INSULATION AROUND A CRYOGENIC VESSEL

Field of the Invention

[0001] The present application relates to wrapping cryogenic thermal insulation around a vessel of a cryogenic tank.

Background of the Invention

[0002] Cryogenic tanks store cryogenic fluid and function to reduce or minimize the transfer of thermal energy from the external environment into the internal environment of the tank. This allows cryogenic fluid to be maintained at a temperature substantially lower than the ambient tank temperature without having to actively cool the fluid. Thermal energy is transferred into the tank by convection, conduction, and thermal radiation. Convection is the movement of molecules within fluids and occurs when molecules transfer heat from the outer tank environment to the surface of the tank. Conduction is a mode of energy transfer between bodies of matter due to a temperature gradient. When constituent particles in the tank collide and exchange random kinetic energy conduction occurs such that heat flows from the outer surface of the tank to the inner surface of the tank. Thermal radiation is electromagnetic energy emitted by all matter due to thermal motion of charged particles that does not require a physical medium in which to travel. When thermal radiation enters the cryogenic tank it interacts with molecules of the cryogenic fluid, increasing their random motion thereby increasing the temperature of the fluid. Various techniques have been developed to reduce or minimize convective, conductive and radiated heat transfer into cryogenic tanks.

[0003] Cryogenic tanks are known to comprise an inner vessel located within an outer vessel and a vacuum that is established therebetween. The vacuum insulates the inner vessel reducing heat leak in the form of convective and conductive heat transfer. Since a perfect vacuum is difficult to achieve, to further reduce heat leak it is known to wrap the inner vessel with cryogenic insulation, which reduces convective, conductive and radiated heat transfer. [0004] Cryogenic insulation is comprised of an insulating tape, such as an industrial paper or fiberglass, superimposed with a metal foil, such as aluminum foil. The insulating tape reduces heat leak due to temperature differences between the inner and outer vessel (convective and conductive heat transfer). The metal foil has a low emissivity coefficient and serves to reduce heat leak due to thermal radiation. Additionally, the metal foil can have a high reflectivity, such as aluminum foil, which further reduces heat leak by reflecting radiation away from the tank. In practice the insulating tape and metal foil can be collated onto one roll and dispensed together while wrapping the inner vessel, or they can be wound on separate rolls and dispensed separately from each other but simultaneously such that the foil superimposes the insulating tape while wrapping the inner vessel. Since metal is an excellent conductor of heat, to reduce low resistance heat paths to the inner vessel the insulating tape contacts the inner vessel but the metal foil does not. To further reduce low resistance heat paths the width of the metal foil is less than that of the insulating tape such that the metal foil of one wrap around the inner vessel does not come into contact with the metal foil of previous or successive wraps. When the metal foil of successive wraps accidentally comes into contact, it creates a thermal short circuit that allows heat to efficiently transfer past a layer of insulation towards the cryogenic fluid. [0005] In some cryogenic applications it is known to suspend the inner vessel within the outer vessel by fixing ends of the inner vessel with respective ends of the outer vessel with supports. The supports are aligned with a common longitudinal axis of both the inner and outer vessels. This is beneficial in mobile applications where the inner vessel needs to be securely arranged with respect to the vehicle for safety and operational reasons. The support at one end of the inner vessel comprises a port through which a conduit passes for introducing cryogenic fluid into the inner vessel, and also through which cabling is routed for instrumentation equipment such as level sensors. This port is left uncovered during the wrapping of the inner vessel such that an opening exists for routing the conduit and cabling internally. In a preferred embodiment a manifold coupling is connected with the port after wrapping. An external conduit for cryogenic fluid and a cabling conduit for instrumentation connects with the manifold coupling outside the tank, and a main passageway to the inner vessel is provided from the manifold coupling inside the tank. One method of wrapping the inner vessel to leave the port opening uncovered involves suspending the vessel vertically while it rotates about its longitudinal axis and successfully wrapping the vessel in a generally longitudinal orientation. A pivot arm having at least one roll of cryogenic insulation at a distal end of a central axis about which the arm is rotated in a plane slightly offset from the longitudinal axis of the tank is employed to dispense the cryogenic insulation. With this technique port openings at either end of the cryogenic tank can remain uncovered.

