WO1993025842A1

WO1993025842A1 - Process for evacuating a thermally insulating jacket in particular the jacket of a dewar or of another cryogenic device

Info

Publication number: WO1993025842A1
Application number: PCT/IT1992/000141
Authority: WO
Inventors: Claudio Boffito; Antonio Schiabel; Alessandro Gallitognotta
Original assignee: Saes Getters S.P.A.
Priority date: 1992-06-08
Filing date: 1992-11-09
Publication date: 1993-12-23
Also published as: IT1258958B; ITMI921416A0; CN1080039A; KR950701056A; CN1029154C; ITMI921416A1; KR100253841B1

Abstract

Process for evacuating the thermally insulating jacket (5) of a dewar (1) having an inner wall (2) and an outer wall (4), with the inner space between said walls completely partially filled with an insulating material (9), containing also a moisture sorbing material (10) and a getter material (11), in which said moisture sorbing material is a chemical drying agent, and said getter material is a Ba-Li alloy.

Description

PROCESS FOR EVACUATING A THERMALLY INSUUTING JACKET IN PARTICULAR THE JACKET OF A DEWAR OR OF ANOTHER CRYOGENIC DEVICE.

Background_of_the_Inventi_on 1) £ie^d_of_the_Invent|on

The present invention relates to an improved process for evacuating a thermally insulating jacket and in particular the jacket of a dewar or of another cryogenic device, like for instance dedicated pipings for the storage and/or the transport of cryogenic gases such as nitrogen, oxygen, hydrogen, helium, argon, etc. or of other substances requiring the main¬ tenance of a temperature different from room tempera¬ ture, usually lower, the insulating features of said jackets being notoriously reached by means of vacuum and of an insulating material.

The new process is particularly advantageous, as it allows said jacket to be put in condition to work in a very short time. 2) Ihe_Prior_Art

It is known, according to the common practice, to use for the purposes above dewars or pipings having a vacuum jacket in order to reach an adequate thermal insulation. As a further thermoi nsulat i ng measure, there is usually inserted in the jacket an insulating material like glass wool, expanded organic polymers ⁽e.g. polyurethane and resins having various composi¬ tions⁾ and, very frequently, the so called "multi¬ layers". These latter are consisting of alternated sheets made from organic polymers (sjuch as polyolefi- nes), preferably showing a cross-linked structure, and of aluminized plastics, obtained for instance, as it is known, by coating a plastic film with aluminium by means of evaporation under vacuum.

It is also known that the vacuum, in said jacket, has the tendency to degrade along the time, because of the outgassing of the inner components and even of the walls (namely by emission of gaseous species like CO, N , H , H O, 0 etc.) and owing to possible "leaks" (namely penetration of an atmospheric gas). In order to maintain the vacuum, there is normally uti lized a gas sorbing material, placed in the jacket and generally consisting of zeolites, molecular sieves, silica gel, activated carbon (charcoal) and other sorbing material having physical action. These substances have to be kept at a very low temperature, e.g. the temperature of liquid nitrogen or a lower one, in order to allow the sorption of the most part of such gaseous species. Moreover, the physical sorbing mate¬ rials hereinabove do not effectively sorb hydrogen. Therefore it is required, for instance, to insert in the jacket palladium oxide, as disclosed by GB-A-921273. Pd oxide in fact converts hydrogen into water, accord- ing to the reaction:

PdO + H -* Pd + H O (α) Water is then physically sorbed by the zeolites or by the other physical adsorbents present in the jacket. ^A first drawback, coming from the use of these techniques, is due to the fact that the preparation of the insulating jacket of the dewars or of the pipings above requires a very long time, because the activation of the physical absorbents (zeolites and the like) requires an extended thermal treatment under pumping, which can even last several days.

The process length is mainly determined by the necessity of reducing as far as possible the residual amount of water contained in the physical adsorbents and in the insulating materials. A shorter ti e could be reached by drastically increasing the temperature of the thermal treatment of the system (e.g. beyond 150-200°C), but frequently this treatment can not be carried out because the involved materials are not consistent with such temperatures or because of practi- cality grounds or of grounds bound to the process economi cs .

