US 20030127313 A1
A pot-like digestion vessel (1), which is closed by a cover (3), is provided with adjustable automatic internal pressure relief by means of a pressure applying plunger (6) which is held in a screwing part (7) via a resilient pressure generating mans (8) to press against the cover (3). In another embodiment the plunger (6) holds a valve body (33) in an outlet channel (31) which extends through the cover (3).
1. A device for closing a pot-like digestion vessel having an opening which is covered by a cover, said device comprising
a pressure applying plunger;
a screwing part arranged coaxially with said pressure applying plunger; and
a resilient pressure generating means arranged between the pressure applying plunger and the screwing part,
said screwing part and said pressure-applying plunger being configured such that one fits around the other in a socketed arrangement.
2. A device according to
3. A device according to
4. A device according to
5. A device according to
said screwing part has an external thread.
6. A device according to
said screwing part is screwable into an abutment which is supported at a location such that a digestion vessel is disposed below said screwing part and said pressure-applying plunger, said abutment being formed by an upper wall of a box-like housing for containing a digestion vessel.
7. A device for closing a pot-like digestion vessel having an opening which is covered by a cover, said device comprising:
a pressure-applying plunger;
an abutment arranged to surround a digestion vessel; and
a screwing part which fits around said pressure-applying plunger in the manner of a cap and which includes a lower edge portion formed with a screw-thread and arranged such said lower edge portion can be screwed to said abutment,
said abutment being formed by a supporting ring which can be fixed to a digestion vessel at its circumference.
8. A device according to
a digestion vessel having a flange at an edge thereof;
said supporting ring being fastened to an outer circumferential surface of said digestion vessel and/or fits under the flange and/or is formed on the flange.
9. A device according to
said pressure-applying plunger is axially guided by a cylindrical sliding face of an internal hollow cylinder, which is formed in said screwing part, to move slidingly along said face.
10. A device according to
at least one annular groove is formed in said cylindrical sliding face of said pressure-applying plunger.
11. A device according to
said pressure-applying plunger has a U-shaped cross-sectional shape and wherein
said resilient pressure-generating means is arranged in a recess created by said U-shaped cross-sectional shape.
12. A device according to clam 1, wherein
said resilient pressure-generating means is formed by at least one of a disc spring and a helical spring.
13. A device according to
a coaxial hole is formed in said pressure-applying plunger
14. A device according to
said screwing part includes in an upper region thereof, a guidance section which guides an extension of said pressure-applying plunger.
15. A device according to
said screwing part is provided at its upper end with an element for applying rotation thereto.
16. A device according to
said pressure-applying plunger forms a cover of a digestion vessel.
17. A device according to
a position-indicating marking, is arranged between said screwing part and said abutment.
18. A device for closing a pot-like digestion vessel, said device comprising:
a cover which closes an opening in such digestion vessel; said cover being formed with an outlet channel extending therethrough
a closing part which bears against said cover to hold said cover over an opening of a digestion vessel;
a closing part which bears against said cover; and
a valve which closes said outlet channel, said valve being arranged to open automatically in response to an internal pressure in such digestion vessel exceeding a given value.
19. A device according to
said outlet channel is disposed axially of said cover, and wherein
said valve includes a valve body which is larger than said outlet channel, said valve body being preloaded by means of a spring against an edge of said outlet channel.
20. A device according to
said edge of said outlet channel is formed by the step surface of a widened portion of said outlet channel.
21. A device according to
a section of said outlet channel branches off inside said cover above said step surface.
22. A device according
said closing part is a screwing part which is screwed into an abutment and which is also arranged to be screwed against said cover.
23. A device according to
said screwing part forms a guide for said valve body.
24. A device according to
said screwing part is a sleeve into which said valve body extends.
25. A device according to
said spring is arranged in said sleeve.
26. A device according to
a stop arranged to limit outward extension movement of said valve body, said stop being arranged at a lower edge of said sleeve, said stop being formed by a flange which is detachably fixed to said lower edge.
27. A device according to
said valve body is detachably connected to a valve body base by means of a screw connection.
28. A device according to
a valve body spigot arranged to fit into said outlet channel with a small clearance for movement, said valve body projecting downwardly from said valve body.
