GB2487643A

GB2487643A - Helically convoluted double layer plastics tubing

Info

Publication number: GB2487643A
Application number: GB1200814.0A
Authority: GB
Inventors: Stuart Grant Jones
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-01-29
Filing date: 2012-01-10
Publication date: 2012-08-01
Anticipated expiration: 2032-01-10
Also published as: GB201200814D0; GB201101634D0; GB2487643B

Abstract

The tubing comprises two heat set helically convoluted layers, the inner 1bC with shallower convolutions than the outer layer 1aC which has an external elongate retaining element 2. The element 2 is wound from a fixing point 3a on a sleeve 3 rotating in a direction 3d and moving axially in a direction 3c from external tube heating 4 to soften the plastics. The heating 4 has associated temperature monitoring 8. The retaining element 2 forms the convolutions in the outer layer 1aC when being helically wound around the two heated layers 1a, 1b. also forming the lesser convolutions 1bC the inner layer. Clamps 5a1 and 5a2 substantially surround the tubing with external cooling 7.

Description

?Qniiintbeicallv tonvpluted di4pkz' tubinE: This invention relates to a method & apparatus for forming from two plastic tubes, heat set helically convoluted duplex' tubing which has an inner tube with shallower convolutions than the outer one which has an external retaining element.

The internal tubes or liners' used in flexible hose assemblies should ideally have a smooth inner surface as this allows the materials being conveyed to experience less pressure drop, less turbulent flow patterns and higher flow rates, as well as being easier to be drained/cleaned with less likelihood of residual material being left trapped' within them. However smooth tubes are prone to kinking' or collapsing when flexed & are unable to achieve tight bend radii, especially when they have larger diameters and this can cause them to fail prematurely, often with disastrous results.

Patent EP 1141605 discloses a single plastic tube or liner' with a convoluted outer surface occupied by an external helical wire, & a ribbed' but essentially smooth inner surface. The convoluted outer surface helps to reduce the traditional flexibility problems associated with liners' with smooth inner surfaces. But this liner' suffers from a drawback -it is be unable to safely contain the material being conveyed if the tube material experiences a structural failure, as there is no external pressure resistant outer tube to provide containment.

Other flexible hose products have a smooth plastic inner tube which is typically inserted within a convoluted plastic outer tube or corrugated metal outer line?, to provide a second layer of containment if the inner tube fails. But they are subject to the limitations of any flexible hose with a totally smooth inner tube -a lack of flexibility and the risk oflinking' & premature failure if flexed too severely.

It is an object of the present invention to overcome these problems.

According to the present invention there is provided apparatus for forming from two plastic tubes, heat set helically convoluted duplex' tubing which has an inner tube with shallower convolutions than the outer one which has an external retaining element comprising: tube heating means arranged to heat at least a portion of both tubes to predetermined temperature(s) which islam sufficient to cause the chosen tube material(s) to at least partially lose elastic memory; control means for controlling the tube heating means so as controllably heat and maintain a part of both tubes at the predetermined temperature(s); means to substantially maintain the overall diameter of the unconvoluted outer tube; an elongate convolution fonning element which is operable to urge the heated portion of the outer tube against the heated portion of the inner tube when both portions have reached the predetermined temperature(s) to form helically convoluted tubing; a convolution forming element source for delivering the elongate convolution forming element; means for rotating both tubes relative to the convolution forming element source and means for moving both tubes axially relative to the convolution forming element source; whereby the convolution forming element is wound around and urged against the heated outer tube which is urged against the heated inner tube, whilst the overall diameter of the unconvoluted outer tube is substantially maintained, to form helical convolutions in both tubes when the tube material(s) has/have at least partially lost its/their elastic memory, the convolution forming element being retained on the newly convoluted portion of the outer tube to form a convolution retaining element as that portion of both tubes is moved away from the tube heating means.

