WO1988003548A1

WO1988003548A1 - A joint and a sealing element for it

Info

Publication number: WO1988003548A1
Application number: PCT/SE1987/000513
Authority: WO
Inventors: Rolf Westin
Original assignee: Studsvik Energiteknik AB
Current assignee: Studsvik Energiteknik AB
Priority date: 1986-11-03
Filing date: 1987-11-02
Publication date: 1988-05-19
Anticipated expiration: 1989-05-03
Also published as: SE8604703D0; SE8604703L

Abstract

An elastic sealing strip includes a cross-linked thermoplastics element (1') deformed from its memorised shape, such that when it is fitted in place it can be caused to strive towards its memorised shape by heating it to the fusion interval of the cross-linked thermoplastics material. The element can also be covered with a rubber elastic surface layer (9). A joint includes two support surfaces between which the sealing strip is deformed by compression. Relative to its memorised shape, the strip has a remaining compression deformation in a direction between the two support surfaces.

Description

TITLE OF THE INVENTION

A joint and a sealing element for it

TECHNICAL FIELD The invention relates to a joint including two support surfaces, between which a sealing element is elastically deformed by constriction between the supporting surfaces. The invention also relates to a sealing element for the joint.

BACKGROUND

In many types of joint, an elastic sealing strip or ring of rubber is inserted between two solid bodies, and the bodies are urged towards each other, e.g. by a screwed connection, so that the strip or ring is elastically deformed to sealingly fill out irregularities in the opposing surfaces of the bodies, thereby providing an efficient seal between the bodies. However, such strips or rings gradually lose their resilience, particularly in the presence of oxygen-rich media, their engagement pressure against the surfaces in question being reduced and the risk of leakage increasing. It is often a complicated and expensive procedure to change the sealing strips or rings. Often operation of a plant in which the joint is included must be interrupted in connection with changing the seal or gasket. It would therefore be desirable to be able to extend the effective life of a sealing strip or ring of the kind in question. It would also be desirable to reduce the risk of leakage if the bodies should become more mutually remote.

Naturally, it could be possible in the joint described above, to retighten the bodies more closely to each other, thus re-establishing the engagement force of the sealing strip or ring against the surfaces, but such a procedure is not always practicable or possible, and if this kind of tightening is performed, the risk is great that the safety margins for the joint with respect to sealing and strength are greatly reduced. An object of the invention is thus to provide a technique permitting the establishment or re-establishment of a desired engagement pressure for the sealing strip against the surfaces of the bodies coacting with it.

CHARACTERISATION OF THE INVENTION

The object mentioned is achieved with a joint and a sealing element of the kind disclosed in the accompanying independent claims. Accordingly, the invention is based on the knowledge that a body made from cross-linked polyethylene (PEX) has a memory of the shape it had when it was cross- linked. Above a given temperature boundary, 110ºC, i.e. the lower limit of the material fusion interval, the material strives to regain the shape it had at the occasion of cross-linking, if relative to said shape.

The body shall thus be given a deformation which is durable at temperatures below the material fusion interval. The body may then be deformed at a temperature below the fusion interval, e.g. at room temperature (T_R), and substantially retains its shape obtained at the deformation (the elastic return to shape after the deforming forces have been unloaded from the body is relatively small). Alternatively, the body may be deformed at a higher temperature in, or above, the fusion interval, lower deformation forces then being required, although it is then necessary to maintain the deformation while the body is cooled to a temperature below the fusion interval, if the deformation is to be permanent.