A second drawback comes from the physical nature of the sorption, which makes the reaction reversible. Therefore, for instance, as a dewar is emptied and its inside is at room temperature, the sorbed gases are re-emitted thus giving rise to a certain pressure in the jacket. Consequently, during the fi lling of the inner vessel (of the dewar or of the piping), already returned to room temperature, there is observed a vigorous boi ling, with a gas loss, because of the worsened insulation conditions. These are re-established after a certain time, once the zeolites have been cooled down to cryogenic temperature and have sorbed again the gases. Such a problem, known as "boi l-off" are reboi ling is a serious one, especially in the case of liquefied gases like H and He, which are relatively expensive and endowed with a Low heat of evaporation. It should also be underlined the fact that a few of the known materials used so far do react with the liquefied gas, with which they can occasional- ly get into contact, as soon as there is a leakage from the inside of the vessel, e.g. because of the formation of cracks. PdO in particular cannot be utiliz^¬ ed along with liquefied H , because of possible explo¬ sions, in the case of breaking of the inner wall. The same occurs in the case of liquid 0_, when activated carbon (charcoal) is used.

It was suggested by C. Boffito et al., in J. Vac. Sci . Technol. A5(6), 3442 (1987), to use a getter- ing material, based on a Zr-V-Fe alloy disclosed in GB-A-2043691. This material is actually solving a few of the problems hereinabove, as it can chemically sorb the different gases, responsible for the degrading of the vacuum, and especially hydrogen.

The insulating material however gives rise, during the manufacturing process, to a heavy release of water, which can considerably jeopardize the effecti¬ veness of the sorption, with respect to the other gaseous species, all along the life of the device.

It is therefore a first object of the present invention to provide a process allowing to shorten the time required for evacuating and making ready the insulating jacket of a dewar or of an other cryogenic device, such as for containing and/or transporting liquefied gases. Another object of the present invention is to provide a process of the kind hereinabove, free from said "boi l-off" problems, during the fi lling of the vessel with liquefied gases, once the vessel has got empty and after it has come back to room temperature. A further object of the present invention is to provide a process allowing the elimination of the hazard bound to some of the materials used for the maintenance of vacuum, in particular palladium oxide and activated carbon (charcoal) . These materials can react in fact in an explosive way with hydrogen and oxygen, respecti ely, if either of the two gases are present in a liquefied form, inside the vessel to be insulated and if such a vessel is undergoing a breakage.

Still another object of the present invention is to provide a process for obtaining an insulated device (dewar or piping) granting an effective chemical pumping, with respect to the undesired gases, all along the life of the same device. Disclosure These objects can be accomplished by an improved process for evacuating and making ready a thermally insulating jacket and in particular the jacket of a dewar or of another cryogenic device, having an inner wall and an outer wall and having the inner space between said walls completely or partially filled with an insulating material, wherein said inner space also contains a moisture sorbing material and a getter material, characterized in that said moisture sorbing material is a chemical drying agent, preferably select¬ ed from the sorbing materials showing an H O vapour pressure lower than 1 Pa and in particular from barium oxide, strontium oxide, phosphorus pentoxide and mixtu- re s t h e r eof .

According to a preferred very effective embodi¬ ment, the new process comprises the following subsequent steps : A. the inner space of the jacket is first evacuated down to a pressure lower than 100 Pa by means of a vacuum pump;

B. said inner space is contemporaneously exposed to said chemical drying agent, while keeping the getter in an inactivated form;

C. said inner space is further evacuated down to a pressure lower than 5 Pa, by means of a vacuum pump;

D. said getter is activated; and E. the jacket is isolated from the pump, by sealing the connection between said pump and the inner space of the j acket .

Optionally the pumping under item A hereinabove can be discontinued during the operation under item ^B and during the operation under item A and B the inner wall of said dewar, or of an other cryogenic device, is kept hot, at a temperature not higher than 150°C and preferably 120°C, thus promoting the release of water from the insulating material. The operation under item B does normally last no more than 48 h and preferably from 2 to 48 h nstead of the several days required in the past.