According to a second aspect of the present invention there is provided a method of producing from two plastic tubes, heat set helically convoluted duplex' tubing which has an inner tube with shallower convolutions than the outer one which has an external retaining element comprising the steps of: locating a smaller diameter tube inside a larger diameter tube; heating at least a portion of both tubes to predetermined temperature(s) which causes both tubes material(s) to at least partially lose itsftheir elastic memory; controlling tube heating means so as to controllably heat and maintain a part of both tubes at the predetennined temperature(s); rotating and moving both tubes axially relative to a convolution forming element source for delivering an elongate convolution forming element which is operable to urge the heated portion of the outer tube against the heated portion of the inner tube when portions of both tubes have reached the predetermined temperature(s) to form helically convoluted tubing; whereby the convolution forming element is wound around and urged against the heated outer tube which is urged against the heated inner tube, whilst the overall diameter of the unconvoluted outer tube is substantially maintained, to form helical convolutions in both tubes when the tube material(s) has/have at teast partially lost its/their elastic mezany, the convolution forming element being retained on the newly convoluted portion of the outer tube to form a convolution retaining element as that portion of both tubes is moved away from the tube heating means.

The invention produces heat set, stable, stress free, helically convoluted duplex' tubing which has an inner tube with shallower convolutions than the outer one, which has an external retaining element; in a single continuous process which requires no further treamient in order to stabilise it.

This duplex' tubing is formed by helically convoluting two smooth plastic tubes at the sante time -the smaller one being located within the larger one. The overall diameter of the smaller one is less than the internal diameter of the larger one.

Because the duplex' tubing has an inner tube with a shallow convoluted inner surface, it is much more flexible than a similar sized tube with a completely smooth wall'; yet it is easier to clean & drain, has less pressure drop, less turbulent flow patterns & higher flow rates for the materials being conveyed, than a traditional' helically inner convoluted tube. The more convoluted outer tube together with the external retaining element, offer the inner tube great loop' support when severely flexed, helping to substantially maintain its circular cross-sectional shape In the unlikely event of any structural failure in the inner tube, the material being conveyed is unable to escape as the outer tube can safely contain the leak until the hose assembly can be replaced. This provides an extra level of protection which is most valuable when the substances being conveyed may be harmful to lifr or/and the environment and any leak is most undesirable.

Typically, the duplex' tubing is used as a line? in flexible hose assemblies, which may be externally covered along their lengths by steel braiding, rubber covering or similar, & have suitable fittings securely fastened at each end. An external element is typically retained around the outer tube, although it can also be removed if required..

Both the inner & outer tubes of the duplex' tubing may be flared-out' together at each end of a hose assembly onto suitable flat-faced end-fittings such as flanges.

The inner tubes' shallow convolutions typically give its inner surface a ribbed' appearance, & whilst the outer tube has deeper convolutions & an external retaining element. The smooth plastic tubes used to form the duplex' tubing are typically made from PTFE, or similar plastics/thermoplastics. Both inner and outer tubes may be made from the same material, or both may be made of different materials. The wall thickness' of both inner and outer tubes may be equal or different.

Typically, the apparatus to form the duplex' tubing is arranged either vertically or horizontally.

Preferably, one end of both tubes are first securely fastened to the end of a rigid sleeve' (or similar), and then the loose' end of the elongate convolution forming element is also secured to it at a predetermined location point The sleeve' may be a thin-wall steel tube or similar, & is operable to be controllably rotated & axially moved.

When an internal support is employed & part of it is located within the external tube heating means, parts of the sleeve' may be cut away & removed along some of it's length to allow the internal support to be controllably heated to a predetermined temperature range by the external tube heating prior to convolution formation beginning. The heated internal support is then able to provide assistance in heating the inner tube; prior to, and during convolution formation.

When the tube heating (external & internal if required) have reached the required temperature(s) for the process to begin, the sleeve' is rotated whilst moving axially away from the convolution forming element source, and the elongate convolution forming element is first wound around the sleeve' in a helical manner before being wrapped around the heated outer tube, During its first few rotations the rigid sleeve' offers substantial loop' support to both tubes as the elongate element is wrapped around the outer tube under controlled tension, preventing either of them from defonuing or collapsing. As the convolution formed in the outer tube becomes deeper it quickly trap& and centres' the inner tube, and then begins to form a shallow convolution in it. At this point both tubes are permanently squeezed' together greatly increasing their ability to withstand the torque exerted upon them by the rotating sleeve acting against the tension in the convolution fomiing element, and they are together able to substantially maintain their cross-sectional shape whilst moving away from the tube heating means. The newly formed duplex' tube now provides continual loop? support to both unconvoluted tubes as the elongate element is wrapped around them. By ensuring that the overall diameter of the unconvoluted outer tube is substantially maintained around 360' of its circumference when the element first comes into contact with it, (typically by using a pair of suitably shaped clamps), the outer tube is adequately supported immediately prior to, durIng, & immediately after convolution fonnation. Preferably, the elongate convolution forming element comes into contact with the outer tube, adjacent to, & in very close proximity to the clamps.

