid product confined within a partially filled container is shipped, whether by highway, rail, sea or air, the substantially unending disturbances imparted to the liquid as a consequence of its inertia and of container vibration, changes in the velocity thereof and in the direction of its movement, etc., imparts kinetic energy to the liquid causing it to move about or slosh within the container. The kinetic energy represented by such motion of the liquid is at least partially dissipated by conversion into heat which has the consequence, often undesirable, of elevating the temperature of the liquid product.

The amount that a liquid sloshes within a container, and the resultant conversion of kinetic energy to heat, is to a considerable extent a function of the geometry of the container; and, in this reference, conversion of the kinetic energy to heat varies with the square of the length of the uninterrupted wave motion (i.e., the distance between abutments in the direction of wave motion or travel). In view of this, a common means for reducing the kinetic energy and hence, heat imparted to the liquid, is to install multiple-baffle structure in the container so as to reduce the free distance the waves can travel in the direction of the greatest expected changes in velocity of the container (i.e., the direction of greatest expected positive and negative accelerations). However, the hydraulic forces that develop during the shipment of relatively dense liquids under the accelerations expected in rail transport is very large, and by way of example, a typical specification for a dense liquid such as liquid oxygen requires the structural design of the container and any baffle structure to accommodate an acceleration of 7 g's. Thus, installation of multiple baffles within a container for dense liquids is quite expensive since such structures must be quite substantial.

Relatively large volumes of cryogenic liquids are necessarily transported in partially filled containers because such liquids must be maintained at very low temperatures as, for example, temperatures of the order of -200.degree. F.; and as a consequence, a great temperature difference exists between such liquids and the ambient environments. Therefore, it is to be expected that there will be some increase in the heat content of such liquids when they are shipped by rail or other transport over relatively long distances, and this increase will be accompanied by an increase in the volume of the liquid. As a result, it is customary to maintain substantial ullage space within a container in which cryogenic liquids are shipped; and by way of example, in containers having a capacity in excess of 20,000 gallons, it is customary to fill the same only to about 90 percent to 95 percent of capacity.

Accordingly, the problem of reducing the rate at which the heat content of a cryogenic liquid increases as a consequence of its being shipped in a partially filled container is one to which considerable attention has been directed for several reasons. Increases in the heat content result in product loss because of the operation of pressure relief valves and rupture discs in transit, which operation permits the escape of the product to protect the container against excessive pressures, and because of flashoff of temperature-induced product vapors that accumulate in transit when the product is unloaded at its destination. So far as is known, such efforts to reduce the rate of increase in the heat content in a cryogenic liquid during shipment thereof has resulted in the inclusion of multiple-baffle structures to reduce the distances of wave motion, as explained hereinbefore.

An object, among others, of the present invention is to provide an improved method of and apparatus for transporting large quantities of liquid products, cryogenic products for example such as liquid oxygen, nitrogen, etc., and which method and apparatus minimize the development of kinetic energy within the liquid and conversion of such energy into heat; and which improved method and apparatus accomplishes such minimization without the requirement for multiple-baffle or other structures of high mass, complexity and strength.

Such object is generally accomplished by confining at least a major fraction of the liquid which ordinarily would be loaded into a shipping container therefore, within a first compartment of the shipping container having a volumetric capacity substantially equal to that of the major fraction. While ideally, all of the liquid is confined within the first compartment, a minor fraction of it may be confined within a second compartment of the shipping container, but the second compartment therefor has a greater volumetric capacity than is required by such minor fraction, sufficiently greater so as to accommodate any increases in the volume of the major fraction as a consequence of increases in the heat content thereof. The two compartments are interconnected so that any increases in the volume of the major fraction are withdrawn from the compartment therefor and are delivered into the space afforded in the compartment confining the minor fraction. The increase of kinetic energy of the liquid as a consequence of motion imparted thereto is significantly minimized because there is substantially no ullage space within the compartment confining the major fraction so that motion thereof is substantially inhibited, and the compartment containing the minor fraction is constructed so that the free distance for wave motion is quite small, thereby materially restricting the wave motion and heat increase developed therefrom in the minor fraction.