 This is a continuation-in-part of application Ser. No. 10/099,443, filed Mar. 15, 2002.
 Field of the Invention
 This invention relates to a device for closing a pot-like digestion vessel and more particularly it concerns novel closing arrangements by which excessive internal pressure in such vessels is relieved.
 To perform an analysis of materials, it is known for one or more materials, e.g. a solid or liquid sample material, to be heated in a digestion vessel, whereby in most cases a pressure above atmospheric is produced in the digestion vessel and this in turn promotes the digestion of the sample material or the like.
 A device of the type to which this invention may be applied is described in EP 0 830 891 A1. In this generic design a resilient pressure-generating means acting between a screwing part and a pressure-applying plunger is formed by a stack of metal disc springs arranged one above the other, the pressure-applying plunger taking the form of a U-shaped component in which is arranged a dome-like cage part which is of metal and forms a Faraday cage for the disc springs. This device has a large number of parts and is structurally large and in particular tall.
 A device for closing a pot-like digestion vessel is also described in GB 2 184 040 A. In this known device the pressure-applying plunger, the screwing part and the resilient pressure-generating means arranged therebetween are contained in a hat-like screwed cover of a pot-like supporting vessel into which is fitted a holding vessel against the cover of which the pressure-applying plunger acts. The screwing part is mounted to be vertically displaceable with a pressure-applying piece in a vertical guide hole in the screwed cover and is in the form of a nut which is screwed on to a pressure-applying piece extending coaxially upwardly from the pressure-applying plunger and having a threaded shaft which passes through a guide hole. Arranged in the annular wall of the hat-like screwed cover in the screwed-on state is at the same level as the upper edge of the supporting vessel an internal annular groove from which a venting passage extends radially outwards through the annular wall. This known design has a large number of parts, is structurally large, is complicated and costly to manufacture and is time-consuming to manipulate. There is also the risk that the pressure-applying piece and the nut will move away from one another owing to fouling, for example, as a result of which different pressure ranges will arise when the cover of the holding vessel is opened and closed.
 It is the object of the invention to simplify a device of the above-mentioned types and to design it in such away that a small or low construction is obtained. Furthermore, the device is to be improved with respect to its operation. An attempt is also made to reduce the number of separate parts in the device and the amount of material it consumes.
 This object is achieved by means of the features described and claimed hereinafter.
 In a device according to one aspect of the invention, a pressure-applying plunger and a screwing part fit around one another in a socketed arrangement by means of at least one annular wall which may be part of the pressure-applying plunger and which extends upwardly therefrom preferably as an integral part thereof and fits around the screwing part like a socket, or which may be part of the screwing part and extends downwardly therefrom preferably as an integral part thereof and fits around the pressure-applying plunger like a socket. It is also possible for two circumferential walls to be provided of which one annular wall extends upwardly from the pressure-applying plunger and the other extends downwardly from the screwing part and for the annular walls to fit around one another in a socketed arrangement. Not only does this allow guidance and/or radial support to be obtained for the pressure-applying plunger but the interior space between the pressure-applying plunger and the screwing part which holds the resilient pressure-generating means is closed by the at least one annular wall, so that contaminants cannot enter the interior space for the resilient pressure-generating means from the holding space of the holding vessel in case of lifting of the cover of the holding vessel through an overpressure reaction, nor can other contaminants enter from the surroundings of the device. In this way the interior space is also protected against aggressive treating or digesting chemicals, whereby the movement function of the device is protected and long service life is achieved. If screw-thread guidance is used, positive guidance is provided in both axial directions of movement, so that in case of an adjustment a secure adjustment of the screwing part is achieved even if a certain stiffness is present in the guidance means.
 The seal between the pressure-applying plunger and the screwing part can be improved by arranging between the guidance faces which co-operate in a socketed arrangement a seal, preferably an annular seal, which may, for example, be an O-ring which can be inserted in an external annular groove in the part around which the other part fits or in an internal annular groove in the part which fits around the other.
