WO2003071181A1

WO2003071181A1 - Pressurised container comprising a tubular connection

Info

Publication number: WO2003071181A1
Application number: PCT/CH2003/000126
Authority: WO
Inventors: Thomas Jeltsch
Original assignee: Ems-Chemie Ag
Priority date: 2002-02-22
Filing date: 2003-02-20
Publication date: 2003-08-28
Also published as: ES2279091T3; JP2005527415A; EP1476694B1; DE50305953D1; JP4257212B2; EP1476694A1; KR100718945B1; DK1476694T3; KR20040105722A; DE10207613C1; ATE348288T1; AU2003205485A1

Abstract

The invention relates to a pressurised container (1) comprising at least one tubular connection (2) to an opening in the container wall. According to the invention, the cross-sections of the opening and at least the tubular connection (2) in the vicinity of the opening are oval-shaped, the large axis of the oval shape being aligned with the direction of the first primary stress, which would be generated if the pressurised container (1) were subjected to an internal pressure at the connection point without the opening and the connection (2). This permits both the local and global maximum comparison stress to be advantageously reduced in the component when the latter is subjected to a pressure and/or the stress to be displaced into areas, in which it is not as critical. The invention can be used in a particularly advantageous manner for injection-moulded parts consisting of plastic, as joint lines in many geometrical forms cannot be avoided in precisely those areas where the most stress occurs as a result of the geometry of said parts.

Description

DESCRIPTION

TITLE

Pressure vessel with a tubular connection

TECHNICAL AREA

The present invention relates to a pressure container with at least one tubular connection at an opening in the container wall.

If a pressure vessel e.g. If an internal, pneumatic or hydraulic overpressure is applied, tensions occur in the container wall. To minimize the stresses and to maximize the ratio of volume to surface, pressure vessels are constructed as spherical or cylindrical as possible. If a connection is attached to such a container, the stresses concentrate in the area of the connection, as a result of which this area is subjected to higher loads and forms a weak point of the pressure container.

In the case of plastic injection-molded pressure vessels with tubular connections, such as those used for water meter housings, there are weld lines in the direction of flow of the injection molding material behind the connections due to the manufacturing process, and thus in an area in which the stresses also concentrate. This means a considerable weakening for the component, since an increased load occurs precisely where the weakest material properties have to be expected at the same time. Even in the case of T-pieces or the like, in which a pipe socket branches off from a pipe section with approximately the same cross-section, significant stress concentrations arise on the inside of the bend of the branch when subjected to internal pressure. In the case of injection-molded T-pieces, there are weld lines in this area and the demolding concept creates sharp edges. The collapse of the stress concentrations with the weld lines and the edges again leads to a considerable weakening of the component.

Fundamentally similar problems occur with pipe bends.

The stresses for each point of the container wall can be described mathematically by stress tensor. The invariant of the stress tensor against coordinate transformations are three so-called main stresses, one of which is defined as the first main stress in the technical terminology of stress tensor mechanics. In the case of a container under an internal overpressure, the first main stress corresponds to the greatest tensile stress. This term is also used in the following. So-called main directions are assigned to the main stresses.

STATE OF THE ART

In order to compensate for the weakening of the pressure vessels through the installation of connections, it is known to provide reinforcements for the container wall and / or the connection. However, the attachment of such reinforcements is not always possible, in particular retrospectively, and / or causes additional effort and use of materials.

In WO 01/69187 A1, in relation to a pressure vessel which can be used in particular as a water meter housing, it is proposed to design the bottom part of the latter to be elliptical in order to reduce the voltage. It remains unclear which floor shape WO 01/69187 A1 is based on, since hemispherical or at least approximately elliptical floor shapes (so-called bobbin floors) have of course long been known in the prior art and would have at least equally good properties. In addition, WO 01/69187 A1 proposes to shift tensions away from weld seams in the area between two connections and the lower part of the housing, which is identical to the base part Make connections elliptical. 2 shows an example of this. WO 01/69187 A1 does not make any statements about which would allow an understanding of the stress shift claimed, nor does it make any statements about where the voltages are shifted to.