An exemplary structural embodiment of the invention is illustrated in the accompanying drawing, in which:

FIG. 1 is a schematic diagram of a container and the various flow connections thereto embodying the present invention;

FIG. 2 is a broken side view in elevation of the inner wall of a container embodying the invention, the outer wall thereof being broken away and shown in section; and

FIG. 3 is a transverse sectional view taken along the line 3--3 of FIG. 2.

As respects the present invention, the constructional features and characteristics of the relatively large containers in which cryogenic liquids are stored and transported may be conventional; and as a consequence, such constructional details are neither illustrated nor described since containers of this general type are well known in the art. Accordingly, for purposes hereof the container illustrated may be taken to be a railway tank car used for transporting cryogenic liquids and it is essentially conventional except to the extent that such tank car is specifically modified as explained herein. The container shown is denoted in its entirety with the numeral 10, and it comprises an inner wall structure 11 defining a liquid-receiving compartment 12 therewithin and an outer shell or wall structure 13 surrounding the wall structure 11 to enclose the same. The wall structures 11 and 13 are separated and define a chamber or space 14 therebetween which is provided with a vacuum and thermal insulation so as to retard and minimize the rate of heat migration into the liquid contents confined within the compartment 12.

As explained hereinbefore, cryogenic products likely to be shipped within the container 10 are products such as liquified oxygen and nitrogen and, evidently, these products must be maintained at a very low temperature, for example, of the order of -200.degree. F. As a consequence of the temperature differential established between the low temperature liquid within the compartment 12 and the ambient air temperatures exteriorly of the outer shell or wall structure 13, there is always a slow inward migration of heat through the wall structures and insulated space 14 to the cryogenic product within the compartment 12. It will be apparent that the temperature increases of the cryogenic liquid resulting from such heat migration thereto will cause the liquid to expand and, therefore, provision for such expansion must be made, usually by not completely filling the compartment 12 (for example, only filling the same to about 95.degree. of its volumetric capacity).

The relatively large ullage space thereby provided within the compartment 12 has the disadvantages of affording considerable room for the liquid to slosh or move about within the compartment as the container is transported; and since the hydrokinetic energy represented by such moving liquid must be dissipated, a considerable component of it appears as unwanted heat, thereby accelerating the temperature rise of the liquid. As explained heretofore, previous practice has resulted in the construction of multiple baffles within the compartment in an effort to reduce the motion of the liquid therewithin and, therefore, the hydrokinetic energy which is caused and which is dissipated, at least in substantial part, as heat.

In the container 10 shown in the drawing, the compartment 12 is subdivided into major and minor compartments or compartment sections 15 and 16 as by means of a bulkhead 17 mounted within the compartment 12 intermediate the ends thereof. The major compartment or compartment section 15 is significantly larger in a volumetric sense than the minor compartment 16, and in a typical installation the order-of-magnitude ratio is in the range of about 9to 1. As will become more apparent hereinafter, the precise ratio may vary substantially, but ordinarily the greatest advantage is realized when the minor compartment is made as small as possible without being so small that it cannot accommodate the ordinarily anticipated expansion-caused overflow of liquid from the major compartment.

Any suitable materials may be made to construct the container 10, and in a usual instance the inner wall structure 1 will be stainless steel whereupon the bulkhead 17 is advantageously formed of stainless steel welded or otherwise secured to the wall structure 11 so as to flow-isolate the major and minor compartments 15 and 16. The bulkhead 17 may be initially provided with a centrally located access opening so that entrance to the compartment 16 can be gained through the interior of the compartment 15 as necessary while the container is being constructed. This opening is sealed in the late stages of the construction by a cover 18 which is secured to the bulkhead 17 such as by welding.