Moreover said chemical drying agent and said getter are preferably lying in separate zones, contra- ry to the previous teachings of the prior art, against the outer wall of said dewar or of other cryogenic d evi c e .

As to said getter, it may consist of the alloys which can be activated at a relatively low temperature, like those based on barium and/or zirco- nium and preferably the alloys Ba-Li as disclosed in the European Patent Application No.92830186 in the name of the same applicant, in particular the alloy having raw formula Ba Li ..

In other more detai led words the new improved process allows to evacuate and to make ready a thermal insulating jacket of a cryogenic device (dewar, trans^¬ fer pipings and so on) for the storage and/or the transport of a stuff which has to be kept at a tempe¬ rature different from room temperature, in particular lower. Such a process allows to sorb the gases originat¬ ed during the same process and during the subsequent life of the cryogenic device, by combining the action of a particular (chemical) water adsorbing agent, prevailingly acting in the first stages of the process (by providing a water pumping "in situ", which accele¬ rates the same process) along with the action of a getter, activated only in a subsequent step, which is prevai lingly acting as a chemical sorbing material with respect to the other gases different from water, like for instance 0 , N , CO, H , etc.

The invention is hereinafter described more clearly with reference to the following drawings, which are supplied for merely i llustrating purposes, without limiting in any way the scope of the same invention, in which:

£I§ ϋi_l shows a schematic sectional view of a commonly used metal dewar for the storage of liquefi¬ ed gases, having an insulating jacket according to the invention;

FIGURE_2 shows a graph reporting the results of the tests carried out by means of said dewar of Fig. 1 ; and

FJ GJUJR E__3 shows a preferred arrangement of the drying agent and of the getter inside the vacuum jacket. The series of operations hereinbelow, embodying the process according to the invention, is described referring to a vessel like the dewar of Fig. 1, but the same operations could refer to an insulated piping conveying liquefied gases as well. As it is known, the dewar 1 consists of an inner container or vessel 2, preferably made from metal, e.g. steel, defining an inner volume or useful space 3, suitable to contain a liquefied gas, which can communicate with the surroundings by means of a "neck" 6, normally closed but not sealed. An outer wall or mantel 4 defines, along with the inner wall 2, a jacket 5, partially filled, at least in the portion surrounding the inner wall 2, with an insulating material 9, preferably the "multi layer" type herein above. The jacket 5 can communicate with an outer pumping system (not shown in the drawings) by means of a connecting fitting 8 and a valve 7 for switching off or disabling the pump.

According to the present invention, there are inserted in the jacket 5 a chemical moisture adsorber 10 and a gettering material (11) lying in separate zones, against the outer wall 4, contrary to the known prior art, describing the getter material to be positioned against the inner wall, at a lower temperature. Then a first pumping step of the jacket 5 starts, through the tubular fitting 8, until reach¬ ing a pressure of 100 Pa or lower, which is anyhow requiring only a few minutes. The valve 7 is subsequent¬ ly closed, thus isolating the wall vessel from the pumping apparatus, and the water sorbing material 10 exerts, for a time between 2 and 48 h a selective pumping action with respect to the water vapour out- gassed from the insulating material 9.

During this step, the valve 7 can be also kept open all along said induction time. In such a case, however, the drying material 10 is always sorb¬ ing the most part of the water vapour, because the action of the pump is limited by the flow conductance allowed by connection pipe fitting 8. Again in this phase of water vapour sorption (with or without any pumping from outside) the inner wall 2 can undergo a smooth heating, not above 150°C, for instance by penetration into the space 3 of hot air or of hot water, in order to accelerate the removal of the water vapour from insulating material 9, in particular from the innermost layers, lying near the wall 2 more than the other layers.