PRESENTATION OF THE INVENTION

The object of the present invention is to specify how the problems mentioned in the connection area of tubular connections can be reduced in the case of pressure vessels and thereby pressure vessels can be produced which can be loaded with a higher pressure difference or which have a higher safety reserve with the same load.

To achieve this object, the invention proposes ovalizing the connection in the connection area. The cross-sections of the opening and the tubular connection at this opening are oval and aligned with the major axis of the oval shape in the direction of the first main stress, which would result if the pressure vessel were subjected to an internal overpressure at the connection point without the opening and the connection ,

The assumption of an internal overpressure serves only as a thought model for determining the direction of the first main stress and thus the direction of the large axis of the oval shape. The resulting geometry generally also applies to an external overpressure. To simplify the description, however, essentially only the internal pressure case is considered below.

The ovality O can be expressed by the ratio of the length of the large (D) and the small (d) axis of the oval shape and is, depending on the application and geometry, approximately in the range between 1, 1 and 3.0, preferably in the range between 1, 1 and 2.5, and particularly preferably in the range between 1, 1 and 1, 9.

As a simple example of the direction of the greatest main stress, an essentially circular-cylindrical pressure vessel can be considered, in which stresses in the vessel wall result in internal pressure loading which are approximately twice as great in the circumferential direction as in the longitudinal direction. A tubular, at least in According to the invention, the connection area of oval connecting pieces would therefore have to be connected to the container wall with its large axis aligned in the circumferential direction.

In the case of purely spherical containers or those with a relatively large, flat container wall, the invention is not used since, due to the geometry, no major main stress occurs.

In the context of the invention, an oval shape is to be understood quite generally as an elongated round shape, of which e.g. an elliptical shape would represent a special case.

The ovalization of the connection in the mentioned main voltage direction not only has a significant influence on the reduction of the maximum voltages that occur in the pressure vessel wall. They can be used to shift the voltages that occur to other areas, e.g. in those where there are no weld lines or sharp edges and where they have a less adverse effect.

A certain disadvantage is the reduction in the cross-section available, for example, for a flow, which is associated with ovalization, if this is brought about from a circular cross-sectional shape by flattening this shape or by reducing an axis. In order to achieve the advantages according to the invention, however, it is sufficient to carry out the ovalization only directly in the connection area and to continuously change back to a circular tube shape within a relatively short distance. The length of this transition section L can e.g. are in the order of magnitude of the diameter of the circular shape of the connecting pipe, or may also be very short. Such a limited cross-sectional reduction only causes small pressure losses, which are usually negligible in relation to the losses of an overall system.

However, the invention naturally also includes pressure vessels which are not flowed through and are under static pressure.

The term pressure vessel and / or the tubular connection is to be understood very generally within the scope of the present invention. In addition to that Example already given in principle also combinations of pressure vessels and connections, in which the pressure vessel and the connection have approximately the same cross-section, as is the case, for example, with so-called T-pieces. A pipe bend can be understood as a limit case of a cylindrical pressure vessel closed on one side and reduced in diameter with a pipe socket attached to the side at the closed end, whereby there is no longer even a difference between the part serving as pressure container and the part serving as pipe socket. On the other hand, a pipe bend can also be regarded as a one-armed T-piece.

Furthermore, as already mentioned, the term pressure vessel should not be interpreted restrictively in the sense that it always only has an internal overpressure compared to e.g. external normal pressure is loaded. The inventive principle and the resulting geometry remain the same for every pressure difference situation and thus also for an external overpressure compared to e.g. an internal normal pressure, such as this e.g. in the case of submarines or the like. This means that it can be used both as an internal pressure vessel and as an external pressure vessel.

In a special embodiment, the pressure container according to the invention can be a gas generator housing for at least one airbag, as is increasingly used in automobile construction. This is a pressure container through which a gaseous medium is directed into the corresponding air bags after the airbag has been triggered. This creates very high pressures in a very short time, which the housing has to withstand.

