Threaded pressure vessel

Abstract

The present invention relates to a pressure vessel (10) for receiving a gas at elevated pressure, comprising a vessel housing (12), which has a first opening (14) at at least one side and is equipped in the region of the first opening (14) with a first internal thread (14a) that has a first thread pitch, and a vessel cover (18), which is equipped with a first external thread (18a) and is configured to be screwed onto the first opening (14) of the vessel housing (12), wherein the first external thread (18a) of the vessel cover (18) has a second thread pitch. According to the invention, a first pitch difference is provided between the first thread pitch of the first internal thread (14a) and the second thread pitch of the first external thread (18a).

Description

[0001] Bolted pressure vessel

[0002] Description

[0003] The present invention relates to a pressure vessel for containing a gas under increased pressure, comprising a vessel housing which has a first opening on at least one side and is provided in the region of the first opening with a first internal thread having a first thread pitch, and a vessel lid which is provided with a first external thread and is adapted to be screwed onto the first opening of the vessel housing, wherein the first external thread of the vessel lid has a second thread pitch.

[0004] Such pressure vessels must be designed to withstand high internal pressure and be pressure-tight. They can be used for various applications, such as separating media based on their physical state, for example, to separate a liquid from a gaseous phase, storing gaseous media at high pressure, or as containers for filter elements that, for example, process previously compressed gas. Due to their use in corresponding systems, these vessels are frequently exposed to pressure fluctuations, which they must be able to handle without damage over their service life.Depending on the application, operating pressure and pressure fluctuation range of such pressure vessels, their service life is limited to a certain permissible number of load cycles and such pressure vessels must be replaced as soon as the maximum permissible number of load cycles has been reached during their operational service life.

[0005] In the prior art, a widely used design for pressure vessels, particularly for separating condensate after compression and for filtering corresponding gas, is to provide screwed pressure vessels. These essentially consist of two or three components, namely a vessel housing, a vessel lid, and possibly also a vessel base. Accordingly, the vessel housing each comprises either a single or two separate openings into which the vessel lid and, if applicable, the vessel base are screwed in the assembled state. For this purpose, the vessel housing is hollow with at least one internal thread in the region of the corresponding opening or openings, and the vessel lid and, if applicable, the vessel base have corresponding complementary external threads to allow them to be screwed onto the vessel housing.

[0006] In particular, standard threads are often used for this purpose today, for example, metric threads according to DIN 13 or UN threads according to ASME B1.1. In such standard threads, the radius at the notch root of the external thread is larger than the radius at the notch root of the internal thread, since in a conventional screw connection, the bolt or the screwed-in component is the more heavily loaded component.

[0007] However, the design method just described has several shortcomings, as the corresponding threads are not specifically adapted to the application described here, but rather rely on standard threads. Experience has shown that, during operation of such state-of-the-art pressure vessels, a crack develops over time in the innermost thread of the vessel housing, depending on the operating pressure and pressure fluctuation range. The reason for this crack formation and subsequent crack propagation is the notch effect at the thread notch root, as the geometry of a metric standard thread is not designed for the pulsating load of the female thread or internal thread.

[0008] Vibration or fatigue cracks resulting from this type of cracking increase with continued operation of the pressure vessel until the remaining material can no longer absorb the resulting force and fails with a violent crack. During operation with compressible media, this poses significant hazards, as the corresponding vessel lid, along with the annular fragment of the vessel housing, is thrown into the air, and the remaining vessel is pressed into the ground by the counterforce.

[0009] To prevent such accident scenarios, the number of permissible load cycles is determined during pressure vessel testing and specified in the corresponding operating instructions for the respective pressure vessel type. When the maximum permissible number of load cycles is reached, the pressure vessel in question must be scrapped, and regular tests must be conducted on the vessel, even during its intended service life.

[0010] As a further measure to prevent such accidents, various standards have stipulated safety factors for the permissible operating pressure of such pressure vessels, some of which can be as high as a factor of 4. Tests have shown that, in addition to leaks, two main causes of vessel failure occur during burst tests: longitudinal fracture of the vessel casing and thread shearing.