 Within the scope of the invention there are many possible ways for the screwing part to be supported against an abutment. Such an abutment may be formed, for example, by a supported disc which fits over a location for the digestion vessel in a space to which microwaves can be applied. If a disc of this kind is open in all directions it can also be used to cover a plurality of circumferentially distributed locations for digesting vessels which include the device. In this case the disc may, in a manner known per se, be a turntable which is mounted to be rotatable in the microwaving space together with the locations.
 The abutment may also be formed by the upper wall of a pressure-absorbing body which delimits a location for the digestion vessel on both sides, at the bottom and at the top. In this embodiment the digestion vessel with the device may be placed on the location and removed therefrom optionally from one side.
 It is also possible, in accordance with one aspect of the invention, for the abutment to be formed on or supported against the circumferential surface of the digestion vessel. For this purpose the screwing part may be formed by the upper wall of a cap, the annular wall of which can be connected, in its free edge region, for example, to the digestion vessel, e.g. by screws. It is particularly advantageous in this case if an externally threaded ring is fitted on to the circumferential surface of the digestion vessel and is supported against said vessel, preferably against the underside of a flange in the edge region of the digestion vessel, so that the flange absorbs the reaction forces of the device and ensures positioning of the threaded ring by contact. This embodiment is distinguished in particular by its small structure which takes up little space, is handy, consumes little material and is also of low weight.
 It is possible and advantageous in all embodiments for a marking or scale to be so arranged between the screwing part or parts attached thereto and the abutment that, with respect to the resilient closing force of the pressure-generating means, the screwing part can be preset in steps or steplessly in such a way that a given opening pressure is obtained in the interior space of the digestion vessel, above which pressure the cover of said vessel automatically lifts and thus opens. This allows the operator to preset desired maximum internal pressures in the holding space of the holding vessel while taking account of the material or materials to be treated.
 If, in a device of the present type, the cover of the digestion vessel forms a valve body of a valve which opens at a predetermined internal pressure of the vessel as a result of the elasticity of the resilient pressure-generating means, a relatively large pressure surface is available on the inner face of the cover and a relatively large pre-loading force is therefore required to retain the cover in its closed position on the digestion vessel. Consequently, a relatively large expenditure of force is required to change the pre-loading force and thereby to adjust the internal pressure value above which the cover is automatically to open. Moreover, it can be regarded as disadvantageous that when such opening of the cover by overpressure takes place the actual point of opening and venting is uncertain and can extend over an indeterminate portion of the circumference or over the entire circumferential portion.
 It is a further object of the invention to provide a device in which the digestion vessel can be opened with a small expenditure of force. It is also to be achieved that the device is so configured that on venting of the digestion vessel at an internal pressure exceeding a given value the venting takes place at a precisely specified point.
 This object is met by means of a digestion vessel cover which is formed with an outlet channel extending therethrough and a valve which closes the outlet channel. The valve is arranged to open automatically in response to an internal pressure in such digestion vessel exceeding a given value.
 Advantageous refinements of the invention are also described and claimed herein.
 In the device according to an aspect of the invention, the cover has passing through it an outlet channel closed by a valve which automatically opens at an internal pressure in the digestion vessel which exceeds a given value.
 Present in the area of the cover in this embodiment is a special valve which automatically opens at an internal pressure in the vessel which exceeds a given value, while the cover remains in its closed position. In this embodiment the size of the opening of the digestion vessel is not relevant; what is relevant is the size of the outlet channel which is considerably smaller than the size of the opening of the digestion vessel or the size of the cover. A considerably lower pre-loading force is therefore required to close the valve, and changes to this pre-loading force can be carried out with a relatively small expenditure of force and with ease of operation, so that the handling convenience of the device according to the invention is considerably improved.
 Furthermore, in the embodiment of the invention according to a specific embodiment, the venting point is restricted to the end aperture of the outlet channel. Because sample material can escape during a venting process and fouling can thereby take place, measures for preventing or eliminating fouling can be arranged or taken more simply and precisely in this embodiment.
 Further refinements of the invention give rise to simple, small and operative structures and serve to prevent fouling of the valve mechanism and to permit precise and easily operated setting of different pressure values at which the valve opens.
 Advantageous embodiments of the invention will be explained in detail below with reference to a plurality of embodiments and drawings.