[0006] In practice a pump extracts fluid from the inner vessel and delivers it to a downstream consumer. The pump can be located externally of the cryogenic tank and connected to it by insulated piping. One drawback to this approach is heat leak through the insulated piping that tends to accelerate boil-off of the cryogenic fluid within the tank leading to premature venting and waste. An alternative technique locates the pump in an additional off- axis port that is adjacent to the manifold port such that an inlet to the pump is disposed in the inner vessel and an outlet from the pump is located external to the cryogenic tank, as disclosed in the Applicant's own Canadian Patent 2,362,844. This arrangement increases the thermal isolation of the cryogenic fluid within the tank from the external environment, reducing heat leak and slowing down boil-off In a preferred embodiment a pump assembly comprising a pump and a downstream vaporizer is located in the off-axis port such that an outlet of the pump assembly delivers fluid from the cryogenic tank in a gaseous phase to the downstream consumer. [0007] The off-axis port in the tank complicates the wrapping process. Employing the wrapping technique described above, this port tends to get covered by insulation, which then needs to be trimmed such that the pump assembly can be inserted. Trimming is a manual, time consuming and comparatively expensive operation. It can also degrade the thermal isolation of the inner vessel by creating thermal short circuits when aluminum foil in adjacent layers accidentally contact each other. [0008] The present method and apparatus provide an improved technique for wrapping cryogenic insulation around a vessel comprising on-axis and off-axis ports.

Summary of the Invention

[0009] An improved method of wrapping insulation around a vessel comprises rotating the vessel about an axis; wrapping insulation around the vessel in a plane offset from the axis as the vessel rotates; and translating one end of the vessel relative to the plane as the vessel rotates, the relative translation between the one end and the plane is along a path of translation; a portion of the vessel at the one end remains uncovered and a shape of the portion is influenced by the path of translation. The axis is a longitudinal axis in a preferred embodiment, but is not required to be in all embodiments. The insulation can comprise a first layer superimposed on a second layer. When the axis is a longitudinal axis, the insulation is wrapped longitudinally around the vessel. In a preferred embodiment successive wraps of the insulation overlap respective previous wraps and the insulation forms a multi-layer covering. The path of translation is one of a linear path and a non-linear path. The one end translates for at least a part of one complete rotation of the vessel. The path of translation is one of a closed path and an open path. In a preferred embodiment the shape is oblong; however in other embodiments various arbitrary shapes are possible according to application requirements. The shape is further influenced by when the one end translates relative to the rotation of the vessel. In another preferred embodiment the method can comprise the further steps of translating an end opposite the one end of the vessel on a second path of translation as the vessel rotates such that a second portion of the vessel remains uncovered. The second portion is located at an end opposite the one end. In yet another preferred embodiment the method further comprises wrapping tape around the axis at a perimeter of the vessel as the vessel rotates at at least one location along the axis such that the tape crosses the insulation. In another embodiment, instead of translating the one end the method comprises a step of changing an orientation of the plane relative to the one end. A preferred embodiment of the method further comprises tracking a template that rotates with the vessel and is representative of the shape whereby the one end translates along the path of translation due to the tracking. A perimeter of the template can be tracked.

[0010] An improved apparatus for wrapping insulation around a vessel comprising a first end opposite a second end along an axis thereof comprises a rotation mechanism for rotating the vessel about the axis, the rotation mechanism is rotatably coupled to at least one of the first end and the second end; an insulation wrapping mechanism for wrapping the insulation around the vessel in a plane offset from the axis as the vessel rotates; and a translation mechanism for translating the second end relative to the plane, the translation between the second end and the plane is along a path of translation; a portion of the vessel at the second end remains uncovered and a shape of the portion is influenced by the path of translation of the second end. The rotation mechanism and the translation mechanism cooperate to support the vessel. The vessel is pivotably supported at the first end and rotatably supported by the rotation mechanism at the second end. The translation mechanism can comprise at least one of a linear actuator, a CNC machine and a CNC program that controls the path of translation, a mechanical mechanism for effecting translation such as a cam follower. There is a pivotable connection at the first end for pivotably and rotatably supporting the first end while the second end translates. In a preferred embodiment the translation mechanism comprises a controller, a translational device, a template and a limit switch; the template is representative of the shape and rotates with the vessel; the limit switch is responsive to the template to produce at least one limit signal representative of a position of the template; the controller is responsive to the at least one limit signal to command the translational device to translate the second end. The controller can be an electronic controller. The limit switch can be a proximity sensor or a two-position limit switch, and can be responsive to a perimeter of the template. The rotation mechanism comprises a mechanical linkage rotatably supporting the second end. The mechanical linkage can comprise a universal joint and a shaft having a shaft axis, such that due to the universal joint an orientation of the shaft axis remains fixed as the second end translates. In a preferred embodiment the apparatus further comprises an auxiliary role wrapping mechanism for wrapping tape around the axis at a perimeter of the vessel at at least one location along the axis. The tape is interleaved with the cryogenic insulation around the vessel. The tape can be cryogenic insulation. Brief Description of the Drawings