The various connections to and interconnections between the compartments 15 and 16 are shown in FIG. 1, and the numerous valves, gauges and conduits, and the manner of connection of the conduits with the respectively associated compartments may be largely conventional. Thus, the major compartment 15 is provided adjacent each end thereof with valve-equipped conduits 19 and 20 that are used selectively to fill the compartment 15 with a cryogenic liquid and to withdraw such liquid therefrom. Usually, the conduits 19 and 20 are arranged with the wall structure 11 and compartment 15 so as to enter the same adjacent the bottom thereof as shown. The provision of the two valve-equipped conduits 19 and 20 serves as a convenience so that the compartment can be loaded and unloaded from either of its ends. The compartment 15 is also provided with a valve-equipped vent conduit 21 having a safety valve 22 located therealong which, by way of example, may be a burst disc designed to relieve the pressure within the compartment 15 should it exceed a value of about 45 p.s.i.g.

The compartment 15 has a conduit 23 communicating therewith which is equipped with a manually manipulatable valve, and the conduit is so arranged with respect to the compartment that liquid can be withdrawn through the conduit when the compartment contains a predetermined volume of liquid. For example, assuming a typical situation in which the compartment 15 comprises approximately 90 percent of the total capacity of the composite compartment 12, wherefore the compartment 16 comprises about 10 percent of such total capacity, the conduit 23 may be arranged to enable liquid to flow therethrough when the compartment 15 is substantially full. Therefore, the conduit 23 and the valve therefor might be referred to as a "90 percent full trycock." A pair of conduits 24 and 25 respectively connected to the bottom and top of the compartment 15 define liquid and vapor lines, respectively, and each is equipped with a valve and terminates in the respectively associated liquid level and pressure gauges 26 and 27. The conduits 24 and 25 are interconnected intermediate the valves and gauges therealong by an equalizing valve 28, and a bleed valve 29 associated therewith can be used to withdraw quantities of liquid from the line 24. The gauges 26 and 27 may be coupled, as shown in FIG. 1.

The compartment 16 is provided with analogous connections thereto, and in this respect a valve-equipped conduit 30 is used to supply liquid to the compartment, and to withdraw liquid therefrom. The compartment 16 has a conduit 31 communicating therewith which is equipped with a manually manipulatable valve, and the conduit is so arranged with the compartment that liquid can be withdrawn through the conduit when the compartment contains a predetermined volume of liquid. For example, assuming the exemplary ratio of about 9 to 1 heretofore stated, the conduit 31 might be arranged to enable liquid to be withdrawn when the composite compartment 12 is filled to about 95 percent of its total capacity, whereupon the compartment 16 would be about 50 percent filled. It may be observed that customarily the containers in which cryogenic liquids are shipped are filled to about 95 percent of capacity to provide sufficient excess volume or ullage space to accommodate thermal expansion of the liquid. Accordingly, the conduit 31 and the valve therefor might be referred to as a "95 percent full trycock."

A pair of conduits 32 and 33 respectively connected to the bottom and top of the compartment 16 define liquid and vapor lines, respectively, and each is equipped with a valve and terminates in the respectively associated liquid level and pressure gauges 34 and 35. The conduits 32 and 33 are interconnected intermediate the valves and gauges therealong by an equalizing valve 36, and a bleed valve 37 associated therewith can be used to withdraw quantities of liquid from the line 32. The gauges 34 and 35 may be coupled, as shown in FIG. 1.

The compartment 15 is connected with the compartment 16 by an overflow conduit 38 running generally from the top of the compartment 15 to a conduit 50 which is connected to the top of compartment 16. Since conduit 50 is connected to the top of compartment 16, reverse flow of liquid from the minor compartment to the major compartment is prevented. This can be important, especially if the major compartment should happen to be vented to the atmosphere. For example, if the burst disc or safety valve 22 should fail prematurely, liquid within the minor compartment cannot flow into the major compartment through the overflow conduit, thus minimizing the amount of liquid lost through the safety valve.

