DE10207613C1 - Injection molded pressure vessel, with a pipe connection at a side wall, has an oval shape for the wall opening and the end of the pipe at the connection for increased strength - Google Patents

Abstract

The pressure vessel (1), with at least one tube connection (2) at a side wall opening, has an oval wall opening and the end of the round tube is shaped into a matching oval for bonding in place. The longest axis of the oval shape is on the line of the maximum stress when the interior is under pressure at the bonding point. The assembly is of injection molded plastics.

Description

TECHNICAL AREA

The present invention relates to a pressure vessel with at least one tubular Connection to an opening in the tank wall.

If a pressure vessel z. B. with an internal, pneumatic or hydraulic pressure stresses occur in the container wall. To minimize tension and to maximize the volume to surface ratio become pressure vessels constructed as spherical or cylindrical as possible. Will be on such a container Attached connection, the stresses concentrate in the area of the connection, which places a higher load on this area and a weak point in the pressure vessel forms.

In injection molded plastic pressure vessels with tubular Connections, such as those used for water meter housings, result Due to the production, weld lines in the direction of flow of the injection molding material behind the Connections and thus in an area where the tensions also concentrate. For the component, this means a considerable weakening, since an increased load occurs precisely where the weakest material properties are expected at the same time must become.

Even with T-pieces or the like, in which a pipe socket with a pipe piece branches about the same cross section, arise on the inside of the curvature  Branch with internal pressure load significant stress concentrations. at Injection-molded T-pieces are again weld lines in this area exists and there are sharp edges due to the demolding concept. The The concentration of tension coincides with the weld lines and the edges here too a considerable weakening of components.

Fundamentally similar problems occur with pipe bends.

The voltages for each point on the container wall can be mathematically calculated Describe stress tensors. Invariant of the stress tensor Coordinate transformations are three so-called principal stresses, one of which is in the Technical terminology of the stress tensor mechanics is defined as the first main stress. in the In the case of a container under an internal overpressure, the first corresponds Main tension of the greatest tension. This term is also used in the following used. So-called main directions are assigned to the main stresses.

STATE OF THE ART

The weakening of the pressure vessels through the installation of connections to compensate, it is known to reinforce the container wall and / or the connection provided. The attachment of such reinforcements is not always, in particular subsequently, easily possible and / or causes additional effort and Use of materials.

PRESENTATION OF THE INVENTION

The object of the present invention is to specify how, in the case of pressure vessels, the ones mentioned Problems in the connection area of tubular connections are reduced and as a result Pressure vessels can be manufactured, which are loaded with a higher pressure difference can be or which with the same load over a higher safety reserve feature.

To achieve this object, the invention proposes an ovalization of the connection in Connection area in front. The cross sections of the opening and the tubular connection This opening is oval and with the major axis of the oval shape in  Direction of the first main stress aligned, which is when the Pressure vessel with an internal overpressure at the connection point without the opening and Connection would result.

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

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, depending on the application and geometry, lies 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 principal stress, an im essential circular cylindrical pressure vessel are considered, in which at Internal pressure load in the container wall result in stresses in the circumferential direction are about twice as large as in the longitudinal direction. A tubular, at least in According to the invention, the connection area of the oval connecting piece would therefore have to be on the container wall its major axis connected in the circumferential direction.

For purely spherical containers or those with a relatively large, flat one The invention does not apply to the container wall, since it is not the largest here due to its geometry Main tension occurs.

Within the scope of the invention, an oval shape is generally intended to be an elongated round one Form are understood from the z. B. an elliptical shape is a special case would.

The ovalization of the connection in the mentioned main voltage direction has not only have a significant impact on lowering the maximum voltages in the Pressure vessel wall occur. The voltages that occur can also be  move other areas, e.g. B. in those where there are no weld lines or sharp edges are present and where they have less of an adverse effect.

A certain disadvantage is the reduction, for example, for a flow to the Available cross-section associated with ovalization if this starting from a circular cross-sectional shape by flattening this shape or is brought about by reducing an axis. To achieve the inventive However, it is sufficient to have ovalization only directly in the connection area carry out and within a relatively short distance again to a circular Pipe shape. The length of this transition section L can, for. B. in the Order of magnitude of the diameter of the circular shape of the connecting pipe, or may also be very short. With such a limited cross-sectional reduction only small pressure losses are caused, which in relation to the losses of a Overall system are usually negligible.