[0011] FIG. 1 is an elevation view of an inner vessel in a cryogenic insulation wrapping apparatus according to one embodiment. [0012] FIGS. 2a through 2g are top views of the inner vessel of FIG. 1 shown at progressive rotational positions respectively in a conventional cryogenic wrapping apparatus.

[0013] FIGS. 3a through 3m are top views of the inner vessel of FIG. 1 shown at progressive rotational positions respectively in the wrapping apparatus of FIG. 1.

[0014] FIG. 4a is a top view of the inner vessel of FIG. 1 shown wrapped with cryogenic insulation after 180° of rotation of the inner vessel in the wrapping apparatus of FIG. 1.

[0015] FIG. 4b is a top view of the inner vessel of FIG. 1 shown wrapped with cryogenic insulation after 360° of rotation of the inner vessel in the wrapping apparatus of FIG. 1.

[0016] FIG. 4c is a top view of the inner vessel of FIG. 4b shown with an opening formed by the cryogenic insulation.

[0017] FIGS. 5a through 5g are top views of the inner vessel, a template and a two position limit switch of FIG. 1 shown at progressive rotational positions respectively in the wrapping apparatus of FIG. 1.

[0018] FIG. 6 is a side elevation schematic view of one embodiment of a limit switch according to the embodiment of FIG.1. [0019] FIG. 7 is a flow chart diagram for controlling a translational device in response to a limit signal from a limit switch according to the embodiment of FIG. 1.

Detailed Description of Preferred Embodiments)

[0020] Referring to FIG. 1, apparatus 10 wraps cryogenic insulation 20 around inner vessel 30 of a cryogenic tank. Inner vessel 30 comprises port 40 at end 50 and collar 60 at end 70, both centered on longitudinal axis 80. In other embodiments port 40 and collar 60 can be situated along an axis which is not a longitudinal axis. In the present example ends 50 and 70 are convex in shape. After inner vessel 30 is wrapped with cryogenic insulation 20, port 40 and collar 60 serve as the mounting points for suspending inner vessel 30 within an outer vessel (not shown). In use cryogenic fluids and instrumentation cabling are typically routed through port 40. Port 90 at end 50 is centered on axis 100 that intersects axis 80 and is generally referred to as an off-axis port. Off-axis port 90 can be used to install additional apparatus such as a pump assembly (not shown) for extracting cryogenic fluid from vessel 30 for a downstream consumer. [0021] Apparatus 10 comprises rotational device 110, which can be an electric motor, that is supported by translational device 120. Rotational device 110 is rotatably coupled with vessel 30 through mechanical linkage 130 and serves to rotate vessel 30 about longitudinal axis 80. Vessel 30 is supported at end 50 by mechanical linkage 130, and at end 70 by a pivotable connection between collar 60 and support post 140. Translational device 120 serves to move rotational device 110 along track 410 such that end 50 moves back and forth when required as vessel 30 rotates and insulation is applied, which will be described in more detail below. Translational device 120 can comprise a linear actuator, or other types of apparatuses that provide translation. In other embodiments translational device 120 can comprise any mechanical mechanism for effecting translation, such as a cam follower. Mechanical linkage 130 comprises universal joint 420 that allows shaft 430 to maintain a fixed axis relative to rotational device 110 as translational device 120 moves along track 410. Rotational device 110 and mechanical linkage 130 cooperate together as a rotation mechanism. Support post 140 comprises end 200 that allows collar 60 to pivot as end 50 translates back and forth, and end 210 that is rigidly connected with frame 220. Pivot arm 150 has roles 160 of cryogenic insulation at distal ends 170 and 180 that rotate about central axis 190 in a plane 500 angularly offset from longitudinal axis 80 such that cryogenic insulation is wrapped in a longitudinal orientation around vessel 30. As vessel 30 rotates cryogenic insulation that is being applied to the vessel overlaps between 30% and 60% of the previous wrap. Pivot arm 150 is rotatably coupled to rotational device 230 in housing 240 and is made to rotate by device 230. Controller 250 is operatively connected with rotational devices 110 and 230 and translational device 120 and commands these devices to rotate and translate respectively. In the present embodiment controller 250 comprises an electronic controller. In a preferred embodiment the electronic controller is a microcontroller configured with a program to control apparatus 10. In other embodiments the electronic controller can be an analog controller. Controller 250 is shown in housing 240 in the present embodiment. In other embodiments controller 250 can be located elsewhere. Rotational device 230 is the lead drive and rotational device 110 is the follower drive. The lead drive precisely determines the speed of the follower drive and in this manner the overlap of insulation 20 is always exactly the same at any speed from start to finish. [0022] Referring now to FIGS. 2a through 2g the problems associated with conventional wrapping apparatuses is discussed. When vessel 30 rotates counter-clockwise about axis 80 without translational movement of end 50 relative to plane 500 insulation 20 covers port 90. As can be seen in the progression from FIGS. 2a through to 2g, where for each of the figures only the currently applied wrap of insulation 20 is shown, port 90 rotates in a counter-clockwise direction and becomes substantially covered with insulation. This insulation needs to be removed such that the pump assembly can be installed, which is a labor intensive process that results in reduced thermal performance due to accidental thermal short circuits introduced when trimming.