[0011] Of these, longitudinal fracture of the container casing can be prevented relatively easily by sufficiently increasing the wall thickness, with the only disadvantage being that the material required is increased. In a similar way, resistance to thread shearing could also be improved by enlarging or lengthening the thread. However, such an enlargement would result in a lower permissible number of load cycles for the same container geometry because this would reduce the material thickness of the internal thread, which is again undesirable. At the same time, it has been shown that the effect that can be achieved by lengthening the thread is only slight because the stresses that occur in the thread are distributed inhomogeneously and corresponding simulations have shown that the innermost thread is always the one subject to the greatest load.Furthermore, extending the thread would require significantly more material and increase manufacturing costs, which is undesirable for cost reasons. It is therefore desirable to increase the permissible number of load cycles in such pressure vessels while maintaining the same vessel geometry. Should the appropriate measures increase the number of permissible load cycles to such an extent that the economically desired number of load cycles is exceeded, consideration could even be given to reducing the wall thickness of the pressure vessel. The resulting material savings could reduce the costs of manufacturing such pressure vessels.In any case, however, a higher number of permissible load cycles has the advantage that the service life of the corresponding vessels can be increased and the prescribed inspection intervals can be extended, which could already save costs during the service life of such pressure vessels.

[0012] Accordingly, it is the object of the present invention to further develop a generic pressure vessel of the type described above in such a way that its service life or the number of permissible load cycles is increased and thereby, in the manner described above, more efficient operation and cost-effective production of corresponding pressure vessels is enabled, while at the same time the risk of serious accidents is to be reduced.

[0013] For this purpose, it is proposed according to the invention that there is a first pitch difference between the first thread pitch of the first internal thread of the container housing and the second thread pitch of the first external thread of the container lid.

[0014] The use of a thread pitch difference has only been used in a very small number of special cases due to increased technical requirements and the associated costs, for example due to the need for special tools and special testing equipment as well as generally non-standardized parts. However, it is clear that in the specific application at hand in the field of pressure vessels for gases, the advantages of such a solution clearly outweigh its disadvantages, as will be explained in more detail below. The characteristic feature of a thread pitch difference is the ratio of the thread pitch of the first internal thread to that of the first external thread. For example, the thread pitch difference would be 1% if one of the two pitches is 2 mm and the other is 2.02 mm.

[0015] It is to the credit of the inventors of the present invention to have recognized that by increasing the thread pitch, in particular of the first external thread of the container lid, or by reducing the thread pitch of the first internal thread of the container housing, an advantageously modified load distribution within the thread is achieved. Since in this context the maximum stress at the operating pressure of the corresponding pressure vessel is shifted from the thread notch root of the innermost thread to the thread notch root of one of the outer threads, a resulting fatigue crack will form at the corresponding axially further outward location, i.e. in particular at one of the outer threads. Furthermore, the improved distribution of stress level prevents or reduces the formation of stress peaks.Resulting stress peaks are reduced, so that an increased service life can be expected when using materials susceptible to stress corrosion cracking.

[0016] By changing the position of a fatigue crack from the innermost thread to one of the outer threads while maintaining the same vessel geometry, the safety risk caused by a crack or breakage of the corresponding thread is significantly reduced. In such a case, the pressure vessel would continue to function for a short time, as the undamaged part of the thread would continue to bear the load caused by the gas pressure and would not fail immediately.

[0017] In the event of a complete rupture, only the severed ring- or spiral-shaped vessel would fall, not shoot through the air. Furthermore, during regular inspections of corresponding vessels or the higher-level system, which are already required anyway, such a crack or rupture would be detected early, and appropriate measures could be taken; in particular, the corresponding pressure vessel could be withdrawn from service in good time before a catastrophic failure occurs. Another conceivable scenario in this context would be a fatigue crack in the vessel lid, which could lead to leaks before a vessel failure, i.e., a leak before a rupture, which would also represent a significantly lower risk than the catastrophic failure of state-of-the-art pressure vessels described above.

[0018] Furthermore, it has been shown that the aforementioned more homogeneous load distribution at higher pressures also allows the corresponding thread to achieve a higher burst pressure. Accordingly, for vessels that fail due to thread shearing, the burst pressure can be increased while maintaining the same vessel geometry, which is why existing standard requirements could still be met even with a shortened thread and corresponding material savings.