FIG. 1 is a vertical section through a digestion vessel having a device according to the invention for closing the vessel;
FIG. 2 is an enlarged view of a detail marked X in FIG. 1;
FIG. 3 is a vertical section through a modified vessel arrangement having a device for closing a digestion vessel, which is also modified;
FIG. 4 is a vertical section through a digestion vessel arrangement having a modified device for closing the vessel, according to the invention.
 A pot-like digestion vessel 1 having a circumferential wall 1 a which is in the form of a hollow cylinder and is, for example, relatively thin, and a flat bottom wall 1 b, is surrounded, and supported in the radially outward direction, by a supporting shell 2 in the form of a hollow cylinder which may extend up to a flange 1 c on the upper edge of the circumferential wall 1 a, so that the flange 1 c rests on the upper end face of the supporting shell 2.
 The upper opening of the digestion vessel 1 is to be closed and opened by means of a cover 3 which is, for example, flat and in the form of a stepped cylinder of which, in the embodiment shown, a lower cylindrical cover extension 3 a fits into the digestion vessel 1 with little play and is sealed, preferably by an annular seal 4. The latter may be formed by an O-ring 4 a which is mounted in an annular groove in the circumferential surface of the cover extension 3 a. The shoulder face 3 b of the cover 3 thus rests on the flange 1 c.
 The device, designated as a whole by reference numeral 5, for closing the digestion vessel 1 even when a pressure above atmospheric is present in the interior space 1 d of said digestion vessel, consists of three main parts, namely a pressure-applying plunger 6, a screwing part 7 at least part of which is arranged above the latter, and a resilient pressure-generating means 8 which is arranged between an upwardly directed supporting face 6 a on the pressure-applying plunger 6 and a downwardly directed supporting face 7 a on the screwing part 7 and which in functional operation is preloaded with a given compressive force. In the embodiment shown the pressure-generating means 8 comprises a plurality of disc springs stacked on top of one another in opposite directions. The screwing part 7 has at least in its upper portion an external thread 7 b with which it is screwed into a threaded bore 9 a of an abutment 9 as a screwing part. To allow the screwing part 7 to be turned an element 11 for applying rotation is arranged in its upper face and is intended for a rotation tool which can be positively engaged therewith in rotation. In the embodiment shown the element 11 for applying rotation is formed by a noncircular socket.
 To cover the space 12 occupied by the pressure-generating means and thus to protect it from fouling, there is provided an annular wall 7 c which surrounds the space 12 and which fits at least partially around the pressure-applying plunger 6 and/or the screwing part 7 in a socketed arrangement. In the embodiment shown the annular wall 7 c, which is preferably in the form of a hollow cylinder, extends downwardly from the screwing part 7 as an integral part thereof, fitting at least partially over the preferably cylindrical circumferential surface of the pressure-applying plunger 6 with clearance for movement and thus making the space 12 inaccessible from the sides.
 The amount of the vertical overlap a can be sufficiently large to allow a guidance function to be performed between the pressure-applying plunger 6 and the screwing part 7 and/or an annular seal 13 when the device 5 telescopes in a manner which has yet to be described. Preferably arranged in the gap at the overlap is a sealing ring 13 a, e.g. an O-ring, which is mounted in an annular groove in the circumferential surface of the pressure-applying plunger 6 and is covered by the inner surface of the annular wall 7 c in any telescoped position. The maximum amount of telescoping movement b in the vertical direction is determined in the present embodiment by the distance between the annular wall 7 c and the cover 3. If the pressure-applying plunger 6 is formed to have a flange 6 b as indicated in ghost lines, the maximum amount of telescoping movement b is determined by the distance from the flange 6 b. The flange 6 b allows the area over which the pressure-applying plunger 6 acts against the cover 3 to be enlarged and the pressure per unit area effective in operation to be reduced.
 The pressure-generating means 8, or the spring discs, may be centered by the inner circumferential surface of the annular wall 7 c. However, to avoid the frictional wear which occurs between the two during telescoping, the preferably annular pressure-generating means 8 is centered internally with a small amount of clearance for movement by means of the centering spigot 6 c which in the embodiment shown extends upwardly from the pressure-applying plunger 6 as an integral part thereof and may then engage in, or terminate shortly before, a guide hole 7 d present between the supporting face 7 a and the element 11 for applying rotation, thus providing additional guidance, which, however, is not absolutely essential, for the pressure-applying plunger 6.