When the cross-linked thermoplastic body, which has been deformed from its memorised form, is then heated to a temperature above the lower boundary of the fusion interval it begins to regain its memorised shape. If the body is cooled to a temperature below the fusion interval, this regainment of shape is interrupted. This effect is utilised in accordance with the invention in a particularly favourable way in the inventive joint. This favourable effect is obtained with a material comprising cross-linked polyethylene, but there should be an entire series of different materials, e.g. cross-linked polypropene, having properties corresponding to those for polyethylene. Accordingly, it may be conceived that primarily cross-linked thermoplastics have the indicated properties, and that those skilled in the field of plastics can point to other materials, the properties of which are substantially equivalent to those of cross-linked polyethylene in the respects touched on above, so that they are usable for sealing strips and joints in accordance with the invention. A simple form of sealing strip according to the invention may be produced from an extruded polyethylene section, which is cross-linked with a circular cross-sectional shape, and then deformed to an oval shape at room temperature (T_R) or at a temperature greater than 130ºC, followed by rapid cooling to T_R. This oval strip is then constricted between two bodies, being deformed visco-elastically in the direction of the minor axis of the oval. If the engagement pressure between strip and bodies should decrease after a time, the pressure can be re-established by giving the strip a temperature of greater than 110ºC, the oval strip then striving to return to its circular shape, which is its memorised shape, but is prevented from doing so by the surface pressure then established between strip and bodies.

It will be understood that the inventive strip can be covered by a surface or outer layer over the whole of its periphery, or one or more parts thereof, this layer pererably being of a rubber elastic material and/or fluoroplastics, for the purpose of giving the sealing element desired surface friction or resilience in its surface layer and/or to give it a suitable resistance to its environment.

A surface or outer layer or casing as discussed above can be provided such that the inner strip or core of a cross- linked thermoplastics can be inserted into the separately prepared surface layer or casing. Such a layer or casing can be made of a material requiring, for instance, a thermal treatment, such as a vulcanization, at a temperature that would have triggered a premature change in shape of the core towards its memorised shape had the core been present during the preparation of the layer or casing.

When it is desired to trigger the change in shape of the strip towards its memorised shape, the strip must be given a temperature within the fusion interval for a time which may be one or some minutes. If the strip is used in a joint of a container structure through which there flows a fluid (e.g. sealing strips in flat heat exchangers, gaskets in flanged pipe joints and the like), the necessary heating can be provided by passing a fluid at increased temperature through the container. It should be quite clear, however, that the sealing strip can be given the desired temperature directly or indirectly by blowing a hot gas towards it, by spraying a hot liquid on it, by passing a hot fluid through an interior duct in it, by passing a current through electric heating means incorporated in it, by energy-rich radiation which directly heats the material of it or first heats an energy-absorbent material that has been mixed into it, or by heating it in some other conventional way. Hereinbefore the invention has been described with respect to the situation where the contact pressure between the strip and the bodies between which it is constricted were to drop for some reason, a condition here being that the bodies retain their shapes and mutually relative positions. However, situations are conceivable where, for some reason, the bodies could assume a mutually angular relationship, or move away from each other to an extent that the direct resilience of the strip is not sufficient to fill the increased gap between the bodies. In such a situation, the gap can be filled by the strip being caused to expand by heating in the fus ion interval .

The invention will now be described in the form of an example, which should not be regarded as restricting the invention, and with reference to the accompanying drawings. DRAWINGS

Figure 1 schematically illusstrates the cross-sect i on o f s t ock f or the invent ive s eal ing s tr ip . Figure 2 i s the cross-section of a strip in accordance with the invention. Figure 3 is a schematic illustration of a cross-section of a joint in accordance with the invention. Figures 4-7 are sections corresponding to that of Figure 3 for different conditions of a joint in accordance with the invention. Figure 8 schematically illustrates an embodiment of a sealing strip in accordance with the invention. Figure 9 is a cross-section through a further development of a sealing strip in accordance with the invention.