[0023] With reference to FIGS. 3a through to 3g, the covering of port 90 can be prevented if end 50 of vessel 30 can be made to move back and forth as port 90 rotates through the fourth and first quadrants IV and I respectively. In these figures only the currently applied wrap of insulation is shown for clarity. In FIG. 3a vessel 30 has rotated such that port 90 is at the 270° location and as vessel 30 continues to rotate counter-clockwise end 50 begins to move to the left, as illustrated in FIGS. 3b and 3c by arrow 250. In this manner port 90 moves away from the currently applied wrap of insulation 20 at end 50 such that it does not get covered. As port 90 rotates through the fourth quadrant IV to the 0° location end 50 stops moving to the left, as illustrated in FIG. 3d, and begins to move back to the right as port 90 continues to rotate through the first quadrant I, as illustrated in FIGS. 3e and 3f by arrow 260. When port 90 rotates to the 90° location end 50 stops moving to the right as shown in FIG. 3g. For the remaining 180° in one complete revolution of vessel 30 about longitudinal axis 80, end 50 remains stationary (does not move back and forth) as vessel 30 continues to be rotated by rotational device 110. Port 90 rotates through second quadrant II and third quadrant III and returns to the 270° location, as illustrated in FIGS. 3h through to 3m. [0024] Referring now to FIG. 4a, the pattern of insulation that develops at end 50 as vessel 30 begins rotation as in FIG. 3a and port 90 rotates through the fourth and first quadrants to location 90° as shown in FIG. 3g is illustrated. FIG. 4b shows the pattern of insulation at end 50 after completing one complete revolution. Note that in the present embodiment multiple layers of cryogenic insulation 20 are applied to vessel 30 such that the vessel completes multiple revolutions about axis 80. In other embodiments vessel 30 rotates at least as much to be sufficiently covered by cryogenic insulation depending upon application requirements. As can be seen in FIG. 4c, where the wraps of insulation 20 have been removed for clarity, an oblong shaped opening 270 is formed around ports 40 and 90. A portion 510 of vessel 30 is left uncovered by opening 270. The space between port 40 and port 90 in portion 510 of vessel 30 can be manually pre- wrapped with insulation if required. [0025] Referring back to FIG. 1 and to FIGS. 5a through 5g the manner by which opening 270 is formed will now be described. Template 280, also known as a cam, is rigidly connected with mechanical linkage 130 such that it rotates with vessel 30. Template 280 is located above ports 40 and 90 such that it is aligned with opening 270, as seen in FIG. 4c. Perimeter 290 of the template is similar but not necessarily the same as desired perimeter 300 of opening 270. A two-position limit switch 310 comprises a ram 320 that contacts perimeter 290 of template 280. In the present embodiment controller 250, translation device 120, template 280 and limit switch 310 cooperate together as a translation mechanism. In other embodiments the rotation mechanism can comprise any apparatus that can rotate vessel 30 while either end 50 or 70 translates, and the translation mechanism can comprise any apparatus that can translate either end 50 or 70 while vessel 30 rotates. Controller 250 is responsive to limit signals from switch 310 to command translational device 120 such that the position of ram 320 is controlled. A schematic representation of limit switch 310 is shown in FIG. 6, and as will be understood by those familiar with the technology there are a variety of other techniques for creating two position limit switch 310. Spring 330 urges ram 320 away from housing 340 such that contact 350 meets contact 360 when ram 320 is unobstructed forming a short circuit between wires 370 and 380 whereby a first limit signal is transmitted to controller 250 on wire 380. Similarly, when ram 320 is urged towards housing 340 such that contact 350 meets contact 390 a short circuit forms between wires 370 and 400 whereby a second limit signal is transmitted to controller 250 on wire 400. In other embodiments limit switch 310 can be in the form of a proximity sensor wherein there is no contact between template 280 and the proximity sensor. When template 280 is more than a first gap length from the proximity sensor the first limit signal is sent to controller 250 by the proximity sensor. When template 280 is less than a second gap length from the proximity sensor the second limit signal is sent to controller 250 by the proximity sensor. In this manner the gap between template 280 and the proximity sensor can be maintained within a predetermined range by controller 250 commanding translational device 120 according to the present method. In still further embodiments limit switch 310 can be a rotary position sensor that detects the angular position of template 280 around axis 80 whereby controller 250 can be made aware of this position to command translational device 120 accordingly.