 Arranged in the pressure-applying plunger 6, preferably coaxially therewith, is a through hole 14 which may, for example, be lined with an annular wall 15 which may have at its lower end a flange which allows it to be inset into a recess in the pressure-applying face. To allow it to be secured axially in the pressure-applying plunger 6, the annular wall 15 may be a press-fit therein.
 The hole 14 may be used to hold functional elements, e.g. sensor elements, which pass through the screwing part 7 and which are functionally connected via said hole to associated control or regulating devices. A temperature sensor, for example, may the provided, which is inserted in the hole 14. A pressure measuring element may also be inserted in the hole 14, the vertical position, or rather the change in position, of which pressure measuring element, on lifting of the cover 3, serves as an indication of the level of the internal pressure prevailing in the digestion space 1 d in operation.
 In the embodiment shown in FIG. 1 the abutment 9 is formed by the upper wall 16 a of a supporting housing 16 which may be of box-like configuration with a bottom wall 16 b and two side walls 16 c and forms a holding space 17 for the digestion vessel 1, which space is open at the front or is horizontally continuous and in which said vessel can be installed and removed again from the appropriate side. In its installed state the digestion vessel 1 rests with its bottom wall 1 b, which is flat in the embodiment shown, on the bottom wall 16 b of the supporting housing 16, whereby the first bottom wall 1 b is supported against the internal pressure. An intermediate layer 18, which is preferably made of a thermally insulating material, e.g. plastics material, is arranged between the bottom walls 1 b and 16 b to reduce the dissipation of heat from the digestion vessel 1.
 The relatively great length of the annular wall 7 c, which extends downwardly from the screwing part 7, enables the screwing part 7 to have a large range of vertical adjustment and thus to be adapted to digestion vessels 1 of different heights, if the external thread 7 b extends at least to the lower end region of the annular wall 7 c or, for example, over its entire vertical length.
 To prevent the pressure-applying plunger 6 from dropping out of the position in which it overlaps the screwing part 7 when there is no digestion vessel 1 under the device 5, a limiting device 21 is provided between the pressure-applying plunger 6 and the screwing part 7 or the annular wall 7 c to limit the maximum extension movement of the pressure-applying plunger 6 and which in the embodiment shown is formed by a stop device having a stop element 22 a in the lower end portion of the annular wall 7 c. The opposing stop element 22 b is preferably formed by the sealing ring 13 a which projects somewhat from the circumferential surface of the pressure-applying plunger 6 in the radial direction and can thus form the opposing stop element 22 b without the need for any special opposing stop element. As a result, the sealing ring 13 a performs a sealing and stopping function. The stopping element 21 is formed by an internal annular bead on the inner circumferential surface of the annular wall 7 c. To allow a small clearance for movement or sliding, the diameter of the annular bead is made larger than the diameter of the pressure-applying plunger 6, thus enabling the latter to move in the annular bead. The internal diameter d1 of the annular bead is made somewhat smaller than the internal diameter d2 of the rest of the annular wall 7 c, thus enabling the internal bead to be formed. At the same time, the internal diameter d2 is made only sufficiently large for the sealing ring 13 a to form a resilient seal against the inner circumferential surface 7 e of the annular wall 7 c. In the embodiment the outer flank of the stop element 22 or the annular bead is formed by an oblique or rounded lead-in face 23. The inner flank 24 is the face of a step which preferably extends approximately at right angles to the center axis of the pressure-applying plunger 6. In the embodiment the internal annular bead is pointed in cross-section, producing a pointed annular edge. The sealing ring 13 a will thereby be sheared off if the pressure-applying plunger 6 is forcibly extracted from its socketed connection and protection is provided against dismantling in view of the damage that would be caused to the sealing ring 13 a.