BRIEF EXPLANATION OF THE FIGURES

The invention will be explained in more detail below on the basis of exemplary embodiments in connection with the drawing. Show it:

Figure 1 is a perspective view of a cylindrical container with an oval pipe socket attached.

Fig. 2 under a) in a sectional view the housing of a water meter with two opposite tubular connecting pieces, under b) a side view and under c) a top view of the same housing; 3 under a) - d) each quarter segments of a housing corresponding to that of Fig.

2, however, with connection pieces of different shapes, the graded gray tint identifying the resulting stress condition in the case of an internal pressure load; white tones mean high and black tones mean low voltage;

Fig. 4 in a bar chart, the local and global maximum

Reference voltages for different connection ovalities in a container of the type of FIG. 2;

5 shows in a view and in several cross sections a T-piece ovalized according to the invention;

6 shows a pipe bend ovalized according to the invention, cut open in a perspective view;

Fig. 7 under a) - d) each half-shells of pipe bends corresponding to that of Fig. 6 but with different ovality, whereby the gradual gray tint shows the resulting stress condition with an internal pressure load;

8 shows the local and global maximum in a bar chart

Comparative stresses for different ovalities in a pipe bend corresponding to that of Fig. 6; and

Fig. 9 sectional drawings through a gas generator housing for a head airbag system.

WAYS OF CARRYING OUT THE INVENTION

1 shows, as a first simple example, a pressure vessel 1 with a circular cylindrical cross section, to which a pipe socket 2 is connected laterally to the lateral surface. If you think away the connection piece 2, then three main stresses can be specified at the connection point of the pipe socket for a load that occurs when the pressure vessel is placed under an internal overpressure. Under the junction of the For example, the raw connector should be understood to be the point of impact P of the connector axis A on the wall of the container 1. Because of the circular cylindrical shape of the pressure vessel 1, the tension in the circumferential direction is approximately twice as great as the tension in the longitudinal direction of the jacket. By making the pipe socket 2 oval in cross-section and by aligning the large axis of the oval shape in the circumferential direction of the container 1 and thus in the direction of the first main stress, a reduction of the stresses and a favorable distribution of the stresses, especially in the connection area of the Pipe socket 2 reached. As a result of the geometry, the small axis of the oval shape is oriented perpendicular to the large axis and thus in the longitudinal direction of the jacket.

Fig. 2 shows under a), b) and c) a pressure vessel 3, as e.g. used for water meters in three views. The pressure vessel 3 has an essentially cylindrical main body 4, which in the present example is closed at the bottom by a spherical cap. Two connecting pieces 5 and 6 are attached to the side of the cylindrical part 4. The connecting piece 5 is oval in the transition region L and the cut surface of its connection point and is therefore designed according to the present invention. The direction of the major axis of the oval shape is, as can be seen under b), oriented in the circumferential direction of the cylindrical part. In the present example, the large axis corresponds to the circular diameter of the connecting pipe, as can also be seen in the top view under c). The maximum reference voltages occurring in the connection area are thereby reduced and at the same time shifted.

Towards the free end of the connecting piece 5, the oval cross section changes into a circular cross section, as a result of which the connecting piece can be connected to other pipelines or the like much more easily, for example also by means of a screw connection. The transition length L between the oval shape and the circular shape is chosen so that there is a relatively smooth transition and no additional stresses are generated. In the example, the transition length L corresponds to approximately half the diameter of the circular shape.

The connecting piece 6 is not ovalized and is therefore not designed according to the invention. It is used here only for a comparative representation of the state of the art. 3 shows under a) - d) each quarter segment 7 of a housing corresponding to that of FIG. 2 but with differently shaped connecting pieces 8.1. - 8.4, whereby the shading indicates the arithmetically resulting stress state with an internal pressure load. The connection stubs are designed to be closed only for computational reasons. The stress state calculation is based on a finite element model. The inherently multidimensional stress state is indicated by a scalar quantity, the so-called comparative or von Mieses stress σ _v , which can be derived from the stress state in a defined manner known to the person skilled in the art. This type of characterization of the stress state is also used in the context of FIGS. 4, 7 and 8 described below.