After the induction time, pumping is started again, should it have been disabled, by re-opening the valve 7, down to a pressure of 5 Pa or less. At this point the getter 11 is activated, e.g. by means of a heat generating device arranged at the outside, at a location corresponding to the inside positioning of the same getter. The heating can simply occur by using a flame, a hot air gun, an electic resistance or other similar means. The temperature to be reached depends on the kind of getter selected for the use. Getters which can be activated at a low or very low temperature are preferred, like the Ba-Li alloys disclosed in the European Patent Application No.92830186, inserted in a blister supplied with a thermoret ractab le cover, according to the European Patent Application No.92830185 also in the name of the same Applicant. Getter materials of this kind do not require a heating at a temperature higher than 120°C in order to be activated. Eventually, during the last step of the process, the jacket 5 is definitely isolated from the outside by closure of the valve 7, isolation of the pump and sealing (e.g. by means of a "pinch-off") in correspon¬ dence to the tubular fitting 8. The same getter is working in a much better way if the sorbed gas does not contain water. This is reason why it is suggested to let the gases get into contact first with the powerful drying agent, such as BaO, and then, in a separate zone, with the getter, e.g. Ba Li ..

4

More in detai l, and according to the embodiment of Fig. 3, said chemical drying agent 10 and said getter 11 can be arranged in a container 12 subdivid¬ ed into an inner and an outer zone by a porous septum 13 and wherein the inner zone contains said getter; the outer zone 15 is communicating with the space containing said insulating material 9 and contains said chemical drying agent 10, which prevents the passage of the water vapour through said septum and towards said getter 11. Container 12 may be a vertical box having an opening at its top and a planar (e.g. horizontal⁾ septum or a toroidal box, wrapping for instance the inner vessel of a dewar or of a cryogenic piping, having a radial or a planar (e.g. horizontal) septum. The same container can be also a rigid, semiri¬ gid or flexible box, made from a substantially water- free material, preferably from metal, glass, ceramics or combinations thereof. It may have a toroidal shape, but also different shapes in cross section could be adopted, i.e. circular, square, rectangular, triangular, elliptical, oval, lobe-shaped and of similar configu¬ ration .

The following examples are supplied for illu¬ strating purposes and do not limit in any way the scope of the invention.

EXAMPLE_1

This example has the purpose to show the behaviour of a dewar like the one illustrated in Fig. 1, ha ing a vacuum jacket prepared without utiliz- ing the technique according to the present invention.

Said vacuum jacket, having a volume of 36 I and containing 500 g of a multilayer insulating mate¬ rial, consisting of polyolefinic tapes alternated with aluminized polyester tapes (traded as MYLAR tapes⁾, was connected to an outer vacuum pump, of the rotary and turbomolecular type, while maintaining the pumping for 5 h. Subsequently the dewar was isolat¬ ed and the pressure increase was started to be recorded a long the time.

The results of these tests are represented by the line 1 of Fig. 2. Line 2, on the contrary, is showing the pressure increase merely due to the penetrat¬ ion of air into the jacket through the Leaks present in the dewar, of the order of 6x10 Pa /s, on the basis of a measurement carried out by means of a mass spectrometer. The tested device was quite well representing the commonly used actual devices; however, in order to perform an accelerated test, there was produced the air leakage as indicated hereinabove, at least 5000 times greater than the value considered as allowable in the case of such an exploitment (normal-

-10 3 ly of the order of 10 Pa m /s) . It was thus possi¬ ble to reproduce in a short time the effects of a long lasting leakage. The difference between line 1 and line 2 is due to the gases, prevailingly water, released by the outgassing of the inner insulating material.

EXAMPLE_2

The test of example 1 was repeated, while inserting however in the vacuum jacket, in the zones corresponding to the drying agent 10 and to the getter 11, respectively 10 g of BaO and 10 g of an alloy Ba Li , according to the European Patent Application 92830186, in the form of granules (small pellets) arranged in the t hermo-ret ractab le containers (vessels) as disclosed in the European Patent Application 92830185. The vacuum jacket was connected to the outer pump and submitted to a short pumping for 10 minutes. Then the system was isolated from the pump by closing the valve 7. Under such conditions, there was an induction time of 24 h, followed by a second stage (re-pumping of the system), by previously opening the valve 7, while contemporaneously activating the getter up to 120°C by means of a heating device, arranged in an outside location corresponding to the inside positioning of the same getter, while utilizing to this purpose a hot air gun. The activation time resulted to be 25 minutes, whereafter the system was isolated and recording of the pressure versus time was caused to start. The results of these tests are plotted as line 3 on Fig. 2. DISCUSSION