Disposed along the conduit 38 is a pressure equalizing check valve 39, which is in the nature of a one way relief valve in that it provides substantially no inhibition of expansion-induced flow of fluid from the compartment 15 into the compartment 16 but prevents reverse flow therepast, usually of gaseous fluid, should conditions within the compartment 16 tend to induce such flow into the conduit 38 via the conduit 50. A differential pressure regulator 40 and a regulator bypass valve 41 connect the overflow conduit 38 with the supply conduit 30 to pressure relate the upper and lower portions of the compartment 16. The compartments 15 and 16 are further interconnected by a flow network that includes a conduit 42 connected via a pressurizer liquid valve 43 and conduit 44 to the bottom of the compartment 15. A pressure relief valve 45 associated with the conduit 42 adjacent the valve 43 is included as a safety device and, for example, may be selected to relieve pressures in excess of 100 p.s.i.g.

The conduit 42 is also connnected as shown at 46 to one end of a pressure building coil 47 which at its other end is connected at 48 through a pressurizer vapor valve 49 to conduit 50 and thereby serving as an ullage space vent therefor. The conduit 50 has a manually manipulatable vent valve 51 therein, and a pressure relief valve 52 is also provided along the line 50 and may be adjusted to relieve pressures in excess of about 20 p.s.i.g.

As shown in FIGS. 2 and 3, for the most part, the various flow conduits are disposed within the space 14 defined between the inner and outer wall structures 11 and 13 except where such conduits enter the composite compartment 12 or extend through the outer wall structure 13. The conduits 19 and 20 through which the compartment 15 is filled and evacuated, selectively, may have expansion loops formed therealong, as shown in FIG. 2; and the conduit 31 through which the quantity of liquid present within the compartment 16 is determined may terminate adjacent the center thereof (as shown in FIGS. 2 and 3) in accordance with the foregoing example in which such compartment is to be filled to about 50 percent of its capacity. The lines 38 and 42 are interconnected by an ullage space pressurizer liquid valve 53.

A typical container 10 of the type being considered might have a capacity of about 24,000 gallons; and to fill the same with a liquid cryogenic product, a supply line is first connected to either of the conduits 19 or 20 and all of the valves in the system are closed except for the regulator bypass valve 41 and ullage space vent valve 51. These conditions realized, the control valve in the conduit 19 (or 20 as the case might be) is opened to initiate the flow of liquid into the compartment 15, and also the liquid valve along the conduit 24 and vapor valve along the conduit 25 are both opened. When the liquid level gauge 26 shows that the volume of liquid within the compartment 15 is beginning to fill the same, the trycock valve in the conduit 23 is opened to permit some escape of fluid or vapor therethrough to allow the conduit to cool. Prior to the compartment 15 being completely filled, the ullage space vent valve 51 is throttled so as to permit a slight pressure to build up within the compartment 16. When operation of the trycock valve in the conduit 31 results in the flow of liquid therethrough, the container is filled to the desired capacity, whereupon the filling operation is terminated, and all of the valves in the system are closed. The tank is then ready for shipment.

When it is desired to withdraw liquid from such typical container 10, a check is first made to determine that all of the valves in the system are closed and a withdrawal line is then coupled to either of the lines 19 or 20. The pressurizer liquid valve 43 and pressurizer vapor valve 49 are then opened, as is the regulator bypass valve 41. When the pressure within the compartment 15 approximates 15 p.s.i.g., as indicated by the pressure gauge 27, the valve within the line 19 (or 20 depending upon which conduit is used) is opened to enable the liquid to be withdrawn and the pressure is regulated by suitable adjustment of either the pressurizer liquid valve 43 or pressurizer vapor valve 49 to maintain the pressure at about 15 p.s.i.g. When withdrawal is complete, the pressurizer liquid valve 43 and valve in the line 19 are closed; and upon the rise in pressure terminating, the pressurizer vapor valve 49 is closed. The withdrawal line is then disconnected and all of the valves are closed, whereupon the car is ready for shipment or other use.