 In the embodiment shown in FIG. 3, in which the same or comparable parts are designated by the same reference numerals, the screwing part 7 with its annular wall 7 c has been made so large that it fits around at least the upper edge portion of the digestion vessel 1 like a cap and is screwed thereto, so that the circumferential wall 1 a or the flange 1 c or an optionally present supporting shell 2, to any of which the annular wall 7 c may be screwed, forms the abutment 9. In the embodiment a supporting ring 25 is fitted on to the digestion vessel 1, preferably in the upper region of the digestion vessel 1, and in particular is arranged to rest against the flange 1 c and to be connected to the outer circumferential surface of the digestion vessel 1, e.g. by pressing on or bonding. A connection of this kind ensures that the forces applied to the abutment are transmitted to the digestion vessel 1. If the supporting ring 25 rests against the flange 1 c a connection to the outer circumferential surface can be omitted as the flange 1 c absorbs the forces from the abutment. However, in this case too it is advantageous for the supporting ring 25 to be retained on the digestion vessel 1, e.g. by bonding or pressing on, so that it cannot be lost.
 The screw connection between the screwing part 7 and the supporting ring 25 preferably comprises an internal thread 7 f in the end portion of the annular wall 7 c and a matching external thread on the supporting ring 25.
 In the two embodiments described above, the pressure-applying plunger 6 can form the cover 3, as shown in FIG. 3. In the embodiment shown in FIG. 1 the cross-sections of the pressure-applying plunger 6 and the screwing part 7 would have to be enlarged as appropriate to allow the pressure-applying plunger 6 to form the cover 3. The hole 14 would also have to end at a distance from the end of the pressure-applying plunger 6 which faces towards the digestion vessel 1.
 In all the embodiments it is also possible for the pressure-applying plunger to have a U-shaped configuration in cross-section as shown in FIG. 3 and thus to be provided with a circumferential wall 6 d which, owing to its axial length, ensures that the pressure-applying plunger 6 is guided in such a way as to be secure against tilting and which can also ensure sufficient sealing. It is also advantageous for one or more grooves 6 e spaced at an axial distance from one another to be arranged in the circumferential surface of the circumferential wall 6 d, which grooves both improve the seal and, in view of the differing thermal expansions, prevent any harmful tensions from arising in the fit when the material of the pressure-applying plunger, in particular at fairly high digestion temperatures, is relatively soft or yielding.
 In the embodiment shown in FIG. 3 the resilient pressure-generating means 8 is formed by a helical spring which is supported against the upper wall of the screwing part 7 and bears against the pressure-applying plunger 6, at the bottom of a recess in the present case.
 In the embodiment shown in FIG. 3, too, there may be provided a centering spigot 6 c which likewise is preferably circular in cross-section and can be guided longitudinally or vertically in the guidance hole 7 d in the upper wall of the cap-like screwing part 7 with clearance for movement. At its lower end the centering spigot 6 c preferably has a flange to reduce the pressure per unit area, against which the pressure-generating means 8 can bear.
 In all the embodiments it is advantageous to apply a marking between the screwing part 7 and a part positioned on the digestion vessel 1 or on the cover 3 or on the pressure-applying plunger 6, which marking shows the axial position of the screwing part 7 with respect to the positioned part and preferably allows the screwing part 7 to be preset in a defined manner. The marking may, for example, be formed by a scale and a pointer which are located in each case on one of the two parts to be compared. In the embodiment according to FIG. 1 the marking 27 can be arranged, for example, on the upper faces of the screwing part 7 and of the abutment 9, preferably close to or at the threaded joint in the screwed assembly, e.g. in the form of an annular scale on the upper wall 16 a and a pointer on the screwing part 7.
 In the embodiment shown in FIG. 3 the marking 27 may be formed, for example, by the front end of the centering spigot 6 c and the lower or upper edge of the guide hole 7 d. It is also possible for the markings or scale divisions to be applied to the inner wall face of the guide hole at axial distances from one another, which allow the position of the centering spigot 6 c and therefore the open or closed position of the pressure-applying plunger 6, and optionally also of the cover, to be seen or read off against the free end of the centering spigot 6 c.