Both tubes are permanently squeezed' together during the convolution process due to the urging' force applied first to the outer tube, then the inner tube, by the elongate convolution forming element, whilst the overall diameter of the unconvoluted outer tube is substantially maintained, & they are being controllably heated at predetermined temperature(s). When squeezed' together both tubes form. a very rigid 3d helical honey-comb' structure which has much greater torsional rigidity than two single unconnected tubes, & with the assistance of the elongate convolution forming/retaining element, they are collectively able to maintain their integrity & cross-sectional shape whilst resisting the torque applied by the sleeve acting against the tension in the element External cooling may also be provided to further assist the newly formed duplex' tubing in resisting the forces acting upon it by reducing its' temperature quickly, increasing its' rigidity. Typically, the external cooling (which may be for example compressed-air or similar), is arranged to surround 360' of the duplex' tubing along a part of its length, beginning in close proximity to where the elongate convolution forming element is first urged' against the outer tube & ending in a direction away from the tube beating means.

When employed an internal support which may be a cable, a mandrel, or similar -is located within the inner tube. It may extend beyond the unconvoluted tubes at one end, or/and extend beyond the convoluted duplex' tube at the other end -providing a means of supporting longer tube lengths without the need for external supports. It can help to center' the heated inner tube during convolution formation & help stabilise its cross-sectional shape. In addition, it may be used to establish and maintain the depth of the inner tubes' convolutions. This is achieved by urging' the elongate forming element to create deeper convolutions in the outer tube & inner tube until a limit' is reached. At this point the troughs of the convolutions formed in the inner tube Will typically be in contact with, (or close to being in contact with), the outer surface of internal support around its circumference. By maintaining the elongate element urging' force acting on the tubes at, or close to this limit' (by controlling its' tension), convolutions with a consistent depth can be formed in the inner tube.

The overall diameter of an internal support is less than the internal diameter of the inner tube. By changing the overall diameter of the internal support, different depths of convolutions may be created in the inner tube of the duplex' tubing. The internal support may be parallel along its' length,, or it may be tapered inwardly to a smaller diameter in a direction away from the tube heating means, to prevent unnecessary friction between the inner tube and the internal support as the newly formed duplex' tubing moves any from the element source & tube heating means. The internal support may have a smooth external surface along its length, or may have a helically configured external surface along part or all of ifs length.

Substantially maintaining the overall diameter of the unconvoluted outer tube when the elongate convolution forming element is urged' against first the outer rube, then the inner tube. (whilst both are maintained within predetermined temperature ranges) may be achieved for example, by two clamps which are operable to enclose & substantially surround the heated unconvoluted outer tube during convolution formation. Preferably both clamps are in very close proximity to where the elongate convolution forming element comes into contact with the outer tube from the convolution forming element source. Typically, both clamps may have concave semi-circular profiles with diameters approximately equal to the overall diameter of the heated outer tube (Ic; their internal diameters are reasonably close-fitting with the overall diameter of the heated outer tube). Preferably, they may also be operable to be advanced from either side of the outer tube towards each other until they substantially surround its circumference -typically after a sleeve or similar (to which both tubes are preferably attached), has moved away from the elongate element source. Both clamps may be controllably heated to maintain the temperature of the unconvoluted outer tube prior to convolution formation.

Preferably, the elongate convolution forming element comes into contact with the outer tube adjacent to, & in very close proximity with the means employed to substantially maintain the overall diameter of the unconvoluted outer tube. A further clamp (or pair of clamps), similar to those typically used to substantially maintain the overall diameter of the unconvoluted outer tube, may be located adjacent to them (in a direction away from the tube heating means). It/they will typically at least partially surround a length of the newly formed duplex tubing, with the elongate convolution forming element located between them. This prevents the element from moving axially along the outer tube during convolution fonnation, whilst helping to stabilise & maintain the shape & integrity of the newly formed duplex' tubing. This clamp or clamps, may also be cooled to further assist the stabilisation of the newly formed duplex' tubing. After the duplex' tubing is formed, the elongate convolution forming element is retained on the outer tube becoming a convolution retaining element.