[0019] Furthermore, it has been shown that introducing a thread pitch difference without simultaneously adjusting the corresponding thread tolerances can result in an increase in the screw-in torque and, in extreme cases, even in thread jamming. This risk increases with increasing thread pitch difference, and it is therefore advisable to adjust the tolerance field position of the corresponding thread accordingly.

[0020] In particular, the corresponding tolerances in the internal and external threads can correspond to tolerance class 4 of DIN ISO 965-1. The individual tolerance changes to be made, which result from a coating or surface treatment, must be designed in such a way that, for a given thread length, there is no overlap between the external and internal threads. In particular, the tolerance adjustment can be made on the thread assumed to be less critical in the specific case. By utilizing an appropriate coating tolerance, the desired tolerance field position can be achieved while taking the thread pitch difference into account. Accordingly, the required adjustment of the tolerance field position depends on the flank angle, thread length and pitch difference, with large tolerances leading to a reduction in the flank overlap.In order to achieve higher thread strength, the flank engagement should be as high as possible, which is why a compromise must be reached between the pitch difference and the flank engagement.

[0021] While so far only a pressure vessel according to the invention with a vessel housing and a vessel lid, i.e. a single screw-on part, has been discussed, in a further embodiment of the invention the vessel housing could further have a second opening on a second side and be provided with a second internal thread in the region of the second opening, which second internal thread has a third thread pitch, and the pressure vessel could further comprise a vessel base which is provided with a second external thread and is designed to be screwed onto the second opening of the vessel housing, wherein the second external thread of the vessel base has a fourth thread pitch and wherein there is a second pitch difference between the third thread pitch of the second internal thread and the fourth thread pitch of the second external thread.

[0022] It is understood that the terms "vessel lid" and "vessel base" are not intended to represent a restriction regarding the orientation of the pressure vessel. Rather, they should be understood in such a way that a pressure vessel with only one screwed-on closure is referred to as the "vessel lid," while two screwed-on closures are referred to as "vessel lid" and "vessel base" in order to distinguish them from each other. The same applies to the first and second internal and external threads, as well as the first to fourth thread pitches, which have been designated in this way solely for the sake of their distinguishability.

[0023] In such embodiments, the first and third, as well as the second and fourth, thread pitches can be essentially identical in pairs, which can make the manufacture of corresponding pressure vessels simpler and more cost-effective, since the corresponding threads can be manufactured in the same way without requiring any corresponding adaptation of the tools used. It is understood that the first pitch difference and the second pitch difference are also identical to each other in such cases.

[0024] On the other hand, applications are also conceivable in which different values ​​are deliberately chosen for the first and third, as well as the second and fourth thread pitches, thus resulting in different pitch differences. In such a case, one of the two resulting threads could be designed with less resistance, making it highly likely that the corresponding thread would fail first, which in turn could represent a safety advantage in certain situations.

[0025] Furthermore, it has been shown that the first gradient difference and / or the second gradient difference can preferably be in the range from 0.2 to 1.0%, in particular in the range from 0.3 to 0.5%, in order to achieve the advantageous effect described above.

[0026] Furthermore, the pressure vessel according to the invention can comprise at least one sealing element between the first opening and the vessel lid and / or between the second opening and the vessel bottom, in particular at least one O-ring located in a sealing groove in the vessel housing or the vessel lid or vessel bottom. In this way, an improved sealing behavior of the pressure vessel according to the invention is achieved in the region of the screwed-on vessel lid and / or vessel bottom.

[0027] As already mentioned above, metric standard threads are not particularly suitable for the pulsating loads on internal threads in this application. Accordingly, the invention can provide the first internal thread and / or the second internal thread with a larger radius in the thread notch root than the corresponding external thread. This can reduce the notch effect in the container housing and increase the number of load cycles until a crack is initiated and the thread breaks. Furthermore, this also reduces the stress peaks that occur in the thread notch root and lowers the risk of stress corrosion cracking when using material susceptible to stress corrosion cracking. Certain minimum radii can be specified for both the respective internal thread and the corresponding external thread.

[0028] Furthermore, the thread notch root or the entire thread of at least one of the internal threads and / or external threads can be hardened, in particular by milling grooves or thread rolling, whereby a higher number of load cycles can also be expected.