EMBODIMENTS In Figure 1 there is illustrated the cross-section of an extruded, cross-linkable polyethylene strip in an unloaded state at the occasion of cross-linking, its appearance denoted by M being the memorised shape of the strip. Figure 2 illustrates the cross-linked strip 1' according to the invention, and which has been formed from the strip 1 in Figure 1 by deforming it at a temperature T_R, or a temperature above the fusion interval 110-130ºC followed by rapid cooling with retained deformation, so that the strip in its unloaded condition has the shape indicated by D in Figure 2, and signifying that the strip in Figure 1 has been deformed from the dimension X ₁ to the dimension X₂ between two opposing surfaces 11 and 12. In Figure 3 the strip 1' is inserted between two positioned surfaces 21, 22, which have been moved towards each other to a spacing of X₃ while elastically compressing the strip 1' between these surfaces 11, 12. Accordingly, the strip 1' will be compressed against the surfaces 21, 22 with a force F_E. This contact force F_E inter alia affords a sealing function in respect of a fluid pressure on one side of the strip 1'. However, the force F_E may drop, due to the resilience of the strip decreasing for some reason, or by the surfaces 21, 22 moving away from each other. The latter case is illustrated in Figure 4, where the surfaces 21, 22 have been sprung apart to such an extent that the distance between them has become X₂, while the temperature of the strip has been kept under 110ºC, signifying that the contact pressure between the strip 1' and the surfaces 21, 22 has fallen substantially to zero. According to the invention, the strip 1' can now be heated in some way so that its temperature exceeds 110ºC, the strip 1' thus striving to obtain the memorised shape illustrated in Figure 1. This will cause the strip 1' to exercise a certain force on the surfaces 21, 22. When the temperature of the strip 1' is then lowered to below 110ºC, this pressure caused by the material trying to regain its memorised shape will remain. The strip 1' is thus given a new shape D₁, and is still resilient, so that it is capable of maintaining a sealing engagement pressure against the surfaces 21, 22, although these may have obtained somewhat increased mutual spacing.

It should be clear, however, that the inventive strip can be utilised in the way depicted in Figure 6. Here it may be assumed that the inventive strip of Figure 2 has been inserted between two positioned surfaces 21, 22 with a spacing greater than the height X₂ of the strip 1' in an unloaded condition, but which is less than the dimension X₁ of the strip 1 in its memorised shape. The strip 1' can now be brought into elast ically compressed surf ace contact with the surfaces 21, 22 by giving the strip 1' a temperature of above 110ºC and keeping the strip at this temperature for a time sufficient to enable the shape of the strip 1 to attempt to come into the memorised shape according to Figure 1, thus to become elastically constricted between the surfaces 21, 22, subsequent to which the strip 1' is cooled to below the fusion interval while retaining its resilient compression between the surfaces 21, 22.

Figure 8 illustrates a strip 1' according to the invention, arranged in the form of a sealing ring 10, which may be used as a sealing gasket in a flanged pipe joint or the like. Also illustrated schematically in the Figure, there is a heating means 30 arranged through the strip and having two outgoing terminals 31, 32. The means 30 may be a fluid duct, and the terminals 31, 32 inlet and outlet stubs for the duct, whereby a hot fluid at a temperature above 110ºC can be taken through the strip to cause it to strive to regain its irtemorised shape.

In connection with Figures 1-7 the memorised shape of the strip 1' has been shown to be rectangular, but it should be quite clear that this shape has been arbitrarly selected for the purpose of illustration, and that the inventive strip may have any shape whatsoever that is suitable for the application in question. It has thus been found that in many cases it is advantageous to let the cross-sectionel shape of the inventive strip be circular in its memorised form and to have an oval cross-section in an unloaded condition, so that after heating to above 110ºC the inventive strip will strive to regain its circular cross-section for establishing or reestablishing a sealing pressure in a joint.

It should also be clear that the embodiment according to Figures 6 and 7 may be used to drive apart two objects such as 21, 22 against surrounding abutments, thus fixing these objects 21, 22 against the abutments.

It should be clear that it is not necessary always to utilise an elasticity of the strip core 1'; if the strip is covered with a rubber elastic surface layer, this can provide the elastic property of the strip below 110ºC, the strip 1' then substantially serving as support to the surface layer. Also in this case a support surface can be displaced by triggering the memory effect of the strip 1'.