The convolution forming element source provides an elongate element which when externally helically wrapped' around the outer tube under controlled tension during convolution formation acts as a convolution forming element, and when retained on the finished duplex' tubing acts as a convolution retaining element The tension required prior to, during, and after convolution formation, may be controllably altered as necessary. By changing the tension applied to the elongate fonning element the urging' force acting on both tubes is changed allowing their convolution depths to be varied. The elongate convolution forming element may for example be a steel wire, steel cable or similar. During convolution formation both tubes are rotated and axially moved relative to the convolution element source. Typically the position & location of the convolution forming element source is fixed & is in close proximity to the means employed to substantially maintain the overall diameter of the unconvoluted outer tube.

The depth of the convolutions fonned in the inner tube along its length, may be varied by controllably changing the element urge' force acting on the outermost rube during convolution formation. Shallow convolutions in the inner tube may give its inner surface a ribbed' appearance, whilst deeper ones can provide it with a more convoluted profile. Changes in the convolution depth of the inner tube directly affect the flexibility of the duplex' tubing, with deeper ones typically providing more flexibility. By accurately controlling the convolution depth of the inner tube, products can be produced with different bend radii to meet the needs of specific customers for specific tasks. The wall thickness' of the inner and outer tubes will also affect the flexibility of the product. The wall thickness of the inner tube is important as its' minimum bend radius is typically the limiting' factor in determining the minimum bend radius of the completed duplex' tubing. The wall thickness' of both inner and outer tubes may be equal or different. The flexibility of the duplex' tubing can also be altered by changing the pitch of the convolutions formed.

During the convolution process at least a portion of both tubes are heated to predetermined temperature(s) which is/are sufficient to cause their chosen tube material(s) to at least partially lose elastic memory, whilst the overall diameter of the unconvoluted outer tube is substantially maintained. Typically the tube heating at least partially circuruferentially surrounds both tubes along part of their lengths ensuring that both tubes are heated to the required process temperature(s) around 360' of their circumferences. The heating is controlled to ensure that a part of both tubes are maintained at these predetermined temperature(s) whilst the overall diameter of the unconvoluted outer tube is substantially maintained.

Where both tube materials are the same there is typically no need to provide internal heat to the inner tube. External tube heating alone will usually ensure that both tubes are heated to the required process temperature(s), with the outer tube typically being hotter than the inner tube as it receives more energy train the tube heating means.

Conveniently, the outer tube is typically required to be convoluted at a slightly higher temperature than the inner tube because it needs to be much more convoluted - (ía; it has to be made to deform more from its original unconvoluted shape). The process temperature of the outer tube is typically monitored by a suitably located infrared thermocouple.

However when both tubes are made from different materials (which require more varied process temperatures), internal heating (or cooling) for the inner tube may be needed in addition to external tube heating. Internal cooling may be required where the process temperature of the inner tube material is lower than the process temperature of the outer tube material. Internal cooling may for example, be provided by passing a suitable liquid coolant, or compressed-air or similar, withinlthrough an internal support located inside the inner tube.

Internal heating may be required where the process temperature of the inner tube material is higher than the process temperature of the outer tube material. Internal heating may for example, be provided by internal heating element(s) or similar, located inside the inner tube. Such heating elements may be part of an internal support, which also has an overall diameter chosen to help establish and control the depth of the convolutions formed in the inner tube. If either internal heating or cooling is used to control the process temperature of the inner tube, it may be monitored to ensure that at least a part of it (it; the inner tube) is maintained within a predetermined temperature range.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows the apparatus prior to the convolution process beginning; with an outer plastic tube la and an inner plastic tube lb securely fastened by a clip 3b to a sleeve' 3 with an elongate convolution forming element 2 also securely fastened to 3 at a fixing point 3a, external tube heating 4 and heat 4a circuniferentially surrounding 3 between Ba and 3b, (optional internal support 6 and heat óa), with two clamps Sal and 5a2 externally partially surrounding 3 between 3a and 4, which are both operable to move inwards; in accordance with the present invention.

Figure 2 shows the apparatus during the convolution process; with a sleeve' 3 rotating 3d and moving axially away 3c from external tube heating 4, heat 4a, and tube temperature monitoring 8, with an elongate convolution forming element 2 securely fastened to 3 at a fixing point 3a, and 2 being helically wound around an outer plastic tube [a and an inner plastic tube lb fonning convolutions laC in the outer tube and comiolutions lbC in the inner tube, with clamps Sal and 5a2 substantially surrounding Ia, and clamp Sb partially surrounding laC with external cooling 7; in accordance with the present invention.