[0026] Referring now to FIG. 5a, when port 90 is located at the 270° location contact 350 is between contacts 360 and 390 and neither the first and second limit signals are transmitted to controller 250. As port 90 rotates through quadrant IV template 280 pushes against ram 320 such that contact 350 meets contact 390 and the second limit signal is transmitted to controller 250. Upon receiving the second limit signal controller 250 commands translational device 120 to move end 50 to the left, in the present figures, along a path away from plane 500 until contact 350 breaks away from contact 390, due to spring 330, at which point the second limit signal is no longer transmitted to controller 250. When port 90 reaches the 0° location shown in FIG. 5d template 280 has reached its furthermost extent along the positive x-axis (the rightward direction in the figures) and no longer moves ram 320 and contact 350 in that direction. The second limit signal is no longer made to be transmitted to controller 250 and translational device 120 is no longer commanded to move to the left accordingly. As port 90 continues to rotate through the first quadrant I template 280 begins to move away from ram 320 such that ram 320 moves to the left to maintain contact with the template due to spring 330. Contact 350 accordingly meets contact 360 and the first limit signal is transmitted to controller 250. Upon receiving the first limit signal controller 250 commands translational device 120 to move end 50 to the right, in the present figures, along a path towards plane 500 such that template 280 moves ram 320 until contact 350 breaks away from contact 360, at which point the first limit signal is no longer transmitted to controller 250. When port 90 reaches the 90° location illustrated in FIG. 5g template 280 stops moving away from ram 320. The first limit signal is no longer made to be transmitted to controller 250 and translational device 120 is no longer commanded to move to the right accordingly. As port 90 continues to rotate through the second and third quadrants template 280 acts on ram 320 to keep contact 350 between contacts 360 and 390 such that the first and second limit signals are not transmitted to controller 250 and translational device 120 is therefore not commanded to move along track 410 accordingly.