[0029] Alternatively or additionally, the vessel housing can comprise a predetermined breaking point in the region of the first internal thread and / or the second internal thread, in particular in the form of a section with reduced wall thickness or a circumferential notch. By providing such a predetermined breaking point on the outside of the vessel housing, a location can be defined near the corresponding thread which can be checked for incipient cracks during regular inspections. Such an inspection is easier to perform on the outside than on the inside and, due to the corresponding simplification, it can be considered to additionally perform such an inspection during maintenance. Furthermore, due to the reduced hazard potential in this way, consideration could be given to reducing safety factors when designing corresponding pressure vessels.

[0030] Although different designs of the at least one thread in the pressure vessel according to the invention are conceivable with regard to diameter, length and other parameters, for example depending on the intended internal pressure in the pressure vessel during its operation and the geometric dimensions of the pressure vessel, in any case the first internal thread and / or the second internal thread can be designed as a single-start internal thread, which in such a case also applies accordingly to the respective associated external thread.

[0031] Furthermore, it should be noted that the pressure vessel according to the invention can be designed and provided for separating media depending on their state of aggregation, i.e., for example, a gaseous phase from a liquid phase, for storing media or as a container for a filter element.

[0032] Further features and advantages of the present invention will become more apparent from the following description of an embodiment thereof, when considered together with the accompanying figures. These show in detail:

[0033] Figure 1 shows a schematic cross section through a screwed pressure vessel according to the invention;

[0034] Figure 2 is a detailed view of the container lid area of ​​Figure 1; and

[0035] Figures 3A to 3C are explanatory schematic views of the threads used in the pressure vessel of Figure 1, in particular the overall thread, the internal thread and the external thread.

[0036] Figure 1 initially shows a pressure vessel according to the invention for containing a gas under increased pressure in a schematic cross-sectional view and is generally designated by the reference numeral 10. In the specific embodiment shown here, it is a filter vessel in which a filter medium can be accommodated, through which a gas under excess pressure can be passed for its filtering.

[0037] The pressure vessel 10 comprises a substantially cylindrical vessel housing 12, which has a first opening 14 and a second opening 16 on its upper and lower sides in Figure 1. The vessel housing 12 is provided with a first internal thread 14a in the region of the first opening 14, which has a first thread pitch, and with a second internal thread 16a in the region of the second opening, which has a third thread pitch.

[0038] Screwed into the first opening 14, the pressure vessel 10 further comprises a vessel lid 18 which is provided with a first external thread 18a complementary to the first internal thread 14a and having a second thread pitch, wherein according to the invention there is a first pitch difference between the first thread pitch of the first internal thread 14a and the second thread pitch of the first external thread 18a.

[0039] Similarly, a container base 20 having a second external thread 20a with a fourth thread pitch is screwed into the second opening 16 of the container housing 12. It should also be noted that conduit elements 20b are provided in the region of the container base 20, which serve to convey pressurized gas into the interior of the container housing 12 for filtration and to discharge it therefrom again.

[0040] Furthermore, it should be noted that, as can be seen particularly in Figure 2, a sealing element 22 is provided between the first opening 14 and the container lid 18, namely an O-ring located in a sealing groove 22a in the container lid 18. Furthermore, a corresponding sealing element 22 is provided in the same way in the region of the container base 20, but is only indicated schematically in Figure 1. It should also be noted that the container housing 12 is provided with a notch 24 in the region of the axially outer end of the first internal thread 14a, which can act as a predetermined breaking point in order to preferably trigger a corresponding crack in the material at a location that represents a lower risk of catastrophic failure of the pressure vessel 10.Furthermore, the provision of the predetermined breaking point facilitates recurring inspection of the pressure vessel, as the location of a potential crack is already determined, eliminating the need to inspect a significantly larger area. This can save time and money, for example, in the case of dye penetrant testing.

[0041] Finally, reference is made to the three views in Figures 3A to 3C, in which the respective threads of the pressure vessel from Figure 1 are shown in schematic detailed views, which result from the interaction of the first internal thread 14a with the first external thread 18a and the second internal thread 16a with the second external thread 20a.