Figure 3 shows the apparatus prior to the convolution process beginning; with an convolution forming element source 2a delivering an elongate convolution fanning eLement 2 which is securely fastened to a fixing point Ba on a sleeve' 3, with an outer plastic tube Ia and an inner plastic tube lb. and clamps Sal and 5a2 which are operable to move inwards towards Ia; in accordance with the present invention.

Claims

Sclaims: 1. Apparatus for forming from two plastic tubes, heat set helically convoluted duplex' tubing which has an inner tube with shallower convolutions than the outer one which has an external retaining element comprising: tube heating means arranged to heat at least a portion of both cubes to predetermined temperature(s) which is/are sufficient to cause the chosen tube material(s) to at least partially lose elastic memory; control means for controlling the tube heating means so as controllably heat and maintain a part of both tubes at the predetermined temperature(s); means to substantially maintain the overall diameter of the unconvoluted outer tube; an elongate convolution forming element which is operable to urge the heated portion of the outer tube against the heated portion of the inner tube when both portions have reached the predetermined temperature(s) to form helically convoluted tubing; a convolution forming element source for delivering the elongate convolution forming element; means for rotating both tubes relative to the convolution forming element source and means for moving both tubes axially relative to the convolution forming element source; whereby the convolution forming element is wound around and urged against the heated outer tube which is urged against the heated inner tube, whilst the overall diameter of the unconvoluted outer tube is substantially maintained, to form helical convolutions in both tubes when the tube material(s) has/have at least partially lost its/their elastic memory, the convolution forming element being retained on the newly convoluted portion of the outer tube to form a convolution retaining element as that portion of both tubes is moved away from the tube heating means.
2. A method of producing from two plastic tubes, heat set helically convoluted duplext tubing which has an inner tube with shallower convolutions than the outer one which has an external retaining element comprising the steps of: locating a smaller diameter tube inside a larger diameter tube; heating at least a portion of both tubes to predetermined temperature(s) which causes both tubes material(s) to at least partially lose its/their elastic memory; controlling tube heating means so as to controllably heat and maintain a part of both tubes at the predetermined temperature(s); rotating and moving both tubes axially relative to a convolution forming element source for delivering an elongate convolution forming element which is operable to urge the heated portion of the outer tube against the heated portion of the inner tube when portions of both tubes have reached the predetermined temperature(s) to toxin helically convoluted tubing; whereby the convolution forming element is wound around and urged against the heated outer tube which is urged against the heated inner tube, whilst the overall diameter of the unconvoluted outer tube is substantially maintained, to form helical convolutions in both tubes when the tube material(s) has/have at least partially lost its/their elastic memory, the convolution forming element being retained on the newly convoluted portion of the outer tube to form a convolution retaining element as that portion of both tubes is moved away from the tube heating means.
3. Apparatus or a method as claimed in claim lot 2 in which, two clamps are operable to enclose & substantially surround the heated outer tube during convolution formation, substantially maintaining the overall diameter of the imconvoluted outer tube.
4. Apparatus or a method as claimed in claim 3 wherein, both clamps have concave semi-circular profiles with diameters approximately equal to the overall diameter of the heated outer tube.
5. Apparatus or a method as claimed in claim 4 wherein, both clamps have internal diameters which are reasonably close-fitting with the overall diameter of the heated outer tube.
6. Apparatus or a method as claimed in claim 5 wherein, both clamps are operable to be advanced from either side of the outer tube towards each other until they substantially surround its circumference.
7. Apparatus or a method as claimed in claim 6 wherein, the clamps are in very close proximity to where the elongate convolution forming element comes into contact with the outer tube from the convolution forming element source.
8. Apparatus or a method as claimed in claim 7 wherein, both clamps are controllably heated to maintain the temperature of the unconvoluted outer tube prior to convolution formation.
9. Apparatus or a method as claimed in any preceding claim wherein, an internal support which may be a cable, a mandrel, or similar, is located within the inner tube.
10. Apparatus or a method as claimed in claim 9 wherein, the internal support extends beyond the uneonvoluted tubes at one end or/and extends beyond the convoluted duplex' tube at the other end.