By comparing lines 1 and 3 of Fig. 2, it can be observed that applying the technique according to the invention allows for a substantial reduction of the pressure increase occurring in the cryogenic device after its sealing. A few analytical tests, carried out by means of a mass spectrometer, showed that the combined use of the two materials, drying agent and getter, does not only allow to quantitatively sorb the water outgassed by the insulating material, but also to face the leaks in the device. Further it should be appreciated that the tests were performed under accelerated conditions, namely under a condition more drastic, as to the load of atmospheric gases, with respect to those occurring in the practice. Therefore an accelerated test lasting 360 h (15 days) is equivalent to at Least 20 years of actual working. It is understood that optional additions and/or changes can be carried out by those skilled in the art, with respect to the operative conditions hereinabove, without departing from the scope and spirit of the claimed invention.

Claims

WHAT_WE_CLAIM_IS

1. An improved process for evacuating and making ready a thermally insulating jacket (5) and in particular the thermally insulating jacket (5) of a dewar or of another cryogenic device (1), having an inner wall (2) and an outer wall (4), and having the inner space between said walls completely or partially filled with an insulating material (9), wherein said inner space also contains a moisture sorbing material (10) and a getter material (11), characterized in that said moisture sorbing material (10) is a chemical drying agent.

2. A process according to claim 1, characterized in that said chemical drying agent is provided with a H O vapour pressure lower than 1 Pa at room tempera- ture and is preferably selected from barium oxide, strontium oxide, phosphorus oxide and mixtures thereof.

3. A process according to claim 1, characterized by the following steps:

A. the inner space of the jacket (5) is first evacuated down to a pressure lower than 100 Pa by means of a vacuum pump;

B. said inner space is contemporaneously exposed to said chemical drying agent (10) whi le keeping the getter (11) in an inactivated form; C. said inner space is evacuated farther, down to a pressure lower than 5 Pa, by means of a vacuum pump;

D. said getter (11) is activated; and

E. the jacket is isolated from the pump, by sealing the connection between said pump and the inner space of the jacket.

4. A process according to claim 3, characteriz¬ ed in that during the exposure of step B the pumping according to step A is discontinued.

5. A process according to claim 3, character¬ ized in that during the steps A and B the inner wall (2) of said dewar, or of another cryogenic device, is kept hot at a temperature not higher than 150°C and preferably 120°C, thus promoting the release of water from the insulating material (9).

6. A process according to claim 3, 4 or 5, characterized in that the step B does not last more than 48 h and preferably from 2 to 48 h.

7. A process according to claim 3, characterized in that said chemical drying agent (10) and said getter (11) are lying, in separate locations, against the outer wall (4) of said dewar or other cryogenic device.

8. A process according to clai 7, characterized in that said chemical drying agent (10) and said getter (11) are arranged in a container (12) subdivided into an inner (14) and an outer (15) zone by a porous septum (13), wherein:

- the inner zone (14) contains said getter (11); - the outer zone (15) is communicating with the inner space containing said insulating material (9) and contains said chemical drying agent (10) which septum (13) and towards said getter (11) .

9. A process according to claim 8, characteriz- ed in that said container (12) is a vertical box having an opening at its uppermost portion and a planar septum (13) .

10. A process according to claim 8, characteriz¬ ed in that said container (12) is a toroidal box having a radial or planar septum (13⁾.

11. A process according to claim 9 or 10, characterized in that said septum (13) is horizontal.

12. A process according to claim 8, characteriz¬ ed in that said container (12) is a rigid, semirigid or flexible box.

13. A process according to claim 8, characteriz¬ ed in that said container (12) has a shape, in cross section, selected among circular, square, rectangular, triangular, elliptical, oval, lobe-shaped and similar configurations.

14. A process according to claim 8, characteriz¬ ed in that siad container (12) is made from a substan¬ tially water-free material, selected from metal, glass, ceramics and combinations thereof.

15. A process according to anyone of the preceding claims, characterized in that said getter

⁽11⁾ is selected from the alloys containing barium and li t i urn.

16. A process according to claim 14, characte¬ rized in that said getter (11) is an alloy having the raw formula Ba Li .