Once the container 10 has been filled as desired, expansion of the liquid within the compartment 15 results in a flow of liquid through the conduit 38 into the compartment 16 which has sufficient excess volume or ullage space therein to accommodate the overage of liquid resulting from thermal expansion thereof in the compartment 15. Evidently, the compartment 15 is maintained in a substantially full condition at all times, wherefore there is no significant motion-accommodating ullage space and hydrokinetic energy otherwise invested within the liquid as a consequence of sloshing thereof within the compartment 15 is significantly minimized. Correspondingly, then, there is a material reduction in the rate at which the heat content of such liquid within the compartment 15 increases.

The liquid within the compartment 16 is permitted to slosh since this compartment is filled initially only to about 50 percent of its capacity. Although sloshing of the liquid within the compartment is tolerated, the rate at which the heat content of this liquid fraction increases is relatively low because the length of the uninterrupted wave motion is quite minimal (i.e., the distance between the bulkhead 17 and facing end of the wall structure 11 which together define the compartment 16). Accordingly, the advantages of providing baffle structure within the compartment 16 is realized without the requirement for such baffle structure. Further, the amount of liquid within the compartment 16 is small relative to the total quantity of liquid being shipped within the container 10, and which total quantity has a major fraction thereof within the compartment 15 which is not associated with a large motion-accommodating ullage space.

Evidently then, the compartments 15 and 16 respectively represent major and minor compartments, and the quantities of liquid therein respectively represent major and minor fractions of the total volume being shipped within the container. The major fraction of liquid is confined within the compartment 15 which has substantially the same volume as that of the major fraction; and the minor fraction of liquid is confined within the minor compartment 16 which has a significantly greater volume than that of the minor fraction. As the major fraction increases volumetrically as a consequence of temperature increases therein, the excessive volume resulting from such expansion is withdrawn from the major compartment 15 and is delivered to the minor compartment 16.

In certain instances, a slight modification or variation in the described arrangement might be provided especially where it is desired to reduce the effects of large-valued impact forces delivered to the container by the liquid therewithin, which impact forces often result from the type of rapid accelerations and decelerations of railway cars caused by coupling cars travelling at appreciably different velocities. In such instances, the overflow line 38 can be arranged with the compartment 15 so that the line terminates a spaced distance downwardly from the top of the inner wall structure 11, thereby leaving a small vapor or ullage space in the compartment 15. Such space should be a very small percentage of the total ullage space which otherwise would be provided substantially entirely within the compartment 16. By way of example, a space only 1 percent as large as the ullage space within the compartment 16 has been found satisfactory.

While in the foregoing specification embodiments of the invention both as to the method and apparatus have been set forth in considerable detail for purposes of making a complete disclosure thereof, it will be apparent to those skilled in the art that numerous changes may be made in such details without departing from the spirit and principles of the invention.

Claims

I claim:

1. A container structure for transporting thermoexpansible liquids including cryogenic liquids and the like, comprising an inner wall structure defining a container chamber therewithin, an outer wall structure spaced from said inner wall structure and defining an insulating space therebetween, an intermediate bulkhead within said chamber subdividing the same into major and minor compartments flow-isolated one from the other, means associated with said major compartment for filling the same with such liquid and for removing the same therefrom, means associated with said minor compartment for introducing liquid thereinto and for removing the same therefrom, and flow conduit means interconnecting said compartments to enable overage from said major compartment to flow into said minor compartment so as to accommodate thermal expansion of such liquid confined within said major compartment, said flow conduit means interconnecting said compartments including valve devices therealong comprising a pressure equalizing check valve permitting flow of liquid from the major to the minor compartment.