First, Figure a) shows the case with a connection piece with a circular cross-section as a reference, whereby a diameter of 40 mm was assumed for the calculation, which is a common pipe diameter e.g. of water pipes. High voltage concentrations can be seen in zones I and II (white areas in FIG. 3a)). If the container is manufactured by injection molding by spraying onto the center of the spherical cap (top in FIG. 3a)), a weld line also results approximately along the cutting edge designated 9 in the direction of flow behind the connecting piece 8.1 by the confluence of two melt streams in the injection mold. Since this coincides with zone II of high voltage, two unfavorable influencing factors with regard to the strength of the container accumulate here.

The sequence of images b) - c) and d) shows how the tensions in areas I and II are reduced and also shifted. In picture b) the ovality is 40:32, in picture c) 40:26 and in picture d) 40:22. The second number corresponds to the length of the small axis d in millimeters, while the large axis D was left at the aforementioned 40 mm of the circular reference cross section. The most favorable case is likely to be shown in Figure c), since there are hardly any stress concentrations and the stresses are fairly evenly distributed at a comparatively low level. In contrast, in Figure d) a zone of high voltage has already formed again in area III.

The influence of the nozzle ovality on the maximum comparison voltage is additionally shown in FIG. 4 in the form of a bar diagram, with additional ones in addition to FIG. 3 Ovalities are taken into account. The ovality varies along the horizontal axis, which is expressed on the one hand by the ratio D / d of the length D of the long axis to the length d of the short axis, for example in millimeters. The resulting ratio for the ovality O is also given in each case. The height of the hatched bars indicates, in relative unit σ _v, on the one hand the maximum reference stress occurring at I or II, referred to as local, while the height of the non-hatched bars indicates the maximum reference stress somewhere, referred to as global, in Figure d) e.g. at the Digit III, indicates. As long as the global maximum is at positions I and II, the local maximum comparison voltage is identical to the global one. As can be seen, the local as well as the global maximum reference stress, starting from the round cross-section (ovality 40:40 or 1), initially decreases with increasing ovality. The global maximum comparison voltage only increases again from the middle of the diagram, because from here a voltage zone begins to form in area III. The local maximum comparison voltage in areas I and II, however, continues to decrease. Depending on the requirements placed on the component, the ovality is suitably dimensioned according to the voltage profiles shown in FIG. 4, an ovality in the range from 1.5 to 1.7 being suitable, for example. Taking into account the reduced material strength along the weld line of injection molded parts and if a maximum burst pressure for the component is to be reached, it may also make sense to choose the ovality in the range in which the global maximum reference stress is already rising again.

2, the global as well as the local maximum comparison voltage can be reduced by more than 50%.

On the far right in FIG. 4, the case is shown for comparison, which would result for a round cross section without ovality with a diameter of 30.6 mm. With this diameter, the cross-sectional area is the same as in the case and 24 mm. It can be seen from the height of the bars in this case that a simple reduction in cross-section without simultaneous ovalization cannot achieve the effect of a well-known stress reduction which is desired according to the invention. Conversely, one could also say that the reduction in the maximum reference stress is primarily due to the ovalization and only to a small extent to the decrease in the free cross-section due to the reduction in the minor axis according to FIG. 4 (variant 40:24) , As a further application example for the teaching according to the invention, FIG. 5 shows a T-piece 10 with three ovalized connections 11, 12 and 13, which all lie in the same plane (drawing plane) and also have approximately the same cross section. The large axis of the oval shape of the three connections is selected according to the invention perpendicular to the plane mentioned. The opposite connections 11 and 12 could, for example, be equated with the container 1 of FIG. 1, the connection 13 in this case corresponding to the pipe socket 2. In contrast to the example of FIG. 1, all three connections are ovalized here, as can also be seen from the cross sections BB and CC shown. In principle, however, it would also be possible to ovalize only two of the three connections, for example the connections 11 and 12, or only one of the connections, for example the connection 13. As with the connecting piece 5 of FIG. 2, the oval cross-section of all three connections is converted back to their free ends over a relatively short distance into a round cross-sectional shape, so that in each case round pipes can be connected to the connections, for example via a screw connection. The round cross sections are denoted by AA or DD in FIG. 5.