11. Apparatus or a method as claimed in claim 10 wherein, the internal support is used as a means to cente? the heated inner tube during convolution formation, & establish and maintain the depth of the inner tubes' convolutions.
12. Apparatus or a method as claimed in claim 11 wherein, the internal support is tapered inwardly to a smaller diameter in a direction away from the tube beating means.
13. Apparatus or a method as claimed in claim 12 wherein, the internal support has a smooth external surfice along its length.
14. Apparatus or a method as claimed in claim 12 wherein, the internal support has a helically configured external surface along part, or all of its length.
15. Apparatus or a method as claimed in claims 9-14 wherein, inner tube heating is provided by heating elements which are part of the internal support.
16. Apparatus or a method as claimed in claims 9-14 wherein, inner tube cooling is provided by passing a suitable liquid coolant within/through the internal support, or by compressed-air which is directed internally through the inner tube.
17. Apparatus or a method as claimed in any preceding claim wherein, the elongate convolution forming element comes into contact with the heated outer tube, adjacent to, & in very close proximity to the means employed to substantially maintain the overall diameter of the unconvoluted outer tube.
18. Apparatus or a method as claimed in claim 17 wherein, the elongate convolution forming element is helically wrapped around the outer tube under controlled tension.
19. Apparatus or a method as claimed in claim 18 wherein, the elongate convolution forming element tension is controllably altered as necessary; prior to, during, and after convolution formation.
20. Apparatus or a method as claimed in any preceding claim wherein, the convolution forming element source is in close proximity to the means employed to substantially maintain the overall diameter of the unconvoluted outer tube.
21. Apparatus or a method as claimed in claim 20 wherein, the position & location of the convolution forming element source is fixed.
22. Apparatus or a method as claimed in any preceding claim wherein, the sleeve' is rigid & may be a thin-wall steel tube or similar.
23. Apparatus or a method as claimed in claim 22 wherein, one end of both tubes are first securely fastened to the end of the sleeve'.
24. Apparatus or a method as claimed in claim 23 wherein, the loose' end of the elongate convolution fonning element is secured to the sleeve' at a pre-determined location point.
25. Apparatus or a method as claimed in claim 24 wherein, the sleeve' is operable to be controllably rotated & axially moved.
26. Apparatus or a method as claimed in claim 25 wherein, parts of the sleeve' are removed along some of its length to allow an internal support, to be controllably heated to a predetermined temperature range by the external tube heating prior to convolution formation. ii
27. Apparatus or a method as claimed in any preceding claim wherein, the process temperature of the inner tube is monitored to ensure that at least a part of it is maintained within a predetermined temperature range.
28. Apparatus or a method as claimed in any preceding claim wherein, the tube heating at least partially circumferentiafly surrounds both tubes along part of their lengths.
29. Apparatus or a method as claimed in claim 28 wherein, both tubes are heated to the required process temperature(s) around 36tY of their circumferences.
30. Apparatus or a method as claimed in any preceding claim wherein, the process temperature of the outer tube is monitored by an infrared thermocouple.
31. Apparatus or a method as claimed in any preceding claim wherein, external cooling is provided.
32. Apparatus or a method as claimed in claim 31 wherein, compressed-air cooling surrounds 360' of the duplex' tubing along part of its length.
33. Apparatus or a method as claimed in any preceding claim wherein, a clamp, or pair of clamps, are used to prevent the (elongate forming) element from moving axially along the outer tube during convolution formation.
34. Apparatus or a method as claimed in claim 33 wherein, this/these clamp(s) at least partially surround a length of the newly formed duplex' tubing.
35. Apparatus or a method as claimed in any preceding claim wherein, the apparatus is arranged to loan the duplex' tubing vertically.
36. Apparatus or a method as claimed in claim 34 wherein, the apparatus is arranged to form the duplex' tubing horizontally.
37. Apparatus for forming forming from two plastic tubes, heat set helically convoluted duplex' tubing which has an inner tube with shallower convolutions than the outer one which has an external retaining element; substantially as hereinbefore described and/or with reference to Figures 1,2 or 3.
38. A method of producing from two plastic tubes, heat set helically convoluted duplex' tubing which has an inner tube with shallower convolutions than the outer one which has an external retaining element; substantially as hereinbefore described and/or with reference to Figures 1,2 or 3.