[0027] In the above described embodiment it is end 50 that is made to translate relative to plane 500 along a path determined by track 410. In other embodiments it is possible to change the orientation of plane 500 relative to end 50 to accomplish the same function of creating opening 270 at end 50. In general it is the relative translation of end 50 to plane 500 that allows opening 270 to be formed, and the path of translation influences the shape of the opening. Different shapes for opening 270 can be formed by translating end 50 relative to plane 500 along paths other than the linear path predetermined by track 410 and by controlling the translation relative to the rotation of vessel 30. In further embodiments end 70 can be made to translate along another path for at least a portion of one rotation of vessel 30 such that an opening with a particular shape is formed at end 70. The paths of translation employed at either end can be along an open path or a closed path. An open path is a path that does not begin and end at the same location, whereas a closed path begins and ends at the same location. The path of translation can be along a nonlinear path. In still further embodiments translation device 120 can comprise a CNC machine and a corresponding CNC program that controls the exact movement of end 50, in which case template 280 and limit switch 310 are not required. The CNC programmed movement of end 50 can be determined by trial and error or can be determined with use of template 280 and limit switch 310 by recording the position of end 50 relative to its rotation. In yet another embodiment template 280 can comprise a groove, and apparatus 10 can further comprise a cam follower. The cam follower is made to be responsive to the groove such that it can effect translation of end 50 as vessel 30 rotates.

[0028] In another embodiment limit switch 310 can be a proximity sensor that generates only one limit signal which indicates that template 280 is within a predetermined distance of the proximity sensor. Based on this one limit signal it is possible for controller 250 to command translational device 120 to move end 50 and template 280 in what is known as a hunting cycle such that opening 270 is formed as previously discussed. With reference to FIG. 7, the hunting cycle will now be described. In step 600 before the wrapping process begins template 280 is more than the predetermined distance from limit switch 310, which is the initial condition. It is understood by those familiar with the technology that other initial conditions are possible and the following method can be adapted accordingly. When apparatus 10 first begins the wrapping process controller 250 commands translational device 120 to move template 280 towards limit switch 310 in step 610. Controller 250 waits to receive the limit signal in step 620. When template 120 is within the predetermined distance limit switch 310 generates the limit signal which is received by controller 250. In response to receiving the limit signal controller 250 reverses the direction of translational device 120 in step 630 such that template 280 moves away from limit switch 310. In step 640 controller 250 waits for template 280 to move beyond the predetermined distance whereby limit switch 310 stops generating the limit signal and is therefore no long received by the controller. The process in step 610 is then repeated. The hunting cycle is repeated for the entirety of the wrapping process. In this manner template 280 is substantially maintained at the predetermined distance from limit switch 310.

[0029] Referring back to FIG. 1, auxiliary rolls 440 located at a distal end of arm 450 are arranged to wrap paper tape 460 axially around a perimeter of inner vessel 30. Two such auxiliary rolls 440 are shown in the present embodiment, and in other embodiments there can be at least one auxiliary roll. Arm 450 is rigidly connected with support post 140 near end 210, and roles 440 are held in place as vessel 30 rotates. In other embodiments auxiliary rolls 440 can be arranged at other locations around the vessel relative to pivot arm 150. The paper tape from roles 440 secures the wraps of insulation 20 to vessel 30 such that they do not slip and fall off vessel 30 at end 50. In particular, as insulation is being applied to vessel 30 as illustrated in FIGS. 5c through 5e there is a narrow shoulder between port 90 and the edge of vessel 30. In this region insulation 20 has a tendency to slide off due to the slope of end 50. By applying paper tape 460 around axis 80 at the perimeter of vessel 30 shortly after wraps of insulation 20 are applied the insulation is secured in place. In this manner interweaved layers of cryogenic insulation 20 and paper tape 460 are formed around vessel 30. It is also possible to use cryogenic insulation instead of paper tape 460 on roles 440. An additional advantage obtained from the use of auxiliary roles 440 is the ability to speed up the wrapping of insulation 20 since the in process wrapping integrity significantly improves with paper tape 460.

[0030] Apparatus 10 is adjustable such that it can accommodate different sizes of vessel 30. For example, frame 220, pivot arm 150 and support post 140 are adjustable to accommodate vessels of varying longitudinal length. Arm 450 can be adjusted to locate auxiliary rolls at varying locations along longitudinal axis 80, and to adjust the relative location of auxiliary rolls 440 with respect to each other. Central axis 190 can be adjusted relative to frame 220 to adjust the angle of plane 500 with respect to axis 80. The location of housing 240 along frame 220 can be adjusted to optimize the wrapping orientation of rolls 160.

[0031] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.