In the application example of Fig. 5, e.g. one of the two opposite connections 11 or 12 away, you get a 90 ° elbow.

FIG. 6 shows a special case, namely a pipe bend 14 which can be produced by injection molding and is cut open in a perspective view, in which sharp edges result from the production. The two connecting pieces 15 and 16 are each ovalized. The big axis of the oval. The shape of both connections is perpendicular to the pipe bend plane. No use has been made of a transition to a round cross section towards its free ends. However, this would be possible and is preferred in itself. The connection 15 is shown closed again for purely computational reasons.

7 shows under a) - d) half shells of pipe elbows corresponding to that of FIG. 6 but with different ovality, the shading, as already shown in FIG. 3, identifying the resulting stress condition with an internal pressure load. Figure a) of Fig. 7 initially shows the case with a round, not as a reference ovalized cross section. Particularly high comparative stress concentrations can be seen in area I (white area).

Based on the sequence of images b) - d), in which only the small axis in the drawing plane has been gradually reduced while keeping the length of the large axis, it becomes clear that the voltages in area I decrease with increasing ovality of the connections 15 and 16 , on the other hand, however, new stress concentrations arise in areas II and III, although in figures b) and c) there is a larger area distribution and overall still at a low level. Only in Figure d) are the comparative stresses in the centers of areas II and III significantly increased. In picture b) the ovality is 20:18, in picture c) 20:16 and in picture d) 20:14.

FIG. 8 shows the four cases of FIG. 7 in a bar diagram corresponding to FIG. 4, the local and global maximum comparison voltages also being shown next to one another as in FIG. 4. The diagram clearly shows that the optimal effect should be around 20:16. In the case of an ovality of 20:16, the global maximum comparison stress is reduced by approx. 47% compared to the round reference shape, and the local maximum comparison stress on the inside of the curve is even reduced by approx. 65%.

9 schematically shows an example of a gas generator housing of an airbag. This is a head airbag module. The ignition device and the gas source are integrated into the housing in the form of a gas cartridge via the installation position 19. After the airbag has been ignited, the gas flows out of the gas cartridge through the oval opening 18 aligned according to the invention, and the gas flow is distributed over the outlets (sockets) 20 and 21, each of which has an airbag (“airbag”) attached a head airbag system for a front and font head airbag is shown schematically, as it is installed laterally in the top center of an automobile, such a gas generator housing could also have an outflow opening 18 for each outlet 20, 21.

The above examples show that the idea of ovalization can be realized on many different container and connection geometries, whereby, as already stated, the concept of the container and the connection can and should be understood in a very general manner. The example of the pipe bend shows in particular that the two Terms container on the one hand and connection on the other hand may even denote the same geometry and can be conceptually interchanged.

The invention can be used with particular advantage in the case of injection-molded parts, because in many geometric shapes, weld lines cannot be avoided precisely where the highest stresses occur due to the geometry.

Suitable materials for the injection molding of the above-described molds include thermoplastic materials, in particular those made from a glass fiber reinforced polymer, the polymer advantageously being selected from the group of polyamides and copolyamides and having a melting point of at least 250 ° C., such as PA66 and PA46. It is particularly preferred to use a polymer which is a partially aromatic, partially crystalline copolyamide with a melting point in the range from 300 ° C. to 350 ° C. Such particularly well suitable polymer is available, for example under the name "GRIVORY ^® HTV-5H1" by the company Ems-Chemie AG / Ems-Grivory, Domat / Ems, Switzerland, in the trade. It is a PA 6T / 6I reinforced with 50% by weight glass fibers (based on the total weight) with a melting point of 325 ° C, i.e. a partially aromatic, partially crystalline copolyamide from the monomer components hexamethylenediamine, terephthalic acid and isophthalic acid. The material is generally particularly suitable for the production of extremely stiff, solid, heat-resistant and dimensionally accurate injection molded parts and is also characterized by very good chemical resistance. The melt temperature during injection molding is approx. 345 ° C.