 In the embodiment shown in FIG. 1 the digestion vessel 1 is made of a corrosion-resistant material, in particular plastics material and preferably polytetrafluoroethylene (PTFE). The other parts of the digestion vessel 1 including the supporting housing 16 are made preferably of material which is permeable to microwaves and preferably plastics material. This may also be true of the device 5. In the embodiment, the spring forming the resilient pressure-generating means 8 or the resilient elements are made of metal, in particular of spring steel or special steel. The screwing part 7 can also be made of metal, e.g. special steel, in which case it acts as a Faraday cage with respect to the spring. Operation is improved if the cage has a polished surface and its edges are rounded. It is also advantageous for the dimensions of the cage, i.e. its diameter and axial length, preferably to be sized at between a quarter and one complete wavelength of the microwave radiation used. In addition, the surface of the metal cage may be coated with a material, such as e.g. PTFE, which is resistant to corrosive chemicals, to prevent the surface of the metal from being attacked by the chemicals.
 For a digestion process a sample material is placed in the digestion vessel 1′ and the digestion vessel 1 is closed with the device 5. As this is done the screwing part 7 is screwed in (FIG. 1) or on (FIG. 3) sufficiently far for the resilient pressure-generating means 8 to exert a closing force which corresponds to a given internal pressure. If the internal pressure rises in operation to more than a given value or the set value, the cover 3 or the pressure-applying plunger 6 automatically opens, in which case the pressure-generating means 8 yields and the internal pressure can escape. In this case the device 5 acts as a pressure-release valve, the opening pressure being adjustable by screwing in the screwing part 7.
 The embodiment shown in FIG. 3, where the same or comparable parts are designated by the same reference numerals, differs from the embodiment described above with respect to a plurality of arrangements.
 Firstly, the abutment 9 is not part of a supporting housing 16 which holds the entire digestion vessel 1 but of a small cap-like supporting housing 22 which, although it similarly transmits the reaction forces to the digestion vessel 1, fits around only the upper portion of the digestion vessel 1, is supported directly or indirectly against this upper portion and therefore transmits the reaction forces to this portion of the digestion vessel 1. The direct or indirect connection to the upper portion of the digestion vessel 1 is effected by a screw connection. A radial venting aperture 7 g is provided in the annular wall 7 c above the flange 1 c or the supporting ring 25. The bottom wall 1 b of the preferably cylindrical holding vessel 1 preferably has a hemispherical configuration, thus enabling the wall to be evenly stressed.
 In the embodiment shown in FIG. 4, in which the same or comparable parts are designated by the same reference numerals, the digestion vessel 1 (apart from the cover 3) and the supporting housing 16 can have substantially the same or in principle the same configuration. The important differences are the following.
 Firstly, the screwing part 7 which forms a closing part is made sufficiently long for it to extend as far as the cover 3 and therefore to be able to be braced against the cover 3, whereby the digestion vessel 1 can be closed in a tightly-sealed manner and the cover 3 rigidly closed.
 Secondly, an outlet channel 31 which is closed by a valve 32 is arranged in the cover 3, the valve 32 opening automatically if the internal pressure in the digestion vessel 1 exceeds a predetermined value. The outlet channel 31 can have an angled configuration, the first section 31 a of the outlet channel which leads from the interior space 1 d preferably extending coaxially in the cover 3 and passing over into a laterally extending outlet channel section 31 b in the upper portion of the cover 3. A valve body 33 is used to close the outlet channel 31 in an arrangement comparable to the pressure-applying plunger 6 of the previous embodiment. The valve body 33, which is, for example, cylindrical, closes the outlet channel 31 with its bottom face; in the embodiment said valve body fits into an upper widened portion 31 c of the channel which has a step face 31 d forming, for example, a right angle, which step face forms a stop for a downwardly directed movement of the valve body 33. The lateral outlet channel section 31 b branches off from the widened channel section 31 c above the step face 31 d. A spigot 33 a which is dimensioned to fit the outlet channel section 31 a with clearance for movement, and which closes the outlet channel section 31 a with small clearance for movement, may project from the underside of the valve body 33. The length c of the spigot 33 a is shorter than the length a of the widened portion 31 c of the channel.