2. A container structure for transporting thermoexpansible liquids including cryogenic liquids and the like and being elongated axially in the direction in which it travels during transport thereof, comprising an inner wall structure defining an axially elongated container chamber therewithin, an outer wall structure spaced from said inner wall structure and defining an insulating space therebetween, an intermediate bulkhead within said chamber in relatively close proximity to one end thereof subdividing the chamber into a long major compartment adapted to be filled substantially to capacity with such liquid to restrict the permissible freedom of movement thereof relative to said wall structure and a minor compartment flow isolated from said major compartment and adapted to be partially filled with such liquid and being short in the axial direction relative to said major compartment so as to restrict the axial extent of the uninterrupted wave motion of any liquid partially filling the same, means associated with said major compartment for filling the same with such liquid and for removing the same therefrom, means associated with said minor compartment for introducing liquid thereinto and for removing the same therefrom, flow conduit means interconnecting said compartments to enable overage from said major compartment to flow automatically into said minor compartment so as to accommodate thermal expansion of such liquid confined within said major compartment, and flow-inhibiting means for prohibiting reverse flow of liquid through said flow conduit means from said minor to said major compartment.

3. The container structure of claim 2 and further comprising pressure relief means connected with said major compartment for limiting the maximum permissible pressure therewithin, the aforesaid prohibition of reverse flow through said flow conduit means being effective during and subsequent to operation of said pressure relief means.

4. The container structure of claim 2 in which said flow conduit means connects with said minor compartment adjacent the upper end thereof so as to be above the elevation of liquid partially filling the same to define at least a part of said flow-inhibiting means and effect the aforesaid prohibition of reverse flow from said minor to said major compartment.

5. The container structure of claim 2 in which said flow conduit means is connected with said major compartment adjacent the upper extremity thereof so that said major compartment is continuously maintained in a substantially completely filled condition when filled with any such liquid.

6. The container structure of claim 5 in which said flow conduit means connects with said minor compartment adjacent the upper end thereof so as to be above the elevation of liquid partially filling the same to define at least a part of said flow-inhibiting means and effect the aforesaid prohibition of reverse flow from said minor to said major compartment.

7. The container structure of claim 6 and further comprising pressure relief means connected with said major compartment for limiting the maximum permissible pressure therewithin, the aforesaid prohibition of reverse flow through said flow conduit means being effective during and subsequent to operation of said pressure relief means.

8. The container structure of claim 7 in which a pressure equalizing check valve permitting flow of liquid from the major to the minor compartment is included in said flow conduit means to define a further part of said flow-inhibiting means.

9. In a method of minimizing the development of kinetic energy and conversion thereof into heat during transport of a thermoexpansible liquid such as a cryogenic liquid or the like, the steps of confining a predetermined large volume of such liquid within an axially elongated major chamber having substantially the same volume as that of the liquid therein so as to restrict the permissible freedom of movement of the liquid relative thereto, confining a substantially smaller volume of such liquid within a minor chamber to partially fill the same and which minor chamber has a much smaller capacity than that of the major chamber but at least as great as the normally expected volumetric increase of the liquid within said major chamber due to any increase in the heat content thereof, restricting the axial extent of the uninterrupted wave motion of any liquid within the minor chamber by minimizing the axial length thereof, withdrawing from the major chamber any volumetric increases in the liquid therein that tend to exceed the capacity of the major chamber and delivering such withdrawn quantities into the minor chamber, and confining liquid within the minor chamber against flow thereof into the major chamber.

10. The method of claim 9 in which any such volumetric increases in the liquid within the major chamber are continuously withdrawn therefrom and delivered into the minor chamber.

11. The method of claim 10 in which the volume of liquid within the major chamber is initially about nine times greater than the volume within the minor chamber.

12. The method of claim 11 in which said minor chamber has a capacity about twice the volume of the liquid initially confined therein.