Finally, it should be pointed out that pressure vessels can not only be made from plastics, but in principle from all suitable materials. In particular, the invention can also be used in metal container construction. The invention can be used particularly advantageously in die-cast metal (eg aluminum). NAME LIST

1 pressure vessel

2 ovalized connecting pieces on the pressure vessel 1

3 pressure vessels

4 cylindrical main body of the pressure vessel 3

5 ovalized connecting piece on the pressure vessel 3

6 round connecting piece on the pressure vessel 3

7 quarter segments of the pressure vessel 3 8.1-8.4 connecting piece on the quarter segments 7

9 cut edge / weld line

10 T-piece

11 - 13 connections of the T-piece 11

14 elbows

15, 16 connections of the pipe bend

17 gas generator housing

18 oval outflow opening

19 Installation position of the gas cartridge

20, 21 outlets (sockets) to the airbags

A-A cutting plane

A nozzle axis

P point of impact of the nozzle axis

I, II, III tension zones

L transition length

D length of the major axis of the oval shape d longer of the minor axis of the oval shape

0 ovality σ _v reference stress (or maximum reference stress)

Claims

1. Pressure vessel (1, 3, 10, 14) with at least one tubular connection (2; 5; 8.2 - 8.4; 11-13; 15, 16) at an opening in the container wall, characterized in that the cross sections of the opening and of the tubular connection at this opening are oval and aligned with the major axis of the oval shape in the direction of the first main stress, which would result if the pressure vessel were subjected to an internal overpressure on the connecting parts without the opening and the connection.

2. Pressure vessel according to claim 1, characterized in that it (3) has an essentially cylindrical main body (4), on which two preferably opposite connection pieces (5, 6) with at least partially oval cross-section are attached to the side of the cylindrical part, wherein the major axis of the oval shape is aligned in the circumferential direction of the cylindrical main body.

3. Pressure vessel according to claim 1, characterized in that it is designed as a T-piece (10) with three in-plane connections (11, 12, 13), at least one of which is at least partially oval in cross section, the major axis of the oval shape is oriented perpendicular to the plane mentioned.

4. Pressure vessel according to claim 1, characterized in that it is designed as a pipe bend (14) with two in one plane, substantially the same shape, at least partially oval in cross-section connections (15, 16), the major axis of the oval shape in each case is aligned perpendicular to said plane.

5. Pressure vessel according to claim 1, characterized in that it is a gas generator housing (17) for at least one airbag.

6. Pressure vessel according to one of claims 1-5, characterized in that the ovality as a ratio of the large (D) to the small (d) axis of the oval shape in the range between 1, 1 and 3.0, preferably in the range between 1, 1 and 2.5, and particularly preferably in the range between 1.1 and 1.9.

7. Pressure vessel according to one of claims 1-6, characterized in that it is a die-cast part.

8. Pressure vessel according to one of claims 1-6, characterized in that it consists of a thermoplastic, in particular a glass fiber reinforced polymer.

9. Pressure vessel according to claim 8, characterized in that it is an injection molded part.

10. Pressure vessel according to claim 9, characterized in that it has a weld line and that the weld line is chosen to run in the direction of the small axis of the oval shape.

11. Pressure vessel according to claim 2 or 10, characterized in that the cylindrical main body (4) is closed on one side by a cap and an injection molded part made by injection from the center of this cap with weld lines in the flow direction behind the connecting piece (5, 6) is.

12. Pressure vessel according to one of claims 8 - 11, characterized in that the polymer is selected from the group of polyamides and copolyamides and has a melting point of at least 250 ° C.

13. Pressure vessel according to one of claims 8 - 12, characterized in that the polymer is a partially aromatic, partially crystalline copolyamide with a melting point in the range from 300 ° C to 350 ° C.

14. Use of the pressure vessel according to one of claims 1-13 as an internal pressure vessel.

15. Use of the pressure vessel according to one of claims 1-13 as an external pressure vessel.