 The valve body 33 extends downwardly from a valve body base 33 b which has an enlarged cross-section and which is dimensioned to fit, with clearance for movement, the internal cross-sectional shape and size of the screwing part 7, which in this area has a hollow cylindrical configuration, and thus is guided to be vertically displaceable therein. A stud 33 c can extend concentrically upwardly from the valve body base 33 b, which stud can for example be formed integrally with the valve body base 33 b and passes through an upper end wall 7 f of the screwing part 7 via a hole 34 and can close said hole at the upper end or project beyond it. Located between the end wall 7 f and the valve body base 33 b is a spring 35, e.g. a helical spring, surrounding the stud 33 c, which spring is supported against the end wall 7 f and preloads downwardly the closing part designated as a whole by reference numeral 36. The upper end of the stud 33 c can form with the upper end face of the screw part 7 an indicator for the pressure prevailing at any time within the movement range of the length c, or for the opening pressure.
 The closing part 36 is formed preferably in two parts with the valve body 33 and is detachably connected to the latter by means of a screw connection. In the embodiment a threaded pin 33 d projects downwardly from the valve body base 33 b, on to which pin the valve body 33 is screwed by means of a threaded bore 33 e and is thereby retained detachably and exchangeably. For the purpose of adaptation to different covers 3 a plurality of different valve bodies 33 adapted in each case to the associated cover 3 and which can be exchangeably attached to the valve body base 33, can be provided.
 A stop 37 to limit the downward movement of the closing part 36 can be arranged at the lower end of the screwing part 7. In the embodiment a disc flange 38 is preferably detachably fixed to the lower portion of the screwing part 7, which disc flange has a coaxial bore 39 by means of which the valve body 33 passes through the disc flange 38 and can also, for example, be guided therein, and projects downwardly beyond the disc flange 38.
 For a digestion process the filled digestion vessel 1 is placed below the valve body 33 and the screwing part 7 is screwed manually against the cover 3 using a rotation tool 41 which is positively connectable by means of a non-circular socket connection or two socket connections 42 to the upper end of the screwing part 7, whereby the digestion vessel 1 is rigidly closed with an axial loading. In this case the screwing part 7 bears against the cover 3 with its disc flange 38, the pressure per unit area being lower because of the relatively large area of the disc flange 38. When the screwing part 7 is moved towards the cover 3 the valve body 33 contacts the valve seat 33 a, and the valve body base 33 b or the closing part 36 is preloaded by the preload distance V against the spring 35, the spring 35 pressing against the valve seat with a given preload, e.g. 200 N/mm. In the embodiment the first section 33 a of the outlet channel has a diameter of approx. 8 mm.
 In the embodiment the valve body 33 is made of a corrosion-resistant material e.g. ceramic or plastics material, preferably PTFE. The valve body base 33 b can be made e.g. of stainless steel or ceramic material. The screwing part 7 and the disc flange 38 are made preferably of corrosion-resistant metal, in particular special steel. This also applies to the spring 35, which may be made e.g. of spring steel.
 To protect the interior space of the screwing part 7 against fouling an annular seal 42, preferably comprising a sealing ring 42 a mounted in an annular groove in one component, in particular in the valve body base 33 b, is arranged between the valve body base 33 b and the wall of the screwing part 7 and cooperates sealingly with the opposing wall of the other component. An annular seal 42 of this kind can also be provided between the disc flange 38 and the valve body 33.
 For a digesting operation the vessel arrangement is placed in a microwave oven and microwaves are applied thereto. If the internal pressure in the digestion vessel 1 exceeds a predetermined value because of internal heating and, if applicable, a reaction of the material contained in the digestion vessel, the valve body 33 automatically lifts from the valve seat, whereby the outlet channel 31 is opened and the internal pressure is limited by means of venting through the outlet channel 31.
 In all the embodiments it is advantageous for the device 5 or the valve 32 to be configured to be so adjustable that the cover 3 or the valve 32 opens at different pressures in the digestion vessel 1. By this means the opening pressure and therefore the pressure effective during treatment or digestion of the material can be changed and adjusted to a predetermined requirement. This can be achieved, for example, in that a plurality of resilient pressure-generating means 8 or a plurality of springs 35 of differing compressive force which are optionally usable by exchange, are provided. The device 5 or the valve 32 can thereby be adapted to opening pressures of different values.