The term pressure vessel and / or the tubular connection is intended within the scope of present invention can be understood very generally. In addition to that already given example, basically combinations of pressure vessels and Connections in which the pressure vessel and the connection are approximately the same Have cross-section as z. B. is the case with so-called T-pieces. As a borderline 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, a pipe bend can be made be understood, with a difference between that as a pressure vessel serving part and the part serving as a pipe socket does not even have to exist anymore. On Elbows, on the other hand, can also be seen as a one-armed T-piece.

Furthermore, as already mentioned, the term pressure vessel must not be used in the same sense be understood restrictively that this only ever with an internal pressure compared to e.g. B. external normal pressure. The inventive principle and the the resulting geometry remains the same for every pressure difference situation and thus also with an external overpressure compared to e.g. B. an internal normal pressure, like this z. B. at submarines or the like. So uses are possible both as an internal pressure vessel and as an external pressure vessel.  

BRIEF EXPLANATION OF THE FIGURES

The invention is intended to be explained below using exemplary embodiments in connection with the drawing are explained in more detail. 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;

Fig. 3 under a) -d) each quarter segments of a housing corresponding to that of Figure 2 but with differently shaped connecting pieces, the graded gray tinting the resultant stress state with an internal pressure load is indicated. white tones mean high and black tones mean low voltage;

FIG. 4 shows the local and global maximum reference voltages for different connection ovalities in a container of the type of FIG. 2 in a bar diagram; FIG.

FIG. 5 is a view as well as in several cross-sections an ovalisiertes according to the invention T-piece;

Fig. 6 is an ovalized according to the invention pipe bend half cut in 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 state with an internal pressure load; and

Fig. 8, in a bar chart, the local and global maximum reference voltages for different ovality at a pipe bend corresponding to that of Fig. 6.

WAYS OF CARRYING OUT THE INVENTION

Fig. 1 shows, as a first simple example of a pressure vessel 1 with a circular cylindrical cross-section, to which a pipe socket 2 is connected to the lateral surface side. 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 connection point of the raw connector, z. B. the point of impact P of the nozzle axis A on the wall of the container 1 can be understood. 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 in 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 z. B. 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 connection piece 5 is oval in the transition area 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 in b), aligned 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 merges 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.

Fig. 3 shows in a) -d) are each quarter sections 7 of a housing according to the but of FIG. 2 with differently shaped connecting piece 8.1.-8.4, being indicated by the shading from the calculated resultant voltage state at 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 voltage state is also used in the context of FIGS. 4, 7 and 8, which are described below.

First, Figure a) shows the case with a connecting piece with a circular cross-section as a reference, whereby a diameter of 40 mm was assumed for the calculation, which corresponds to a normal pipe diameter z. B. corresponds to water pipes. High voltage concentrations (white areas in FIG. 3a) can be seen in zones I and II. 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 the Reduce areas I and II and also shift. 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 each small axis d in millimeters, while the large axis D at the aforementioned 40 mm circular reference cross-section was left. The most favorable case should be in picture c) are present since there are hardly any stress concentrations and the stresses on one comparatively low levels are fairly evenly distributed. In picture d) has in one Area III, on the other hand, has already formed a zone of high voltage.

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 ovalities being taken into account in addition to FIG. 3. 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 z. B. 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 that occurs at I or II, referred to as local, while the height of the non-hatched bars indicates the occurring maximum reference voltage somewhere, referred to as global, in Figure d) z. B. indicates at point III. 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 comparison 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 reference 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. B. offers an ovality in the range of 1.5 to 1.7. 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 achieved, it may also make sense to choose the ovality in the area in which the global maximum reference stress is already rising again.

In the containers of the type of Fig. 2, the global as well as local maximum reference voltage by more than 50% can be lowered.