In Figure 9 there is illustrated a preferred embodiment of the inventive strip. The strip has a core 1' from PEX, and is oval in cross-section in its deformed condition, while having, for example, a circular cross-section for its memorised shape. The core is covered with a casing 9 of a rubber elastic material such as a resilient elastomer, possibly with a protective surface layer such as a fluoroplastic or the like, the casing being selected to provide the strip with properties suitable to the purpose in the intended environment.

Since a cross-linked thermoplastic has relatively low elasticity, particularly at temperatures below the fusion interval, it may be suitable in many applications to cover a deformed body of cross-linked thermoplastic with a rubber elastic material. The composite body will thus give the required elasticity at its surface, at least at temperatures below the fusion interval. A surface layer of fluorplastics or a similar material will give an excellent resistance against unfavourable influence from the environment.

An outer covering or casing of the kind mentioned above, for instance the casing 9, can be provided separately while having a longitudinal slot, for instance, such that the core 1' can be inserted therethrough so as to give the final strip, such as that of Figure 9.

A sealing strip in accordance with the invention can be advantageously used in such as heat exchangers in district heating networks.

Such heat exchangers, of the flat type, have a temperature of about 120ºC and are passed through by oxygenrich water, which leads rather rapidly to sealing strips of rubber type becoming degraded. If inventive PEX sealing strips are used in accordance with the invention, the functional life of such strips will be considerably longer, since PEX has relatively low sensitivity to oxygen. Furthermore, the PEX material is kept at a temperature within the fusion interval, so that the PEX strip immediately strives towards its memorised shape, should the surfaces supporting the strip move apart , leakage thus being avoided .

When using a sealing strip in accordance with the invention at raised temperatures (such as at 80 - 90ºC) although below the temperatures of the fusion interval, a specific advantage can be obtained, because it has been found that at such raised temperatures the inventive sealing strip than slowly will strive towards its memorised shape. This means that a permanent good sealing pressure will be secured and that a possible loss of elasticity will be compensated automatically.

Claims

1. Joint including two support surfaces (21, 22), between which a sealing element (1') is elastically deformed by compression, characterised in that the element (1') includes a body of a cross-linked thermoplastics material, and in that at a temperature below the lower temperature boundary for the fusion interval of the material the body has a remaining compression deformation in a direction between the two support surfaces, this deformation being relative to the memorised shape given to the body when it was cross-linked.

2. Joint as claimed in claim 1, characterised in that the body comprises cross-linked polyethylene.

3. Joint as claimed in claim 1 or 2, characterised in that the body is provided with a duct for the passage of a heating fluid.

4. Joint as claimed in claim 1 or 2, characterised in that the body has bedded into it an element for electrically heating it. 5. Joint as claimed in any one of claims 1-4, characterised in that the body is covered with a surface layer (9) of rubber elastic material and/or fluoroplastics, at least over a part of its periphery.

5. Sealing element including a body of material, the body preferably being a strip or a ring characterised in that the material is a crosslinked thermoplastics material, which in an unloaded and deformed condition has a cross-sectional shape deviating from the memorised shape at the time of cross-linking, whereby the cross-sectional shape of the body can be caused to approach that of the memorised shape by being heated to a temperature above the lower temperature boundary of the material fusion interval.

7. Sealing element as claimed in claim 6, characterised in that the body comprises cross-linked polyethylene.

8. Sealing element as claimed in claim 6 or 7, characterised in that the body has a duct for the passage of a heating fluid.

9. Sealing element as claimed in claim 6 or 7, chara cterised in that the body has bedded into it an element for electrical resistance heating of the body.

10. Sealing element as claimed in any one of claims 6-9, characterised in that the body is covered with a surface layer of rubber elastic material and/or fluoroplastics, at least over a part of its periphery.