[0042] Figure 3A shows the resulting overall thread, Figure 3B the corresponding internal thread and Figure 3C the associated external thread, whereby the relevant sizes or parameters of the corresponding threads are shown in each case, in particular the core diameter of the internal thread Di, the pitch diameter of the internal thread D2, the outer diameter of the internal thread D4, the outer diameter of the external thread (inner diameter) d, the pitch diameter of the external thread d2, the core diameter of the external thread ds, the height of the initial triangle of the thread profile H, the flank coverage Hi, the thread pitch P and the respective radii R of the internal thread and r of the external thread.

[0043] By appropriately selecting the parameters mentioned and in particular a pitch difference in the range of 0.2 to 1.0% between the two threads, the advantages according to the invention described above are achieved, while at the same time the desired functionalities of the corresponding threads can be ensured by an appropriate tolerance design.

Claims

Claims 1 . Pressure vessel (10) for containing a gas under increased pressure, comprising: - a container housing (12) which has a first opening (14) on at least one side and is provided in the region of the first opening (14) with a first internal thread (14a) which has a first thread pitch; and - a container lid (18) which is provided with a first external thread (18a) and is designed to be screwed onto the first opening (14) of the container housing (12), wherein the first external thread (18a) of the container lid (18) has a second thread pitch; characterized in that there is a first pitch difference between the first thread pitch of the first internal thread (14a) and the second thread pitch of the first external thread (18a).

2. Pressure vessel (10) according to claim 1, characterized in that: - the container housing (12) further comprises a second opening (16) on a second side and is provided in the region of the second opening (16) with a second internal thread (16a) having a third thread pitch; and - the pressure vessel (10) further comprises a vessel base (20) which is provided with a second external thread (20a) and is designed to be screwed onto the second opening (16) of the vessel housing (12), wherein the second external thread (20a) of the vessel base (20) has a fourth thread pitch; wherein a second pitch difference exists between the third thread pitch of the second internal thread (16a) and the fourth thread pitch of the second external thread (20a).

3. Pressure vessel (10) according to claim 2, characterized in that the first and third as well as the second and fourth thread pitches are essentially identical in pairs.

4. Pressure vessel (10) according to one of claims 1 and 2, characterized in that the first and third as well as the second and fourth thread pitches each have different values ​​and thus also different pitch differences are implemented.

5. Pressure vessel (10) according to one of the preceding claims, characterized in that the tolerance field position of the thread formed by the first internal thread (14a) and the first external thread (18a) and / or by the second internal thread (16a) and the second external thread (20a) is adapted such that, for a given thread length, there can be no overlap of the respective external thread (18a, 20a) and internal thread (14a, 16a).

6. Pressure vessel (10) according to one of the preceding claims, characterized in that the first gradient difference and / or the second gradient difference is in the range from 0.2 to 1.0%, preferably in the range from 0.3 to 0.5%.

7. Pressure vessel (10) according to one of the preceding claims, characterized in that it further comprises at least one sealing element (22) between the first opening (14) and the vessel lid (18) and / or between the second opening (16) and the vessel bottom (20), in particular at least one O-ring located in a sealing groove (22a) in the vessel housing (12) or the vessel lid (18) or the vessel bottom (20).

8. Pressure vessel (10) according to one of the preceding claims, characterized in that the first internal thread (14a) and / or the second internal thread (16a) are provided with a higher radius in the thread notch root than the associated respective external thread (18a, 20a).

9. Pressure vessel (10) according to one of the preceding claims, characterized in that the thread notch root or the entire thread of at least one of the internal threads (14a, 16a) and / or external threads (18a, 20a) is hardened, in particular by milling or thread rolling.

10. Pressure vessel (10) according to one of the preceding claims, characterized in that the vessel housing (12) comprises a predetermined breaking point (24) in the region of the first internal thread (14a) and / or the second internal thread (16a), in particular in the form of a section with reduced wall thickness or a circumferential notch.

11. Pressure vessel (10) according to one of the preceding claims, characterized in that the first internal thread (14a) and / or the second internal thread (16a) are designed as a single-start thread.

12. Pressure vessel (10) according to one of the preceding claims, characterized in that it is designed for separating media depending on their state of aggregation, for storing media or as a container for a filter element.

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