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 are all 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. B. be equated with the container 1 of FIG. 1, the connection 13 would correspond to the pipe socket 2 in this case. 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 use only two of the three connections, e.g. B. the connections 11 and 12 , or just one of the connections, such as. B. ovalize the connector 13 . As with the connection 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 round pipes can be connected to the connections, for example via a screw connection. The round cross sections are designated in FIG. 5 with AA or DD.

In the application example of FIG. 5, e.g. B. 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 which has been cut open in a perspective view and in which sharp edges result from the production. The two connecting pieces 15 and 16 are each ovalized. The big axis of the oval. Shape is selected perpendicular to the pipe bend plane for both connections. No use has been made here 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.

FIG. 7 shows under a) -d) half shells of pipe bends corresponding to that of FIG. 6 but with different ovality, the shading, as already shown in FIG. 3, identifying the resultant stress state with an internal pressure load. Figure a) of Fig. 7 first shows the case with a round, non-ovalized cross section as a reference. 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) they are distributed over a larger area and overall are 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 finally shows the four cases from 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 shows particularly clearly that the optimal effect should be in the range of an ovality of approx. 20:16. In the case of an ovality of 20:16, the global maximum comparison stress compared to the round reference shape is reduced by approx. 47%, the local maximum comparison stress on the inside of the curve even by approx. 65%.

The above examples show that the idea of ovalization in many different container and connection geometries can be realized, whereby, as already executed, the concept of the container and the connection understood very generally can and should be. 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 have the same geometry designate and can be conceptually interchanged.

The invention can be used with particular advantage in the case of injection molded parts, because this involves many geometric shapes, weld seams cannot be avoided where geometrically the highest stresses occur.

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 being advantageously selected from the group of polyamides and copolyamides and having a melting point of at least 250 ° C., such as, for. B. 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 a particularly well-suited polymer is commercially available, for example, under the name "GRIVORY® HTV-5H1" from Ems-Chemie AG / Ems-Grivory, Domat / Ems, Switzerland. It is a PA 6 T / 61 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, strong, 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 are not only made of Plastics, but in principle be made from all suitable materials can. In particular, the invention can also be used in metal container construction. Especially The invention can advantageously be used in metal die casting (eg aluminum).  

NAME LIST

1

pressure vessel

2

ovalized connecting piece on the pressure vessel

1

3

pressure vessel

4

cylindrical main body of the pressure vessel

3

5

ovalized connecting piece on the pressure vessel

3

6

round connector on the pressure vessel

3

7

Quarter segments of the pressure vessel

3

8.1-8.4

Connection piece on the quarter segments

7

9

Cutting edge / weld

10

Tee

11-13

Connections of the T-piece

11

14

Elbows

15

.

16

Pipe elbow connections
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 the small axis of the oval shape
O ovality
σ _v

Reference voltage (or maximum reference voltage)

Claims (13)

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 are 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. 2. Pressure vessel according to claim 1, characterized in that it ( 3 ) has a substantially 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 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 being vertical is aligned to the named level. 5. Pressure vessel according to one of claims 1-4, characterized in that the Ovality as the ratio of the large (D) to the small (d) axis of the oval shape in the area between 1.1 and 3.0, preferably in the range between 1.1 and 2.5, and particularly is preferably selected in the range between 1.1 and 1.9. 6. Pressure vessel according to one of claims 1-5, characterized in that it is a Die casting is.   7. Pressure vessel according to one of claims 1-5, characterized in that it is made of a thermoplastic, in particular a glass fiber reinforced polymer consists. 8. Pressure vessel according to claim 7, characterized in that it is an injection molded part is. 9. Pressure vessel according to claim 8, characterized in that it has a weld line and that the weld line runs in the direction of the small axis of the oval shape is selected. 10. Pressure vessel according to one of claims 7-9, characterized in that the Polymer is selected from the group of polyamides and copolyamides and one Has a melting point of at least 250 ° C. 11. Pressure vessel according to one of claims 7-10, characterized in that it the polymer is a partially aromatic, partially crystalline copolyamide with a Melting point in the range of 300 ° C to 350 ° C. 12. Use of the pressure vessel according to one of claims 1-11 as Internal pressure vessel. 13. Use of the pressure vessel according to one of claims 1-11 as Outside pressure vessel.